Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled

4
5
6
α-Ketoglutarate + NAD+ +
CoA succinyl CoA + CO2
+ NADH
Succinyl CoA + Pi + GDP
succinate + GTP + CoA
Succinate + FAD (enzymebound) fumarate +
FADH2 (enzyme-bound)
7
Fumarate + H2O
8
l-Malate + NAD+
l-malate
oxaloacetate + NADH +
α-Ketoglutarate dehydrogenase
complex
Lipoic acid, FAD,
TPP
d+e
-7.2
-30.1
f
-0.8
-3.3
e
˜0
0
Furmarase
c
-0.9
-3.8
Malate dehydrogenase
e
+7.1
+29.7
Succinyl CoA synthetase
Succinate dehydrogenase
FAD, Fe-S
H+
*
Reaction type: (a) condensation; (b) dehydration; (c) hydration; (d) decarboxylation; (e) oxidation; (f) substrate-level
phosphorylation.
II. Transducing and Storing Energy
17. The Citric Acid Cycle
17.2. Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled
The citric acid cycle is the final common pathway for the aerobic oxidation of fuel molecules. Moreover, as we will see
shortly (Section 17.3) and repeatedly elsewhere in our study of biochemistry, the cycle is an important source of building
blocks for a host of important biomolecules. As befits its role as the metabolic hub of the cell, entry into the cycle and
the rate of the cycle itself are controlled at several stages.
17.2.1. The Pyruvate Dehydrogenase Complex Is Regulated Allosterically and by
Reversible Phosphorylation
As we saw earlier, glucose can be formed from pyruvate (Section 16.3). However, the formation of acetyl CoA from
pyruvate is an irreversible step in animals and thus they are unable to convert acetyl CoA back into glucose. The
oxidative decarboxylation of pyruvate to acetyl CoA commits the carbon atoms of glucose to two principal fates:
oxidation to CO2 by the citric acid cycle, with the concomitant generation of energy, or incorporation into lipid (Figure
17.16). As expected of an enzyme at a critical branch point in metabolism, the activity of the pyruvate dehydrogenase
complex is stringently controlled by several means (Figure 17.17). High concentrations of reaction products of the
complex inhibit the reaction: acetyl CoA inhibits the transacetylase component (E2), whereas NADH inhibits the
dihydrolipoyl dehydrogenase (E3). However, the key means of regulation in eukaryotes is covalent modification of the
pyruvate dehydrogenase component. Phos-phorylation of the pyruvate dehydrogenase component (E ) by a specific
1
kinase switches off the activity of the complex. Deactivation is reversed by the action of a specific phosphatase. The site
of phosphorylation is the transacetylase component (E2), again highlighting the structural and mechanistic importance of
this core. Increasing the NADH/NAD+, acetyl CoA/CoA, or ATP/ADP ratio promotes phosphorylation and, hence,
deactivation of the complex. In other words, high concentrations of immediate (acetyl CoA and NADH) and ultimate
(ATP) products inhibit the activity. Thus, pyruvate dehydrogenase is switched off when the energy charge is high and
biosynthetic intermediates are abundant. On the other hand, pyruvate as well as ADP (a signal of low energy charge)
activate the dehydrogenase by inhibiting the kinase.
In contrast, α 1-adrenergic agonists and hormones such as vasopressin stimulate pyruvate dehydrogenase by triggering a
rise in the cytosolic Ca2+ level (Section 15.3.2), which in turn elevates the mitochondrial Ca2+ level. The rise in
mitochondrial Ca2+ activates the pyruvate dehydrogenase complex by stimulating the phosphatase. Insulin also
accelerates the conversion of pyruvate into acetyl CoA by stimulating the dephosphorylation of the complex. In turn,
glucose is funneled into pyruvate.
The importance of this covalent control is illustrated in people with a phosphatase deficiency. Because pyruvate
dehydrogenase is always phosphorylated and thus inactive, glucose is processed to lactic acid. This condition
results in unremitting lactic acidosis (high blood levels of lactic acid), which leads to the malfunctioning of many tissues,
most notably the central nervous system (Section 17.3.2).
17.2.2. The Citric Acid Cycle Is Controlled at Several Points
The rate of the citric acid cycle is precisely adjusted to meet an animal cell's needs for ATP (Figure 17.18). The primary
control points are the allosteric enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
Isocitrate dehydrogenase is allosterically stimulated by ADP, which enhances the enzyme's affinity for substrates. The
binding of isocitrate, NAD+, Mg2+, and ADP is mutually cooperative. In contrast, NADH inhibits iso-citrate
dehydrogenase by directly displacing NAD+. ATP, too, is inhibitory. It is important to note that several steps in the cycle
require NAD+ or FAD, which are abundant only when the energy charge is low.
A second control site in the citric acid cycle is α-ketoglutarate dehydrogenase. Some aspects of this enzyme's control are
like those of the pyruvate dehydrogenase complex, as might be expected from the homology of the two enzymes. αKetoglutarate dehydrogenase is inhibited by succinyl CoA and NADH, the products of the reaction that it catalyzes. In
addition, α-ketoglutarate dehydrogenase is inhibited by a high energy charge. Thus, the rate of the cycle is reduced when
the cell has a high level of ATP.
In many bacteria, the funneling of two-carbon fragments into the cycle also is controlled. The synthesis of citrate from
oxaloacetate and acetyl CoA carbon units is an important control point in these organisms. ATP is an allosteric inhibitor
of citrate synthase. The effect of ATP is to increase the value of K M for acetyl CoA. Thus, as the level of ATP increases,
less of this enzyme is saturated with acetyl CoA and so less citrate is formed.
II. Transducing and Storing Energy
17. The Citric Acid Cycle
17.2. Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled
Figure 17.16. From Glucose to Acetyl CoA. The synthesis of acetyl CoA by the pyruvate dehydrogenase complex is a
key irreversible step in the metabolism of glucose.
II. Transducing and Storing Energy
17. The Citric Acid Cycle
17.2. Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled
Figure 17.17. Regulation of the Pyruvate Dehydrogenase Complex. The complex is inhibited by its immediate
products, NADH and acetyl CoA. The pyruvate dehydrogenase component is also regulated by covalent modification. A
specific kinase phosphorylates and inactivates pyruvate dehydrogenase, and a phosphatase actives the dehydrogenase by
removing the phosphoryl. The kinase and the phosphatase also are highly regulated enzymes.
II. Transducing and Storing Energy
17. The Citric Acid Cycle
17.2. Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled
Figure 17.18. Control of the Citric Acid Cycle. The citric acid cycle is regulated primarily by the concentration of
ATP and NADH. The key control points are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.