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Transcript
Metabolism
• Functions of food
• source of energy
• essential nutrients
• stored for future use
• Metabolism is all the chemical reactions of the
body
• some reactions produce the energy which is stored in ATP that other
reactions consume
• all molecules will eventually be broken down and recycled or excreted from
the body
26-1
Catabolism and Anabolism
• Catabolic reactions breakdown complex organic
compounds
• providing energy (exergonic)
• glycolysis, Krebs cycle and electron transport
• Anabolic reactions synthesize complex molecules from
small molecules
• requiring energy (endergonic)
• Exchange of energy requires use of ATP (adenosine
triphosphate) molecule.
26-2
ATP Molecule & Energy
• Each cell has about 1 billion ATP molecules that last for less than
one minute
• Over half of the energy released from ATP is converted to heat
26-3
Energy Transfer
• Energy is found in the bonds between atoms
• Oxidation is a decrease in the energy content of a molecule
• Reduction is the increase in the energy content of a
molecule
• Oxidation-reduction reactions are always coupled within
the body
• w
• Whenever a substance is oxidized, another is almost simultaneously
reduced
26-4
Oxidation and Reduction
 Biological oxidation involves the loss of electron and a
proton (hydrogen atom)
 D
 Dehydrogenation reactions require coenzymes to transfer
hydrogen atoms to another compound
 Common coenzymes of living cells that carry H+
 NAD (nicotinamide adenine dinucleotide )
 NADP (nicotinamide adenine dinucleotide phosphate )
 FAD (flavin adenine dinucleotide )
NAD+ + 2 H  NADH + H+
 Biological reduction is the addition of electron and a
proton (hydrogen atom) to a molecule
 Increase in potential energy of the molecule
26-5
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Carbohydrate Metabolism
• Dietary carbohydrate burned as fuel within hours of absorption
• All oxidative carbohydrate consumption is essentially a matter of
glucose catabolism
C6H12O6 + 6O2  6CO2 + 6H2O+ energy
• Function of this reaction is to transfers energy from glucose to ATP
• Not to produce carbon dioxide and water
26-7
Glucose Catabolism
• Glucose catabolism – a series of small steps, each controlled by a
separate enzyme, in which energy is released in small
manageable amounts, and as much as possible, is transferred to
ATP and the rest is released as heat
• Three major pathways of glucose catabolism
• Glycolysis
• Glucose (6C) split into 2 Pyruvic acid molecules (3C)
• Anaerobic fermentation
• Occurs in the absence of oxygen
• Reduces pyruvic acid to lactic acid
• Aerobic respiration
• Occurs in the presence of oxygen
• Completely oxidizes pyruvic acid to CO2 and H2O
26-8
Mechanisms of ATP Generation
• Phosphorylation is bond attaching 3rd
phosphate group contains stored energy
• Mechanisms of phosphorylation
• Within animals
• Substrate-level phosphorylation - a highenergy from an intermediate in catabolism is
added to ADP.
• In cytosol
• Oxidative phosphorylation - energy is
released as electrons are passed to a series of
electron acceptors (an electron transport
chain) and finally to O2
• In mitochondria
• In chlorophyll-containing plants or bacteria
• Photophosphorylation
26-9
Overview of substrate level phosphorylation
ATP Production
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Key
Carbon atoms
Glucose
ATP
Phosphate
groups
1 Phosphorylation
ADP
Glucose 6-phosphate
Glycogen
Fat
Fructose 6-phosphate
ATP
2 Priming
ADP
Fructose 1,6-diphosphate
3 Cleavage
2 PGAL
2 Pi
2 NAD+
2 NADH + 2 H+
4 Oxidation
2
2 ADP
2 H2O
2 ATP
2
5 Dephosphorylation
2 ADP
2 ATP
2
2 pyruvic acid
2 NADH + 2 H+
2 NAD+
2
2 lactic acid
26-10
Anaerobic fermentation
Aerobic respiration
Steps of Glycolysis (1)
1. Phosphorylation
• glucose enters cell has phosphate
added - ATP used
• maintains favorable concentration
gradient, prevents glucose from
leaving cell
2. Priming
• isomerization occurs
• phosphorylation further activates
molecule - ATP used
3. Cleavage
• molecule split into 2 three-carbon
molecules (PGAL –
phosphoglyceraldehyde)
26-11
Steps of Glycolysis (2)
4. Oxidation
 Removes 2 H atoms
 NAD+ + 2H  NADH + H+
5. Dephosphorylation
 transfers phosphate groups to ADP to form
ATP
 4 ATPs produced (2 ATP used) for a net
gain of 2 ATP
 produces 2 pyruvic acid
26-12
Steps of Glycolysis
• 4 ATP are produced but 2 ATP were consumed to initiate
glycolysis, so net gain is 2 ATP per glucose molecule
• Some energy originally in the glucose is contained in the ATP,
some in the NADH, some is lost as heat, but most of the energy
remains in the pyruvic acid
• End-products of glycolysis are:
• 2 pyruvic acid + 2 NADH + 2 ATP
26-13
Anaerobic Fermentation
• Fate of pyruvic acid depends on oxygen availability
• In an exercising muscle, demand for ATP > oxygen
supply; ATP produced by glycolysis
• Glycolysis can not continue without supply of NAD+
• NADH reduces pyruvic acid to lactic acid, restoring NAD+
• Lactic acid travels to liver to be oxidized back to
pyruvic when O2 is available (oxygen debt)
• then stored as glycogen or released as glucose
• Fermentation is inefficient way to produce ATP
26-14
Anaerobic Fermentation
• Lactic acid is toxic - leaves the cells that generate it and enter the
bloodstream and transported to the liver
• When oxygen becomes available the liver oxidized it back to pyruvic
acid
• Liver can convert lactic acid back to G6P and can polymerize it to form glycogen
for storage
• Remove phosphate group and release free glucose into the blood
• Drawbacks of anaerobic fermentation
• Wasteful, because most of the energy of glucose is still in the lactic acid and
has contributed no useful work
• Lactic acid is toxic and contributes to muscle fatigue
• Skeletal muscle is relatively tolerant of anaerobic fermentation,
cardiac muscle less so
• the brain employs no anaerobic fermentation
26-15
Aerobic Respiration
• Most ATP generated in mitochondria, which requires oxygen as final
electron acceptor
• In the presence of oxygen, pyruvic acid enters the mitochondria
and is oxidized by aerobic respiration
• Occurs in two principal steps:
• Matrix reactions – their controlling enzymes are in the fluid of the
mitochondrial matrix
• Membrane reactions - whose controlling enzymes are bound to the
membranes of the mitochondrial cristae
26-16
Mitochondrial Matrix Reactions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pyruvic acid (C3)
6
CO2
Pyruvic acid oxidation
NAD+
7
NADH + H+
Acetyl group (C2)
8
Acetyl-Co A
Coenzyme A
H2O
9
Citric acid (C6)
Oxaloacetic acid (C4)
H2O
10
NADH + H+
NAD+
(C6)
Citric
acid
cycle
18
H2O
NAD+
11
Citric acid (Krebs) Cycle
NADH + H+
(C4)
12
CO2
17
(C5)
H2O
13
Occurs in
mitochondrial
matrix
(C4)
NADH + H+
14
16
FADH2
NAD+
(C4)
CO2
FAD
(C4)
Pi
15
GTP
GDP
26-17
ADP
ATP
Mitochondrial Matrix Reactions
 Three steps prepare pyruvic acid to enter citric
acid cycle
6. Decarboxylation so that a 3-carbon
compound becomes a 2-carbon compound
 CO2 removed from pyruvic acid
7. Convert that to an acetyl group (acetic
acid)
 NAD+ removes hydrogen atoms from the C2
compound
8. Acetyl group binds to coenzyme A
 results in acetyl-coenzyme A (acetyl-CoA)
26-18
Mitochondrial Matrix Reactions
9. Citric Acid (Krebs) Cycle
• acetyl-Co A (a C2 compound) combines with a
C4 to form a C6 compound (citric acid)-- start
of cycle
• hydrogen atoms are removed and accepted by
NAD+
• another CO2 is removed and the substrate
becomes a five-carbon chain
• previous step repeated removing another free
CO2 leaving a four-carbon chain
• ATP
• two hydrogen atoms are removed and accepted
by the coenzyme FAD
• two final hydrogen atoms are removed and
transferred to NAD+
• reaction generates oxaloacetic acid, which
starts the cycle again
26-19
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Summary of Matrix Reactions
2 pyruvate + 6H2O  6CO2
2 ADP + 2 Pi  2 ATP
8 NAD+ + 8 H2  8 NADH + 8 H+
2 FAD + 2 H2  2 FADH2
• Carbon atoms of glucose have all been carried away as CO2 and
exhaled
• Energy lost as heat, stored in 2 ATP, 8 reduced NADH, 2 FADH2
molecules of the matrix reactions and 2 NADH from glycolysis
• Citric acid cycle is a source of substances for synthesis of fats and
nonessential amino acids
26-21
Membrane Reactions
 Membrane reactions have two purposes:
 to further oxidize NADH and FADH2 and
transfer their energy to ATP
 to regenerate NAD+ and FAD and make them
available again to earlier reaction steps
10. Mitochondrial electron-transport
chain – series of compounds (enzyme
complexes) that carry out this series of
membrane reactions
 most bound to the inner mitochondrial
membrane
 arranged in a precise order that enables each
one to receive a pair of electrons from the
member on the left side of it.
 pass electrons to member on the other side
26-22
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Electron Transport Chain
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
50
NADH + H+
NAD+
FMN
Relative free energy (kcal/mole)
Fe-S
40
FADH2
Enzyme complex 1
FAD
CoQ
30
Figure 26.5
Cyt b
Fe-S
Cyt c1
20
Enzyme complex 2
Cyt c
Cu
10
Cyt a
Cyt a3
Enzyme complex 3
½ O2 + 2 H+
H2O
0
Reaction progress
26-24
Chemiosmotic Mechanism
• Electron transport chain energy fuels respiratory enzyme
complexes
• pump protons from matrix into space between inner and outer mitochondrial
membranes
• creates steep electrochemical gradient for H+ across inner mitochondrial
membrane
• Inner membrane is permeable to H+ at channel proteins called ATP
synthase
• Chemiosmotic mechanism - H+ current rushing back through these
ATP synthase channels drives ATP synthesis
26-25
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Overview of ATP Production
• NADH releases an electron pair to electron transport system and
H+ to prime pumps
• enough energy to synthesize 2.5 ATP
• FADH2 releases its electron pairs further along electron-transport
system
• enough energy to synthesize 1.5 ATP
• Complete aerobic oxidation of glucose to CO2 and H2O produces
32 ATP
• efficiency rating of 40% - 60% is lost as heat
26-27
26-28
ATP Generated by Oxidation of Glucose
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glucose
2 ATP
Glycolysis
(net)
2 NADH + 2 H+
Cytosol
2 pyruvate
Mitochondria
2 NADH + 2 H+
CO2
6 NADH + 6 H+
Citric acid
cycle
2 ATP
2 FADH2
Electron-transport
chain
O2
H2O
3 ATP
25
ATP
Total 32
ATP
26-29
Glycogen Metabolism
 ATP is quickly used after it is formed
 it is an energy transfer molecule, not an energy storage molecule
 converts the extra glucose to other compounds better suited for energy
storage (glycogen and fat)
 Glycogenesis - synthesis of glycogen
 stimulated by insulin
 chains glucose monomers together
 Glycogenolysis – hydrolysis of glycogen
 releases glucose between meals
 stimulated by glucagon and epinephrine
 only liver cells can release glucose back into blood
 Gluconeogenesis - synthesis of glucose from noncarbohydrates,
such as glycerol and amino acids
 occurs chiefly in the liver and later, kidneys if necessary
26-30
Glucose Storage and Use
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Blood
glucose
Extracellular
Intracellular
Glucose
6-phosphatase
(in liver, kidney,
and intestinal cells)
Hexokinase
(in all cells)
Glucose 6-phosphate
Glycogen
synthase
Key
Pi
Glucose
1-phosphate
Glycogenesis
Glycogenolysis
Glycogen
phosphorylase
Glycogen
Pi
Glycolysis
Figure 26.8
26-31
Lipids
• Triglycerides are stored in body’s adipocytes
• constant turnover of lipid molecules every 2 - 3 weeks
• released into blood, transported and either oxidized or redeposited in other fat
cells
• Lipolysis – breaking down fat for fuel
• begins with the hydrolysis of a triglyceride to glycerol and fatty acids
• stimulated by epinephrine, norepinephrine, glucocorticoids, thyroid hormone, and
growth hormone
• glycerol easily converted to PGAL and enters the pathway of glycolysis
• generates only half as much ATP as glucose
• beta oxidation in the mitochondrial matrix catabolizes the fatty acid
components
• removes two carbon atoms at a time which bonds to coenzyme A
• forms acetyl-CoA, the entry point for the citric acid cycle
• a fatty acid with 16 carbons can yield 129 molecules of ATP
• richer source of energy than the glucose molecule
• Lipogenesis - synthesis of fat from other types of molecules
• amino acids and sugars used to make fatty acids and glycerol
• PGAL can be converted to glycerol
26-32
Lipogenesis and Lipolysis Pathways
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glucose
Glucose 6-phosphate
Stored
triglycerides
Glycerol
PGAL
Fatty acids
Glycerol
Beta oxidation
Pyruvic
acid
Fatty
acids
Acetyl groups
New
triglycerides
Acetyl-Co A
Ketone bodies
β-hydroxybutyric acid
Acetoacetic acid
Acetone
Citric
acid
cycle
Key
Lipogenesis
Lipolysis
26-33
Ketogenesis
• Fatty acids catabolized into acetyl groups (by beta-oxidation in
mitochondrial matrix) may:
• enter citric acid cycle as acetyl-CoA
• undergo ketogenesis
• metabolized by liver to produce ketone bodies
• acetoacetic acid
• -hydroxybutyric acid
• acetone
• rapid or incomplete oxidization of fats raises blood ketone levels
(ketosis) and may lead to a pH imbalance (ketoacidosis)
26-34
Proteins
• Amino acids in the pool can be converted to others
• Free amino acids also can be converted to glucose and fat or
directly used as fuel
• Conversions involve three processes:
• deamination – removal of an amino group (-NH2)
• amination – addition of -NH2
• transamination – transfer of -NH2 from one molecule to another
• As fuel - first must be deaminated (removal of -NH2)
• what remains is keto acid and may be converted to pyruvic acid, acetylCoA, or one of the acids of the citric acid cycle
• during shortage of amino acids, citric acid cycle intermediates can be
aminated and converted to amino acids
• in gluconeogenesis, keto acids are used to synthesis glucose
26-35
Learning objectives
• Carbohydrate metabolism
• Describe the principle reactants and poroducts of glucose oxidation
• Contrast thefunctions and products of anaerobic fermentation and aerobic
respiration
• Explain where and how cells produce ATP
• Describe the production, function and use of glycogen
26-36