CHAPTER 16:
The Citric Acid Cycle

基本概念

其他

Conversion of Pyruvate
to Acetyl-CoA

Structure of Coenzyme A

In Eukaryotes, Stages 2 and 3 Are
Localized to the Mitochondria

Structure of Lipoyllysine

Respiration: Stages

Pyruvate Dehydrogenase Complex (PDC)

Cellular Respiration

Overall Reaction of PDC

Only a Small Amount of Energy Available in Glucose Is Captured in Glycolysis

Process in which cells consume O2 and produce CO2

Provides more energy (ATP) from glucose than glycolysis

Also captures energy stored in lipids and amino acids

Evolutionary origin: developed about 2.5 billion years ago

Used by animals, plants, and many microorganisms

Occurs in three major stages:

acetyl CoA production

acetyl CoA oxidation

electron transfer and oxidative phosphorylation

Glycolysis occurs in the cytoplasm.

Citric acid cycle occurs in the mitochondrial matrix†.

Oxidative phosphorylation occurs in the inner membrane.

Except succinate dehydrogenase, which is located in the inner membrane

Net reaction:

Catalyzed by the pyruvate
dehydrogenase complex

oxidative decarboxylation of pyruvate

first carbons of glucose to be fully oxidized

requires 5 coenzymes

TPP, lipoyllysine, and FAD are prosthetic groups.

NAD+ and CoA-SH are co-substrates.

Coenzymes are not a permanent part of the enzymes’ structure.
– They associate, fulfill a function, and dissociate.

The function of CoA is to accept and carry acetyl groups.

Prosthetic groups are strongly bound to the protein.

The lipoic acid is covalently linked to the enzyme via a lysine residue

PDC is a large (up to 10 MDa) multienzyme complex.

Advantages of multienzyme complexes:

pyruvate dehydrogenase (E1)

dihydrolipoyl transacetylase (E2)

dihydrolipoyl dehydrogenase (E3)

The short distance between catalytic sites allows channeling of substrates from one catalytic site to another.

Channeling minimizes side reactions.

The regulation of activity of one subunit affects the entire complex.

Sequence of Events in Oxidative Decarboxylation of Pyruvate

Enzyme2

Enzyme3

Enzyme1

Step 1: Decarboxylation of pyruvate to an aldehyde

Step 2: Oxidation of aldehyde to a carboxylic acid‒ Electrons reduce lipoamide and form a thioester.

Step 3: Formation of acetyl-CoA (product 1)

Step 4: Reoxidation of the lipoamide cofactor

Step 5: Regeneration of the oxidized FAD cofactor
‒ forming NADH (product 2)

The Citric Acid Cycle (CAC)

Step 5
Generation of GTP Through Thioester: Substrate-Level Phosphorylation by Succinyl-CoA Synthetase

Step 6
Oxidation of an Alkane to Alkene
by Succinate Dehydrogenase

Step 4
Final Oxidative Decarboxylation by
α-Ketoglutarate Dehydrogenase

Step 7
Hydration Across a Double Bond: Fumarase

Step 3
Oxidative Decarboxylation by Isocitrate Dehydrogenase

Step 8
Oxidation of Alcohol to a Ketone and Regeneration of Oxaloacetate by Malate Dehydrogenase

Step 2
Isomerization by Dehydration/Rehydration

One Turn of the Citric Acid Cycle

Step 1
C-C Bond Formation by Condensation
of Acetyl-CoA and Oxaloacetate

Net Result of the Citric Acid Cycle

Sequence of Events in the Citric Acid Cycle

Step 1: C-C bond formation between acetate (2C) and oxaloacetate (4C) to make citrate (6C)

Step 2: Isomerization via dehydration/rehydration

Steps 3–4: Oxidative decarboxylations to give 2 NADH

Step 5: Substrate-level phosphorylation to give GTP

Step 6: Dehydrogenation to give FADH 2

Step 7: Hydration

Step 8: Dehydrogenation to give NADH

Net oxidation of two carbons to CO2

Energy captured by electron transfer to NADH and FADH2

Generates 1 GTP, which can be converted to ATP

Completion of cycle

equivalent to two carbons of acetyl-CoA

but NOT the exact same carbons

Regulation of Pyruvate Dehydrogenase

Regulation of the Citric Acid Cycle

CAC Intermediates Are Amphibolic

Anaplerotic Reactions

Direct and Indirect ATP Yield

Intermediates in the citric acid cycle can be used in biosynthetic pathways (removed from cycle).

Must replenish the intermediates in order for the cycle and central metabolic pathway to continue

4-carbon intermediates are formed by carboxylation of 3-carbon precursors.

Regulated at highly thermodynamically favorable and irreversible steps
– PDH, citrate synthase, IDH, and KDH

General regulatory mechanism

activated by substrate availability

inhibited by product accumulation

Overall products of the pathway
are NADH and ATP.

affect all regulated enzymes in the cycle

inhibitors: NADH and ATP

activators: NAD+ and AMP

PDH kinase and PDH phosphorylase are part of
mammalian PDH complex.

Regulation of PDH is somewhat similar to regulation of the glycogen synthase/glycogen phosphorylase complex.

Also regulated by reversible phosphorylation of E1

phosphorylation: inactive

dephosphorylation: active

Kinase is activated by ATP.

high ATP => phosphorylated PDH => less acetyl-CoA

low ATP => kinase is less active and phosphorylase removes phosphate from PDH => more acetyl-CoA

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