Download Lipid Synthesis

Fundamentals I 11:00-12:00
9/4/10
Whikehart
I.
Lipid Synthesis
Scribe: Jordan Ridgeway
Proof: Caleb Landrum
Page 1 of 5
INTRO SLIDE 3 [S3]
a. This lecture is about lipid synthesis
b. We’ll look at the different types of lipids
c. Digestion, transport, storage, some pathology, their use as fuels
d. This is a vast and complicated field
e. We’re going to focus on fatty acid synthesis and glycerol
II. Fatty Acid Synthesis [S4]
a. We take in a large variety of fatty acids in diet
b. Stored and broken down for specific cell needs
c. Why do cells go through process of making fatty acids?
d. In developed nations, it’s not so critical, but it does occur in two different ways
i. De novo synthesis – like making from scratch
ii. Carbon lengthening of Palmitic acid and the formation of double bond
1. Has limitations
III. De Novo Synthesis [S5]
a. De novo synthesis involves making fatty acids from 2 carbon units in form of Acetyl CoA
b. It is important when dietary supply is limited (developing fetus or undeveloped countries)
c. What side effects are produced?
i. If glucose is not used to make ATP – they’ll make fatty acids
ii. This is a one way process – cant make glucose from fatty acids
1. Excess glucose will be converted to fatty acids in a sedentary individual
2. People on high carbohydrate diets (Dannon Yogurt is high carb) will turn to fat
IV. Tissue Sites of De Novo Synthesis [S6]
a. Enzymes that control de novo synthesis are largely in the liver and secondarily in the fat tissues themselves
b. Process goes from cytosol to mitochondria, back to cytosol again (if glucose is source)
c. Looking at metabolic pathway, glucose >pyruvate>acetyl CoA> citrate (TCA cycle) >> Acetyl CoA >> Fatty
Acids
V. What is needed to begin synthesis and where does it come from [S7]
a. 7 steps
b. Step 1 is glucose > pyruvate >> Mitochondrial Matrix >> Acetyl CoA > TCA Cycle
c. What makes it different?
i. Citrate taken from TCA cycle out of mitochondrial membrane and converted to Acetyl CoA and used for
fatty acid synthesis
d.
VI. Part II: Formation of Malonyl CoA [S8]
a. Part 2 is formation of 3 carbon unit Malonyl CoA
i. Comes together with Acetyl CoA, kicks off one carbon and makes a 4 carbon unit
ii. All this begins with Acetyl CoA Carboxylase enzyme
1. The enzyme is actually two enzymes and uses biotin as a cofactor
b. Look at the overall rxn and how it produces Malonyl CoA
c. The biotin acts like a pendulum which bring the CO2 to the acetyl CoA where it will react and be kicked off
d. The CO2 binds to biotin > pendulum swings > Pi is released > CO2 is carried over to Acetyl COA on carboxyl
transferase portion of the enzyme and we end up with Malonyl CoA
e. *** ”That’s a nice little mechanism, you should know something about this for exam purposes” ***
VII. Notes on Malonyl CoA Formation [S9]
f. Malonyl CoA formation and all fatty acid synthesis steps are committed, which means there is no turning
back
g. Powered by hydrolysis of ATP
h. Activation of Acetyl CoA Carboxylase is rate-limiting step/enzyme
i. This is not part of F.A. synthase mega enzyme that we’re going to see later, but you need to know about this
one
j. Activity is tightly controlled through allosteric activity
Fundamentals I 11:00-12:00
Scribe: Jordan Ridgeway
9/4/10
Proof: Caleb Landrum
Whikehart
Lipid Synthesis
Page 2 of 5
i. Many sites for activation/inhibition (textbook page 726,727)
k. The hydrolysis of ATP places Phosphate on residue 1200 of biotin carboxylase subunit.
l. Insulin promotes while Epinephrine and Glucagon inhibit the enzyme. Insulin is one of those hormones that
promotes metabolism – promotes protein, glucose, and F.A. metabolism
m. Generally, the activity of enzyme is determined by the SUM of the bound activators and inhibitors at any
given time. That’s true for any allosteric enzyme. At any time you can have a certain combination of
activators and inhibitors, the “best man wins” and you have specific regulation of activity
VII. Part III General Scheme of De Novo FA Synthesis (Big Picture) [S10]
a. I am not going to go into great depth here
b. What we have is a complex of enzymes called FA synthase
c. In Cytoplasm
d. Complex of 6 kinds of enzymes
e. Acetyl CoA and Malonyl CoA come in and Palmityl CoA comes out other end
VIII. Part V: The Fatty Acid Synthase Structure (AKA The “Megasynthase” [S11]
a. (Lists enzyme subunits on slide) don’t need to memorize these
b. Interesting thing is that this enzyme has multiple activation sites and multiple rxn chambers
c. Fatty acid goes from site to site to site just like a bee on flowers and every time something happens to it to
make it longer than it was before
IX. Part IV: Priming the Pump [S12]
a. Priming the Pump
b. Involves transfer of acetyl and malonyl groups
c. Should know a little about what this means
d. The Phosphoantetheine groups acts as a handle to get Acetyl or Malonyl CoA onto an enzyme itself
X. Part V: The 1st Cycle of Synthesis [S13]
a. Here is this enzyme again, We’ll walk through briefly, but I’m not going to ask you details about this
b. We’re making the F.A. longer here
c. To prime the pump, we need Acetyl CoA and Malonyl CoA
d. Have condensation rxn that produces 4 carbon unit Acetoacyl ACP
i. Anchored to protein – need protein there for solubility purposes
e. Reads rxns/intermediates from slide
f. Final reduction gives 4 carbon carrier protein >> recycle, recycle, recycle until you get a 16 C
molecule
XI. Part VI: Subsequent Cycles and Release [S14]
a. Here’s a diagram of the “recycling”
b. Have a 3 C unit and 2 C unit – gives 4 C unit (-CO2) and malonate is recycled.
c. After 7 cycles, each adding 2 C’s yields total of 16 C’s -> Palmitic Acid
d. This is important because the Megasynthase cannot handle units more than 16 C F.A.’s at a time – as big as
you can make it
e. **** Know the principles here, and initial steps of rxn for Exam purposes ****
XII. CARTOON [S15]
a. Particle Board Furniture cartoon – Note that the devil and his implied role in detailed processes are
mentioned numerous times in this lecture.
XIII. Helpful Comparison b/w FA Oxidation and Degradation [S16]
a. Comparison of B-oxidation (F.A. degradation) talked about in last lecture and F.A. condensation (synthesis)
b. I’m not going to ask you about this – know for your board exams
XIV.
Fatty Acid Synthesis [S17]
a. We just talked about de novo. Now I want to talk about Chain lengthening of Palmitic Acid and double bond
formation
b. This is the second part of Synthesis
Fundamentals I 11:00-12:00
Scribe: Jordan Ridgeway
9/4/10
Proof: Caleb Landrum
Whikehart
Lipid Synthesis
Page 3 of 5
XV. Chain Lengthening and Double-Bond Formation [S18]
a. Palmitic lengthening occurs in mitochondria and Smooth ER by separate but similar mechanisms
b. Limitation – Lengthening occurs usually limited to 2 additional C’s
c. Limitation: Only Single unsaturated F.A’s can be formed
d. Out of all that, you can only get Oleic and steric Acid from Palmitic – you would think you could get more, but
that’s as far as you can go
e. There are many others though
f. You get the others
g. Enzymes involved here are FA Elongases and FA Desaturases
XVI.
Longer Chain, Polyunsaturated FA’s (PUFAs) [S19]
a. In order to get longer polyunsaturated FA’s (2 or more double bonds), you need essential FA’s from diet –
there’s no other way
b. This is the Polyunsaturated FA (PUFA) disconnect for higher life forms
c. In order to get longer chain PUFAs you need Linoleate and a-Linoleate as starting material
d. *He reads the slide basically word for word.
e. These FAs are essential for higher animals since they cannot synthesize these from Palmitic Acid
f. LINOLEATE AND alpha-LINOLENATE MUST BE USED TO MAKE POLYUNSATURATED FATTY ACIDS
(Arachidonate)
g. NOTE: Linoleate and Arachidonate are omega-3 FAs
i. Alpha-Linoleate is an omega-6 FATTY ACID (counting from the methyl end of the fatty acid – a practice
used by nutritional biochemists).
h. ***You need to know these specific essential FA’s for Exam ***
XVII. WHAT ARE SOME OTHER PUFAs AND WHAT ARE SOME OF THEIR PRODUCTS (EICOSANOID
SYNTHESIS) [S20]
a. Again, he reads directly from the slide:
b. Linoleic can be converted to Arachidonic acid
c. These can be converted to an array of Eicosanoids – Prostaglandins, thromboxanes or leucotrienes – shortterm hormones involved in things like inflammation and anti-inflammatory activities.
d. There are drugs that can inhibit the formation of these inflammatory activities – one is steroids, the other ones
are aspirin and NSAIDS – Non-Steroidal Anti-Inflammatory Drugs – important for preventing inflammation
after tooth extraction, for example
e. As you can guess, we are more interested in inhibiting the inflammatory part
f. The process of making Eicosanoids can be two different way:
i. One is the hormone method in which hormone goes to receptor, causes activation of phospholipase a-2
ii. The other is that you can simply “bump”??? The area where the cell is on the skin surface, and that causes
activation as well
g. Phospholipase a-2 is going to cause conversion of phospholipids (taken out of membrane) to Arachidonic
acid
h. The Arachidonic can be acted upon by prostaglandin synthase, which causes formation of PGH2 and PGD2
which has anti-inflammatory activity
i. Leucotriene C sustains inflammatory activity
a. STEROIDS AND “NSAIDS” INHIBIT EICOSANOID SYNTHESIS.
XVIII.
Notes on Pathways to Eicosanoid Synthesis [S21]
a. Reads word for word from slide – explains the importance of tunneling and references Dr. Delucas’s
lecture pertaining to tunneling portions of enzymes.
XIX.
Another PUFA Worthy of Our Attention [S22]
a. DHA (DOCOSAHEXANENOIC ACID aka CERVONIC ACID)
b. Has 6 double bonds
c. Capable of being twisted into different forms – makes for a lot of disorganization within a membrane – makes
membrane more flexible.
d. This particular FA makes for complex reduction processes in nervous tissues and proteins in these
membranes have greater freedom to move
e. About 22% of FA found in the retina is made up of Cervonic Acid b/c a lot of metabolic and electrical activity
going on there
f. See the two possible conformations on slide
Fundamentals I 11:00-12:00
9/4/10
Whikehart
g. ***Could ask you about this on Test**
Lipid Synthesis
Scribe: Jordan Ridgeway
Proof: Caleb Landrum
Page 4 of 5
XX. SUMMARY: Points to Note about FA Synthesis [S23]
a. **Read directly from slide
b. FATTY ACID SYNTHESIS (DE NOVO) AND FATTY ACID ELONGATION, ALONG WITH UNSATURATION,
DO NOT SEEM TO SERVE HUMANS WITH THE BEST EFFICIENCY SINCE:
i. CARBOHYDRATES ARE SYNTHESIZED TO EXCESS FATTY ACIDS WHEN THEY ARE NOT USED TO
MAKE ATP.
ii. THE PROCESSES OF ELONGATION & UNSATURATION OF C-16 FATTY ACIDS IS LIMITED SINCE
CONVERSION TO PUFAs CANNOT OCCUR. (Need essential FA’s for that reason)
c. ESSENTIAL FATTY ACIDS, WHICH WE CANNOT MAKE, ARE NEEDED AS STARTING POINTS FOR
THE SYNTHESIS OF A VARIETY OF USEFUL, POLYUNSATURATED
d. FATTY ACIDS (AKA PUFAs). SOME EXAMPLES OF PUFAs ARE PROSTAGLANDINS (SHORT ACTING
HORMONES) AND DHA (Cervonic Acid - A FATTY ACID THAT MAY BE CARDIOPROTECTIVE, ANTIINFLAMMATORY AS WELL AS A COMPONENT IN NERVOUS TISSUE FUNCTIONS).
e. A PUFA IS ANY FATTY ACID WITH TWO OR MORE DOUBLE BONDS. WITH VIRTUALLY NO
EXCEPTONS IN ANIMAL TISSUE, A PUFA HAS A MINIMAL CARBON LENGTH OF 18.
a. DHA = DOCOSAHEAENOIC ACID = Cervonic Acid
XXI.
Synthesis of Cholesterol [S24]
a. Again, read directly from the slide
b. Cholesterol stiffens the membrane
c. The most important form pictured is the one in the middle, if you took space filling model and rotated it, it is
slimmer – fits w/in certain phospholipids in cell membrane
XXII.
We Obtain Cholesterol from 2 Sources: Diet and Synthesis [S25]
a. Intake can be somewhat controlled in diet – Aim is to avoid high LDL/HDL levels >> blood vessel blockage
b. We have no control on cholesterol synthesis – we’ll talk about later
XXIII.
SITE & ORIGINS OF SYNTHESIS WITH AN OVERALL DIAGRAM [S26]
Know this process in general; know what’s involved in each stage (it’s very complex)
Cholesterol made initially in Liver cells – in cytosol and then in ER
First molecules are Acetyl CoA – familiar, but there similarity disappears
Know the general steps and intermediates
i. Isoprene, remember we said are harmful by themselves but safe in these forms in the body.
e. Each circled step requires multiple enzyme rxns. – Many
a.
b.
c.
d.
XXIV. THE STEPS IN CHOLESTEROL BIOSYNTHESIS ARE LONG AND COMPLEX. WHY SHOULD THEY BE
STUDIED? [S27]
a. Why do we need to study synthesis of cholesterol
i. It’s about control
b. Funding for research was sold on developing ways to prevent high cholesterol
c.
XXV. Abbreviated Cholesterol Synthesis [S28]
a. Most importantly look at committed stage – formation of Mevalonic acid
i. Three Rxns total – read from slide
ii. Most important (RATE-LIMITING) step is reduction of hydroxymethylglutarate (HMG) to Mevalonic acid by
OH-Methyl Glutaryl Reductase (HMG reductase)
b. **Know about this for exam**
Fundamentals I 11:00-12:00
Scribe: Jordan Ridgeway
9/4/10
Proof: Caleb Landrum
Whikehart
Lipid Synthesis
Page 5 of 5
XXVI. INHIBITORS FOR HMG-CoA REDUCTASE – A CLINICAL PAYOFF FOR CHOLESTEROL SYNTHESIS
STUDIES [S29]
a. HMG-CoA Reductase inhibition
1. Reducing keto group to hydroxy group
2. If can stop the intermediate from forming product, can control overall synthesis of cholesterol. This is
the first committed and rate limiting step
3. STATINS – drugs will inhibit HMG-CoA reductase
4. Enzyme itself is controlled by kinases ( add phosphate), phosphotases (remove phosphate), even
cholesterol levels themselves have effect on ½ life of enzyme and ability of mRNA to make enzyme
XXVII. Lipitor – A Statin [S30]
a. Has a structure similar to the HMG intermediate
b. Picture is of the intermediate, show Acetyl CoA, HMG, NADP
c. Lipitor binds active site and prevents production of Mevalonic acid > Cholesterol
XXVIII. Remaining Steps of Cholesterol Synthesis [S31]
a. We’re not going to focus on these
b. Through a series of rxns
i. Take isoprene, bind it end to end > squalene > cyclization steps will forms rings, tail, hydroxy and methyl
groups
ii. We’re not going to focus on these
XXIX.
The LDL Receptor [S32]
a. Read directly from slide
b. Picture is of receptor
c. Defect in or absence of receptor is responsible for Familial Hypercholesterolemia
d. Again, he read directly from slide – you’ll want to read it
e. Statins such as Lipitor used to control cholesterol levels in individuals w/ Familial Hypercholesterolemia
XXX.
Summary – read from slide [S33]
XXXI.
Points to Remember/Summary [S34]
a. **Not all here are important, note the ones here
b. For De novo Synthesis, know:
i. Importance
ii. Malonyl CoA formation and general scheme
iii. Megasynthase Molecule and what it does, not parts
iv. Acyl Carrier Protein – what it is and does
c. Don’t have to compare FA synthesis and B-oxidation
d. The characteristics of carbon lengthening and Desaturation
i. Know Limitations
ii. Need for PUFAs – essential FA’s
iii. Importance of Phospholipase A-2 and Prostaglandin Synthase inhibitors (ex: NSAIDs)
XXXII. Synthesis of Cholesterol [S35]
a. Know general diagram and inhibitor/what inhibitor does
b. Important intermediates involved like mevalonate, squalene
c. Inhibitors of HMG-CoA and LIPITOR
d. Role of LD Receptor and control of Blood Cholesterol
i. Receptor’s normal role
ii. Hypercholesterolemia – “what’s involved”
e. The devil.
[End 43:55 mins]