Download Lecture: Fatty Acids Synthesis Recall the physiological role of

Lecture: Fatty Acids Synthesis
 Recall the physiological role of fatty acid synthesis
o Storage mechanism for excess fuel, starting material for lipid biosynthesis

Relate the cellular conditions that favor fatty acid synthesis, including citrate exiting
the mitochondria to initiate fatty acid synthesis.
o Acetyl CoA starting material
o Acetyl CoA formed in mt matrix condenses w/ OAA to form citrate,
citrate transported out of mt
o Citrate lyase converts citrate back to acetyl CoA and OAA
o OAA reduced to malate by cytosolic malate dehydrogenase using NADH
o Malate oxidatively decarboxylated by malic enzyme forming NADPH,
pyruvate and CO2
o Insulin induces FA synthesis
o NADPH provides reducing power needed to synthesize hydrocarbon chains
 PPP deficiencies= problem, malic enzyme can help some

Recognize the conversion of acetyl CoA to malonyl CoA as the rate-limiting step for
fatty acid synthesis.
o Conversion of acetyl group is endergonic, so 1st step of FA synthesis is
“activation” of acetyl CoA by carboxylation to form malonyl CoA
o Malonyl CoA contributes 2-carbon unit to growing FA chain
o Carboxylation catalyzed by acetyl CoA carboxylase (metabolically irreversible
reaction)
 Commits acetyl CoA to FA synthesis versus other paths
 Consumes energy

Compare and contrast fatty acid synthesis and fatty acid oxidation and relate how
cells keep these two processes separated so that newly synthesized fatty acids are not
immediately re-oxidized after synthesis.
o Synthesis:
 Occurs in cytosol
 Thioester carrier is malonyl CoA
 Complex of enzymes
 Electron carriers are NADPH
o Oxidation:
 Occurs in mitochondria
 Thioester carrier is acetyl CoA
 Individual enzymes used
 Electron carriers are NADH
o FAs synthesized in cytosol, FAs oxidized in mt
o Malonyl CoA key intermediate for FA synthesis
 Malonyl CoA inhibits CPTI (primary player in getting FAs into mt
for oxidation)

Recall the mechanisms of regulation of fatty acid synthesis, and specifically the regulation
of Acetyl CoA carboxylase
o Same hormones that regulate blood glucose levels also regulate FA metabolism
by both short-term and long-term mechanisms
 Long-term involves changing amount of key enzymes in cell
 Short-term involves modulation of activity of enzyme molecules
 Insulin stimulates FA synthesis by stimulating dephosphorylation
of acetyl CoA carboxylase
 Malonyl CoA allosterically inhibits carnitine acyltransferase I so
that acyl CoA is not translocated to mt matrix but remains in
cytosol
 Insulin also causes dephosphorylation of adipocyte hormone
sensitive lipase inactivating it and preventing TG hydrolysis
 Acetyl CoA carboxylase
 Allosteric
o Feed forward and feedback inhibition
 Phosphorylation
o Glucagoninactive
o Insulin active
 Inducible
o Cells can alter amount of enzyme present
Lecture: Synthesis of Triacylglycerols and Major Membrane Lipids
 Compare and contrast the synthesis of phosphatidic acid from glycerol in liver versus
glucose (liver and adipose tissue). Interpret why adipose tissue can’t utilize glycerol
for this purpose.
o Synthesis of phosphatidic acid starts w/ glucose, more locally w/
dihydroxyacetone phosphate (DHAP)
o DHAP reduced to glycerol 3-P by G-3-P dehydrogenase
 Pre-existing glycerol can also be phosphorylated in liver to glycerol 3P
 Adipose lacks enzyme to make G-3-P
 Don’t want adipose tissue to convert glycerol back to lipid
when FA’s are mobilized for energy (use glycerol for glucose
synthesis)

Recall the processing of phosphatidic acid into a triglyceride.
o Acyl groups added at C-1 and C-2 by acyltransferases using acyl CoA,
saturated FAs are preferentially added at C-1 and unsaturated FAs at C-2
o C-3 of phosphatidic acid dephosphorylated, forming diacyl glycerol (DAG)
o 3rd FA attached at C-3 to complete TG synthesis

Relate the process by which VLDLs are made, and identify which organ makes
VLDLs.
o TGs packaged into VLDLs
 Processed in Golgi, then secreted into blood
o Destination is adipose tissue for storage and muscle for energy use
o VLDLs made in liver

Recall triglyceride degradation in adipose tissue by hormone sensitive lipase, and
identify the hormone that regulates the hormone sensitive lipase.
o During fasting, adipose TG broken down (lipolysis)
o Lipases cleave FAs from TG: hormone sensitive lipase starts process
 Signaled by decreasing insulin and increasing glucagon
 cAMP levels rise, protein kinase A is activated
 phosphorylation of hormone-sensitive lipase turns it on
 other lipases cleave remaining FAs from glycerol backbone
o Free FAs and glycerol are released into blood
 FAs oxidized for energy in muscle and kidney
 Free glycerol used by liver for gluconeogenesis
o FAs carried by albumin in blood

Compare and contrast lipoprotein lipase activity versus hormone-sensitive lipase
activity, including the metabolic conditions that support these activities. Interpret
why it is important that these two enzymes not be active at the same time.
o Important that both enzymes are not active at same time b/c ic would be
pointless to be synthesizing FAs when you are trying to use them

Recognize the synthesis of glycerophospholipids from phosphatidic acid, and the
synthesis of sphingolipids and glycolipids from serine and palmitoyl CoA.
o
o
o

Glycerophospholipids
 ethanolamine and choline rapidly phosphorylated upon entering cells
and then react w/ CTP to form CDP-ethanolamine and CDP-choline,
releasing PPi
 Phosphoethanolamine and phosphocholine are transferred from
activated nucleotide derivative to DAG, releasing CMP
 Phospatidylserine is produced from PE
 Liver can interconvert PE to PC by sequential transfer of 3 methyl
groups from S-adenosyl-methionine (SAM)
 SAM is methyl group donor, major player in biochemical
reactions
Sphingolipids
 Sphingolipid synthesis can be divided into 2 parts
 Formation of ceramide
 Conversion of ceramide to sphingomyelin and glycolipids
 Precursors to sphingosine are serine and palmitoyl CoA whose
condensation forms 3-ketosphinganine w/ decarboxylation of serine
 3-ketosphinganine is converted to a ceramide in 3 reactions
 3-keto group reduced to 3-hydroxy group by NADPH
 A delta-4 trans double bond is introduced by oxidation involving
FAD
 Amino group is acylated by transferring an FA (usually
palmitate) from acyl CoA
Glycolipids
 Sphingomyelin formed by transfer of phosphocholine from
phosphatidylcholine to C-1 hydroxy group of ceramide
 Gluco- and galactocerebrosides are formed by transfer of sugar group
from UDP-glucose or UCP-galactose
 Complex glycolipids are formed by adding additional sugars one at a
time
Identify the site of action on a glycerophospholipid of each of the phospholipase enzymes
o Glycerophospholipids are hydrolyzed by phospholipases
 Phospholipase A1 releases FA at postion 1 on glycerol
 Phospholipase A2 releases FA at position 2
 Phospholipase C releases phosphorylated base at position 3
 Phospholipase D releases free base
Lecture: Cholesterol Metabolism
NOTE: Cholesterol synthesis is very complicated, and it is not important to understand it in
detail – just as presented on the lecture slides. However, pay special attention to stage I,
which is the point of regulation for the pathway and the target of drug therapy.

Recall the synthesis of mevalonate from acetyl CoA by the enzyme HMG-CoA
reductase, and interpret the need for NADPH in the synthesis.
o Two acetyl CoAs condense to form acetoacetyl CoA
o 3rd acetyl CoA is added to form 3-hydroxy-3-methylgutaryl CoA (HMGCoA)
o HMG-CoA reductase reduces HMG-CoA to mevalonate using 2 NADPH

Identify the precursor of cytoplasmic acetyl CoA used for cholesterol synthesis (and
fatty acid synthesis). Recall what metabolic conditions favor the movement of citrate
from mitochondria to the cytoplasm.
o Acetyl CoA for cholesterol synthesis formed in mitochondrial matrix
(following glycolysis) and is transported to cytosol by citrate transport system
 Fed state: want to make FAs and make cholesterol
 both involve citrate shuttle to get acetyl CoA out to
cytoplasm

Identify the different mechanisms of regulation of HMG CoA reductase.
 1st committed step and rate-limiting step of cholesterol synthesis
 site of regulation of cholesterol synthesis
 located on cytoplasmic face of ER, has a cytoplasmic catalytic
domain and regulatory membrane domain
 activity is highly regulated
 short-term by phosphorylation/dephosphorylation primarily
by AMP-activated kinase that regulates acetyl CoA
carboxylase (AMP high, HMG-CoA reductase activity
decreased)
 Long-term by modifying the enzyme level in cell (DNA/mRNA)
 Proteolytic degradation:
 removes the enzyme from the pathway
 Reversible covalent modification
 Don’t want to synthesize cholesterol during fasting…
 Glucagon stimulates HMG-CoA reductase phosphorylation
= off
 or when ATP is low…
 or when sterol level is high.
 High [AMP] or high [sterol] turns off HMG-CoA reductase

Recognize the goal of the second phase of cholesterol metabolism. Interpret the
need to activate mevalonate with a pyrophosphate group in this stage of the
synthesis.
o Conversion of mavalonate to isopentenyl pyrophosphate involves a series of
cytoplasmic reactions
o Farnesyl pyrophosphate is formed by condensation of geranyl PPi w/
another isopentenyl PPi w/ PPi release
o 2 farnesyl PPi molecules condense head to head to form squalene and
releases 2 PPi

Enzyme catalyzing squalene synthesis and all subsequent enzymes are
bound to cytoplasmic face of ER

Identify the two enzymes that can esterify cholesterol. Identify where these two
enzymes function and relate why this is important for cholesterol transport.
o Lecithin: cholesterol acyltransferase (LCAT)
 Located in blood
 Esterifies cholesterol associated w/ HDL
o Acyl: cholesterol acyltransferase (ACAT)
 Located in cells
 Concentrated in cells that need to store cholesterol for steroid
synthesis
o Esterification of –OH greatly increases hydrophobicity

Recognize cholesterol breakdown and elimination from the body.
o TGs synthesized in liver are packaged w/ cholesterol from pool in liver cells
into VLDL lipoproteins
o Once in blood, HDL transfers apoCII and apoE and cholesterol esters to
VLDL
o When LPL acts on VLDL, and TG are degraded, IDL is produced
 If TG further degraded, IDL becomes LDL
 Or IDL can be taken up directly by liver by endocytosis
o HDL picks up cholesterol from cells, other lipoproteins
o As enzymes to esterify cholesterol, make it more hydrophobic
o Has more cholesterol esters than cholesterols, not much TG
o HDLs return to liver for recycling
Lecture: Blood Lipoproteins: Look at the “steps” that produce cholesterols and the
“steps” of converting cholesterol to its many products.
 Relate the primary mechanisms for cholesterol transported in the blood - both
dietary cholesterol and that produced by the liver.
o Chylomicron cholesterol: from diet via intestines
 Cholesterol remains associated w/ chylomicrons during LPL action
 Chylomicron remnants that are taken up by endocytosis into liver
cells are digested in lysosomes, which also processes any cholesterol
esters to cholesterol
 Cholesterol stays intact and enters cholesterol pool in liver
o LDL cholesterol: form liver to peripheral tissues
 Cells take up LDLs, incorporate cholesterol inside
 Intracellular cholesterol signals a decrease in cholesterol synthesis
(inhibits HMG-CoA reducatse)
 Cholesterol down regulates production of LDL receptors
 Less receptors bring less cholesterol into cell
 As receptors are endocytosed, fewer receptors remain on cell
surface
o VLDL cholesterol: from the liver



TGs synthesized in liver are packaged w/ cholesterol from pool in
liver cells into VLDL lipoproteins
Once in blood, HDL transfers apoCII and apoE and cholesterol
esters to VLDL
When LPL acts on VLDL, and TG are degraded, IDL is produced
 If TG are further degraded, IDL becomes LDL
 IDL can be taken up directly by liver by endocytosis

Compare and contrast the different forms of lipoproteins in terms of their general
composition and function. (It is not necessary to memorize Table 34.7, but
understanding the trends in the table will help with the understanding of lipoprotein
function. For example, recognizing which lipoprotein has the most triacylglycerol
content, which has the most cholesterol or cholesterol esters, or protein or lipid, etc.
o Classified as chylomicrons, VLDL, IDL (VLDL remnants), LDL and HDL
 Order based on increasing density (greater protein content) and
decreasing size

Recall the function of the LDL receptor and identify what lipoprotein apoprotein(s)
it recognizes.
o All blood lipoproteins have receptors on cells (ex: LDL receptor)
o LDL receptor recognizes apoE and apoB100
 Binds LDL, VLDL, IDL and chylomicron remnants
o Its job is to bind lipoproteins and bring them into the cell by endocytosis
 For LDLs, that means delivering cholesterol and cholesterol esters

Recall the fates of VLDLs and LDLs as they travel through the blood stream.
o As VLDLs lose TGs, they are converted into IDLs, which are smaller and
have a higher protein content, thus a higher density
o Some IDL is taken up by the liver, some is converted to LDL and some give
up remaining cholesterol to nascent HDL

Interpret how LDL import into a cell via the LDL receptor affects cholesterol
synthesis and utilization by the cell, and vice versa.
o LDL receptor delivers cholesterol and CE to cell
o Cholesterol tightly regulates its own metabolism
 Inhibits LDL receptor synthesis
 Controls endogenous cholesterol synthesis
o Activates storage of excess cholesterol
 Re-esterified to form cholesterol ester droplets in cytosol

Recall the role of lipoproteins in the process by which atherosclerotic plaques form
o Minor damage cells lining vessel wall occurs
o Macrophages recruited, uptake LDLs, macrophages get filled w/ lipid =
foam cells
o Foam cells accumulate = fatty streak
o Chemical signals induce local growth, platelet aggregation  clot formation
o More LDL recruited by cells, fat builds up
o Fibroblasts excrete fibrous proteins, make a tough cap
 Cells die, leaving debris
 More macrophages recruited
o Process repeats, adding to buildup until vessel is completely blocked
Lecture: Eicosanoid Metabolism
 Define the term “eicosanoid” and identify the main members of this group.
o Eicosanoid= class of compounds synthesized from 20 carbon FAs
(arachidonic acid most common source in humans)

Consists mostly of prostaglandins (PG), thromboxanes (TX) and
leukotrienes (LT)

Recognize the structural features of prostaglandins, thromboxanes and leukotrienes.
Identify the common starting material for these compounds.
o Prostaglandins are fatty acids
 20 carbon atoms
 an internal, saturated 5-carbon ring
 a hydroxyl group at carbon 15
 a double bond between carbons 13 and 14
 various substituents on the ring
o Thromboxanes structure similar to PG, but they contain a 6-membered ring
 Some TXs have an additional oxygen atom bridging carbons 9 and 11
of the ring
o Leukotrienes are characterized by 3 consecutive double bonds (triene)
 also have an oxygen atom (or two) bound as an OH or as an epoxide
o All synthesized from PUFAs containing 20 carbons and 305 double bonds
(usually arachidonic acid)

Using figure 35.2, relate how cellular signals lead to the release of arachidonic acid
and the subsequent production of eicosanoids.
o PUFAs usually attached to glycerol backbone of phospholipid
 Phosphoatidylcholine or phosphatidylinositol
o PUFA is cleaved from phospholipids by phospholipase A2
 Triggered by stimulus binding to membrane receptors
 Histamines or cytokines
 Steroidal anti-inflammatory agents inhibit phospholipase A2

Identify the three enzymes that convert arachidonic acid to the three major classes of
eicosanoid (prostaglandin, HPETE and epoxides).
o Cyclooxygenase (COX): leads to prostaglandins and thromboxanes via
PGG2
 COX-1 is a constitutive enzyme in gastric mucosa, platelets, vascular
endothelium, and kidney
 COX-2 is inducible and is generated in response to inflammation
 Mainly expressed in activated macrophages but other tissues
also
o Cytochrome P450: leads to diHETE and HETE via epoxides
o Lipoxygenase: leads to leukotrienes, HETE and lipoxins via HPETE
 Lipoxygenase is a dioxygenase that inserts a peroxide

Recall in general terms the synthesis of prostaglandins and thromboxanes from
arachidonic acid.
o Oxygen is added and a 5-carbon ring is formed by a cyclooxygenase that
produces initial prostaglandin, which is then converted to other classes of
PGs or TXs
o PGH2 may be converted to TXA2
o Next step is tissues specific
 Function then degradation

Compare and contrast the mechanisms by which steroidal and non-steroidal antiinflammatory drugs block the formation of prostaglandins and thromboxanes.
o NSAIDs block prostaglandin formation by irreversibly inhibiting COX
 Aspirin acrylates an active site serine
 Aspirin is only COX inhibitor that covalently modifies COX
 Aspirin is more potent against COX-1 than COX-2
 DO NOT give aspirin to any patients that have respiratory
problems (need prostaglandins to help breathe)
 Other NSAIDs bind non-covalently to COX to inhibit them
o Steroidal anti-inflammatory drugs (hydrocortisone, prednisone) block
prostaglandin formation by inhibiting phospholipase A2
 Blocks release of arachidonic acid for PG synthesis
Lecture: The Urea Cycle
 Identify the protein as the major source of nitrogen intake for humans.
o Dietary proteins are the primary source of nitrogen for humans
 Amino acids produced from digestion of proteins are absorbed by
intestinal epithelial cells and enter blood
 AA enter cellular pools and are used for synthesis of proteins and
other nitrogen-containing compounds

Define the relationship between ammonia and ammonium ion. Interpret how
ammonia toxicity can arise in patients despite the very high pKa of ammonia.
o Ammonia is a weak base, and ammonium is its conjugate acid
o Physiological pH is 7.4, so ammonia is nearly 100% protonated
 Ratio of ammonium ion, NH4+ to ammonia, NH3 is 100:1 (more
NH4 means more NH3)
 Ammonia is toxic
 Control NH4 level to control NH3 level
 Deficiency in liver function  several distinct neurological
problems  coma
 Need to clear out excess waste nitrogen (urine)
o Depends on availability of water

Recall the glucose-alanine cycle and the gluatamine cycle for nitrogen elimination.
Identify ammonia the essential biological molecule that moved by these cycles.
Glucose-alanine cycle
Glutamine cycle

Identify the sources of nitrogen for urea synthesis. Recognize which amino acids
“collect” nitrogen atoms?
o Transamination
 Major process of removing nitrogen from amino acids (aa)
 Nitrogen is transferred as an amino group from an aa to alphaketoglutarate to form glutamate
 Catalyzed by transaminases (aminotransferases)
 Readily reversible, involved in aa synthesis and degradation
o All aa except lysine and threonine can undergo transamination

Identify the transaminase reaction and define the key role of pyridoxyl phosphate
(vitamin B6) in the reaction.

Define the functions of ALT and AST, and interpret why are they considered blood
markers for liver damage?
o Liver cell damage leads to leakage into blood of cellular enzymes like AST
(aspartate transaminase) and ALT (alanine transaminase)
o Serum levels of AST and ALT are elevated in chronic alcoholic cirrhosis and
acute viral hepatitis
 It just so happens that two of these, ALT and AST, were discovered
in the blood of patients with liver cell damage and became the first
marker enzymes for liver damage




Recognize the steps of the urea cycle, recall the goal of the cycle, and idenfity where
the cycle takes place in the cell.
Urea cycle steps: Often Careless Campers Are Also Frivolous About Urination
Three phases:
o Synthesize carbamoyl phosphate
o Produce arginine
o Cleavage of arginine to produce urea
Five steps
o 2 in mitochondrial matrix
o 3 in cytosol
Urea is eliminated from cell and eliminated from body via urine

Interpret the critical role of carbamoylphosphate synthetase I (CPS I) in decreasing
blood ammonia levels, and relate how defects in this enzyme would affect blood
nitrogen levels.
o First Step in Urea Synthesis:
 Mitochondrial matrix
 NH4 and bicarbonate are condensed at expense of 2 ATPs to form
carbamoyl phosphate
 1st ATP activates bicarbonate
 2nd donates phosphate
 Catalyzed by enzyme carbamoyl phosphate synthetase I (CPS I)
 If defective, hyperammonemia: NH4+ can’t be cleared from
body

Identify the factors that regulate the urea cycle, including which hormone signal most
strongly boosts the cycle activity.
o N-acetyl glutamate (NAG)
 Formed when arginine is high, stimulates CPS 1 to start moving
nitrogen out
 Increased cell arginine is an indication that the urea cycle is running
and that more ammonium/ammonia needs to be cleared out
o Substrate availability
 Urea cycle enzymes have high capacity to move substrate to product,
allowing liver to clear nitrogen quickly and efficiently
o Induction
 Under conditions of prolonged high nitrogen load (high protein diet,
prolonged fasting)
 Urea cycle enzymes are produced in greater number to help
handle increased load
 Glucagon leads to increased synthesis of CPS I to help handle
increased nitrogen production during fasting

Recall the biochemical basis, in terms of urea cycle enzyme deficiencies, of low blood
urea nitrogen (BUN), hyperammonemia, elevated blood levels of urea cycle
intermediates and orotic acid.
o CPS I: hyperammonemia
 NH4+ can’t be cleared from body
o Ornithine transcabmoylase (OTC):
 Most common deficiency involving urea cycle enzymes
 Increased blood of ammonia and orotic acid
 Carbamoyl phosphate diffuses into cytosol and reacts w/ aspartate
(step in pathway of pyrimidine synthesis)
 Excess orotate (intermediate in pyrimidine biosynthesis) is excreted
in urine
o Argininosuccinate synthase:
 Highly elevated blood citrulline
o Argininosuccinate lyase:
 Argininosuccinate excreted in large amounts
 Elevated blood citrulline
o Arginase: high blood arginine
 low blood urea nitrogen

Recall the types of liver malfunction that can result in high blood ammonia levels.
o Hepatitis: inflammation of liver due to parasites, bacteria or viruses
 can cause hepatic necrosis and thereby reduce liver capacity to
remove toxic nitrogenous waste
o Damage to liver by chronic alcohol consumption-metabolism
o Damage to liver by toxic substances (acetaminophen overdose)
o Bacteria in intestinal tract convert urea back to ammonia and CO2 using
urease enzyme

Fairly normal process (1/4 of urea released by liver recycled by
bacteria)
 Urea is not cleaved by human enzymes
 Normally, ammonia moves to liver, which reprocesses it
 If liver function is impaired  ammonia toxicity
o Hepatic Encephalopathy: brain dysfunction due to liver failure
 Functional and anatomical shunting of nitrogenous metabolites
formed in gut into systemic circulation, bypassing their normal
metabolism by liver cells
 Brain cells become depeleted of alpha-ketoglutarate (and thus ATP)
when high concentrations of ammonia reverses glutamate
dehydrogenase reaction
 Increased synthesis of glutamine causes deficiency of the
neurotransmitter glutamate, which is also precursor of
neurotransmitter gamma-aminobutyrate
Lecture: Special Products Derived from Amino Acids
• Recall the amino acid composition of glutathione and its function in cells
 Composed of glycine, cysteine, and glutamate
 Substrate for gamma-glutamyltransferase, GSH-peroxidase
 Reduces dehydro-ascorbate, which is produced by reaction of ROS + ascorbate
 Glutathione levels can be low due to dietary or environmental factors (chronic
free radical stress)
•
Recall the amino acid composition creatine and its function in cells
 Glycine and arginine (transamidinase) guanidinoacetate and ornithine (Sadenosylmethionine)  creatine
 Reservoir of high energy phosphate: creatine travels from liver to other
tissues (esp muscle and brain) where it reacts w/ ATP to form creatine
phosphate
o Regenerates ATP (CPK reaction reversible)
 Transfer of high energy phosphate: creatine phosphate shuttle of heart and
skeletal muscle allows rapid transport of high energy phosphate (ATP formed in
oxidative phosphorylation) from mitochondrial matrix to cytosol for muscle
contraction
 Amount of creatine in body related to muscle mass
o Constant amount of creatinine excreted in urine daily and reflects muscle
mass
o Serum CK is elevated in patients who had a stroke or MI
• Recognize the general process of heme synthesis and then define the biochemical
basis of porphyrias and lead poisoning. Identify the major organs / tissues that have the
highest rates of heme biosynthesis.
 From succinyl-CoA and glycine
o 1st step (rate limiting step) catalyzed by δ-ALA (δ-aminolevulinate)
synthase
o 2nd step: catalyzed by δ-ALA dehydratase
 2 molecules of δ-ALA condense to form pyrrole,
porphobilinogen
 4 molecules of porphobilinogen condense to form a linear chain
and then a series of phorphyrinogens
 Acteyl groups are first decarboxylated to methyl, then 2 propinoyl
groups are oxidized to vinyl to form protoporphyrinogen IX
 Methylene bridges are then oxidized to form photoporphyrin IX
o Final step: catalyzed by ferrochelatase (heme synthase): iron as (ferrous)
is incorporated to form heme
o Lead poisoning: inactivation of δ-ALA dehydratase (contains zinc) and
ferrochelatase
 γ-ALA and protoporphyrin accumulate
 reduced heme production resulting in
 anemia (lack of hemoglobin)
 reduced energy production (lack of cytochrome for ETC)
o Highest production in liver (cytochromes), bone marrow (hemoglobin)
and muscle (myoglobin)
• Recognize the degradation of heme to bilirubin, define direct versus indirect
bilirubin, and interpret how each form of bilirubin is made in the liver. Using body
bilirubin data (direct, indirect, blood, urine, feces), interpret liver bilirubin function.
 1) RBCs phagocytosed by cells of reticuloendothelial system (RES)
 2) Heme is degraded to bilirubin in RES
o heme oxidized to biliverdin, biliverdin reduced to bilirubin
 3) bilirubin is carried by serum albumin to liver, conjugated w/ glucoronic acid
(bilirubin diglucoronide) and excreted in bile
 4) Globin cleaved to constituent AA
 5) Iron returned to body stores
 Plasma bilirubin:
o Conjugated bilirubin expressed as direct bilirubin
 Couples w/ diazonium salts to form azo dyes
 When bilirubin presented to liver in amounts that exceed its capacity to
conjugate (sever hemolysis), elevated serum bilirubin (unconjugated)
o Conjugated <20% of total
 In hepatocyte dysfunction (viral hepatitis) elevated serum bilirubin (conjugated &
unconjugated)
o Conjugated 20-50% of total
 As a result of hepatic obstruction of drainage ducts from liver to intestine
(gallstornes, cancer at the head of the pancreas)  elevated serum bilirubin
(conjugated)
o Conjugated >50%
• Recall the different isoforms of nitric oxide synthase and compare and contrast how
they are alike and how they are very different.
 Activates guanylyl cyclase (increases cGMP synthesis)
 Nitroglycerin is converted to NO and dilates coronary arteries in treating angina
pectoris
 NO synthesized from arginine by nitric oxide synthase
o NO combines w/ oxygen to form nitrite
o Nitrite converted to nitrate and excreted in urine
• Identify the amino acid precursors that lead to the synthesis of GABA, histamine,
serotonin and catecholamines.
 GABA: inhibitory neurotransmitter synthesized by decarboxylation of glutamate
 Histamine: histadine is decarboxylated to form histamine
 Serotonin: 5-hydroxytrptamine produced by hydroxylation and decarboxylation
of tryptophan
 Melatonin: synthesized by acetylation and methylation of serotonin
 Catecholamines: dopamine, norepinephrine and epinephrine are synthesized
from phelalanine and tyrosine
• Recall the activity of monoamine oxidase, and relate how a malfunction in MAO
activity can lead to Parkinson’s Disease.
 Oxidative deamination by monoamine oxidase (MAO) to form aldehydes
o Too much MAO may cause damage to dopamine receptors when they
breakdown dopamine into it’s by-products (including hydrogen peroxide)
• Identify the amino acid that leads to the synthesis of melanin, and relate how
deficiencies in these enzymes leads to albinism.
 3,4-dihydroxyphenylalanine (DOPA) is oxidized to quinones which polymerize
to form melanin pigments
o tyrosine is converted to DOPA
 hypomelanosis due to heritable defects in Cu-dependent tyrosine hydroxylase
(tyrosinase), or other enzymes that convert Tyr to DOPA or dopaquinone
(which convert to melanin)
Lecture: Single Carbon Transfer
 Recall the transfer of one carbon units by tetrahydrofolate, and identify the various
forms of carbon that FH4 carries.
o Groups containing single carbon can be transferred
o Most oxidized form of carbon (carbon dioxide) is transferred by biotin
o One carbon groups at lower levels of oxidation are transferred by reactions
involving
 Tetrahydrofolate
 Vitamin B12
 S-adenosylmethionine (SAM)
o Serine is the major source of one carbon units

Serine is synthesized from glucose, dietary carbohydrate is the major
source of one carbon units

Interpret the consequences of folate deficiency.
o Causes accumulation of formiminoglutamate (FIGLU, produced during His
degradation)
 Histidine load test is used to detect folate deficiency
 Feed His and measure urine FIGLU
o Results in increased dUMP/dTMP ratio
 Increased incorporation of uracil into DNA
o Inhibition of DNA repair
 Due to lack of dTTP
 Leads to DNA fragmentation
 Macrocytic, megaloblastic anemia: inability of hematopoietic (and
other) cells to synthesize DNA and to divide  insufficient RBCs
o Impaired DNA synthesis and repair leads to large cells w/ abundant
cytoplasm as cells persistently try but are unable to divide

Recall the activity of methotrexate, including the enzyme it inhibits and its effect on
cellular metabolism.
o Methotrexate: folate analog
 Competes w/ FH2 for binding to dihydrofolate reductase
 Strong competitive inhibitor
 Inhibits formation of dTMP from dUMP
o Used in treatment of leukemia
o Methotrexate inhibits dihydrofolate reductase
o 5-fluouracil (pyrimidine analog) inhibits thymidylate synthase

Recall the biochemical action of sulfa drugs on bacteria folic acid synthesis.
o Folate synthesized in bacteria from p-aminobenzoic acid (PABA)
o Sulfa drugs compete w/ PABA
 PABA is required for bacterial growth, folate cannot be absorbed by
bacteria
 Humans can get folate from dietary sources

Recall how vitamin B12 is absorbed and transported in the body.
o Can only be synthesized by bacteria
 Dietary sources are meat, eggs, dairy products, poultry, seafood, and
intestinal flora
o Absorption in ileum requires intrinsic factor, a glycoprotein secreted by
gastric parietal cells
 Lack of intrinsic factor results in pernicious anemia
 Common problem in elderly
 Need to give vitamin B12 by injection, not oral

Recall the two primary single carbon reactions involving vitamin B12 in humans, and
interpret the metabolic result of a B12 deficiency, in terms of these reactions?
o Transfer of methyl group from FH4 to homocysteine to form methionine
 One carbon group transfers from serine to FH4, then to B12, then to
homocysteine, then to SAM, then to other molecules
o Coenzyme in conversion of methylmalonyl CoA to succinyl CoA catalyzed
by methylmalonyl CoA mutase
 Reaction is part of metabolic route for conversion of carbons from
valine, leucine, isoleucine, thymine, and the as 3 carbons of odd chain
fatty acids to the TCA cycle intermediates succinyl CoA
o B12 deficiency precipitates folate deficiency (methyl trap theory: trapping of
FH4 as methyl-FH4)
o Hematopoetic: caused by adverse effects on folate metabolism
 lack of B12 causes folate deficiency by trapping FH4 as methyl-FH4
 no free FH4 available for dTMP synthesis
o Neurological: progressive demyelination caused by inability to convert
methylmalonyl CoA to succinyl CoA in the brain
 Methylmalonyl CoA accumulation interferes w/ myelin formation

Recall S-adenosyl methionine (SAM) function in methyl group transfer. Identify five
reactions where SAM is used as a methyl donor.
o Participates in synthesis of compounds that contain methyl groups
Lecture: Inner-Tissue Relationship of Amino Acid Metabolism
 Recall that the body maintains a large pool of amino acids.
o Ensures continuous availability of individual amino acids for tissues for
synthesis of proteins, neurotransmitters and other nitrogen containingcompounds
o Provides complete pool specific amino acids that can be used as oxidizable
substrates

Interpret why skeletal muscle releases Ala and Gln readily during an overnight fast,
but releases very little branched-chain amino acids during an overnight fast.
o Skeletal muscle uses branched-chain amino acids (BCAA) for energy,
nitrogen sent to the kidney for elimination
 AA, alpha-ketoacids and alanine a sent to the liver for glucose
production

Identify the four principles of amino acid flux between tissues.
o NH4 is toxic
 Transported between tissues as alanine or glutamine
 Alanine primary carrier to liver, where urea is produced and excreted
 Glutamine in blood serves essential metabolic functions
 Utilization of blood glutamine pool is prioritized based on
need
o BCAA can be converted to TCA cycle intermediates and used as fuel by
most tissues
 Apart from BCAA and Ala, Asp and Glu, all AA’s are metabolized
in liver
o Amino acids are major gluconeogenic substrates leading to glucose
 Some AAs used to produce acetyl CoA or ketone bodies
 Hormones control glucose homeostasis and utilization of AAs for
glucose synthesis in liver
o Relative rate of protein synthesis versus degradation (protein turnover)
determines size of AA pool in blood

Recall the two main principles of amino acid utilization by tissues.
o Each organ/tissue will utilize specific amino acids based on metabolic state
and function of organ/tissue
o All tissues have the same requirement for essential amino acids for protein
synthesis during protein turnover

Compare and contrast amino acid utilization by kidney, skeletal muscle, gut and
brain.
o Kidney: takes up glutamine
 Gln is deaminated by glutaminase, forming NH3 and glutamate
 NH3 is released into urine, where it forms NH4+ using protons
from amino acid degradation
 Picks up a lot of free H+
 NH4+ buffers urine
 Alanine and serine are produced from glutamine
 Alpha-ketoglutarate enters TCA cycle  malate, malate  PEP
 PEP enters gluconeogenesis and can form glucose or alanine or
serine, which are released into blood
 Glucose used by kidney for energy
o Skeletal Muscle: muscle releases a lot of Glu and Ala, but not all comes
from protein degradation
 some from metabolic intermediates


BCAA can be used as energy (ketogenic)
Muscle can convert glucose to pyruvate, which can be converted to
alanine and sent to liver
 Liver will use Ala for glucose production, send glucose back to
muscle
o Gut: amino acid utilization is the same whether in the fed state (AA from
ingested proteins) or fasted state (AA from blood pools)

o Brain: AA to neurotransmitters
 Brain and nervous tissue: key function of AA metabolism is
production of neurotransmitters
 Over 40 compounds that contain nitrogen derived from AA
metabolism
 Rapid turnover of many neurotransmitters requires continuous
supply of AA precursors from pool
 Brain is net glutamine producer
 BCAA (esp valine) used to make Glu and Asp and TCA
intermediates
 Glutamate converted to glutamine which is transferred to
neuronal cells or released into blood
 Glutamine is used to make glutamate and GABA in neuronal cells
 GABA breakdown leads to succinate, which is TCA cycle
intermediate
 BCAA cross BBB
 Glutamate to glutamine (Gln can leave or move to neuronal
cells)
 In neuronal cells, Gln to Glutamate to GABA

Recall amino acid flux after a high protein meal for gut, skeletal muscle, liver and
brain.
o After high protein meal, liver and gut use most of the AAs
 Gut uses most of Glu and Asp, so little enter the portal vein




Gut uses some BCAA
Liver takes up 60-70% of AAs in portal vein
Liver converts most of AA’s to glucose
AA entering peripheral circulation are mostly BCAA—liver doesn’t
have transaminases to modify these AAs: muscle does
o Higher levels of glucagon stimulate AA utilization
 Stimulates AA uptake by liver
 Stimulate gluconeogenesis (no glucose in meal)
 Insulin is still released, but not much
 BCAA uptake and protein synthesis continues but
gluconeogenesis is not inhibited

Define “hypercatabolic state” and relate the characteristics of this state to the
general metabolic goals of the body in this state.
o Hypercatabolic state: defined as surgery, trauma, burns and septic stress
 Characterized by increased fuel utilization
 Negative nitrogen balance due to increased protein turnover and
increased degradation to meet AA needs for energy
 Fuel stores mobilized to meet tissue needs when dietary intake is
decreased
 Immune response and wound healing require energy and AAs