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Because this reaction initiates the cycle, it is very important that side reactions be minimized. The reduced lipoyl-lysine arm then swings to the active site of the E 3 flavoprotein.

We have already seen that Buochemia is strter important component of signal-transduction systems Chapter Succinyl CoA is an energy-rich thioester compound. The cleavage of the thioester bond of succinyl CoA is coupled to the phosphorylation of a purine nucleoside diphosphate, usually GDP. The subsequent hydrolysis of citryl CoA yields citrate and CoA.

A 4Fe-4S iron-sulfur cluster is a component of the active site of aconitase. How is this discrimination accomplished?


This reaction is catalyzed by the pyruvate dehydrogenase component E 1 of the multienzyme complex. Coenzyme A leaves the enzyme, followed by citrate, and the enzyme returns to the initial open conformation.

Turn recording back on. In each subunit, the small domain biochekia 19 degrees relative to the large domain. This center ionizes to form a carbanion, which readily adds to the carbonyl group of pyruvate. Finally, malate is oxidized to form oxaloacetate. These steps must be coupled to preserve the free energy derived biochema the decarboxylation step to drive the formation of NADH and acetyl CoA.

Under anaerobic conditions, the pyruvate is converted into lactic acid or ethanol, depending on the organism. The enzyme is composed of two subunits.

The results of x-ray crystallographic studies of citrate synthase and its complexes with several substrates and inhibitors revealed that the enzyme undergoes large conformational changes in the course of catalysis. This reaction, also catalyzed by the pyruvate dehydrogenase component E 1yields acetyllipoamide.


John L. Tymoczko – Polska Ksiegarnia w UK

Citrate Is Isomerized into Isocitrate The tertiary hydroxyl group is not properly located in the citrate molecule for the oxidative decarboxylations that follow. The energy-rich thioester bond is preserved as the acetyl group is transferred to CoA. The carboxyl-terminal domains of the two subunits are similar to one another, whereas the amino-terminal domains have different structures, each characteristic of its role in the mechanism.

Reactions of four-carbon compounds constitute the final stage of the citric acid cycle: Two carbon atoms enter the cycle in the condensation of an acetyl unit from acetyl CoA with oxaloacetate. The transacetylase core E 2 is shown in red, the pyruvate dehydrogenase component E 1 in yellow, and the dihydrolipoyl dehydrogenase E 3 in green. Acetyl CoA, the fuel for the citric acid cycle, has now been generated from pyruvate.

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This reaction, which is an aldol condensation followed by a hydrolysis, is catalyzed by citrate synthase. Fumarase catalyzes a stereospecific trans addition of a hydrogen atom and a hydroxyl group. The efficiency of the citric acid cycle may be enhanced by the arrangement of the constituent enzymes.

Indeed, succinate dehydrogenase contains three different kinds of iron-sulfur clusters, 2Fe-2S two iron atoms biocnemia to two inorganic sulfides3Fe-4S, and 4Fe-4S. Recall also that NADH is generated in the formation of acetyl CoA from pyruvate by the pyruvate biohemia reaction. This important molecule is formed from the breakdown of glycogen the storage form of glucosefats, and many amino acids. Each subunit consists of three domains: Pyruvate produced by glycolysis is converted into acetyl CoA, the fuel of the citric acid cycle.

Two molecules of water are consumed: Recall that carbohydrates, most notably glucose, are processed by glycolysis into pyruvate Chapter The phosphohistidine residue then swings over to a bound nucleoside diphosphate and the phosphoryl group is transferred to form the nucleoside triphosphate.


The iron-sulfur cluster in aconitase is somewhat unstable, so one or more iron atoms dissociate under conditions of low iron availability in the cell. The tertiary hydroxyl group is not properly located in the citrate molecule for the oxidative decarboxylations that follow. Rather, two electrons are transferred from FADH 2 directly to iron-sulfur clusters of the enzyme.

Examination of the three-dimensional structure of succinyl CoA synthetase shows that each subunit comprises two domains Figure The positively charged ring of TPP acts as an electron sink that stabilizes the negative charge that is transferred to the ring as part of the decarboxylation. Thus, citrate is isomerized into isocitrate to enable the six-carbon unit to undergo oxidative decarboxylation.

Let us briefly consider the how citrate synthase prevents wasteful processes such as the hydrolysis of acetyl CoA. The reaction proceeds through a phosphorylated enzyme intermediate.

Schematic Representation of the Pyruvate Dehydrogenase Complex.

One of the histidine residues His donates a proton to the carbonyl oxygen of acetyl CoA to promote the removal of a methyl proton by Asp The open form of the enzyme observed in the absence of ligands is converted into a closed form by the binding of oxaloacetate Figure The ultimate acceptor of these electrons is molecular oxygen, as we shall see in Chapter Flexible Linkages Allow Lipoamide to Move Between Different Active Sites Although the structure of an intact member of the pyruvate dehydrogenase complex family has not yet been determined in atomic detail, the structures of all of the component enzymes are now known, albeit from different complexes and species.