Citric cycle

Most of the metabolites that serve as energy sources for aerobic organisms are broken down (catabo-lized) by various pathways to yield substrates for the central, energy-yielding citric acid cycle (also known as the tricarboxylic acid cycle or the Krebs cycle). The citric cycle is a sequence of enzymatic steps whose net reaction consists of the oxidation of acetate to carbon dioxide. 

The C4 acids give rise in the usual kind of way To oxaloacetic (by the citric cycle, say)... 

Many plants contain a variety of free acids such as acetic acid, citric acid, fumaric acid, malic acid, succinic acid, oxalic acid, glycohc acid, etc. They are components of citric cycle, whereas the others are intermediates in the pathway from carbohydrates to aromatic com-pounds. Following extraction, organic acids can be separated and detected with a variety of techniques. Thin layer chromatographic methods have been also employed to separate certain organic acids,as presented in Table 3. 

Enzymes exert their catalytic activity by bringing reactant molecu together, holding them in the orientation necessary for reaction, and viding any necessary acidic or basic sites to catalyze specific steps, look, for example, at citrate synthase, an enzyme that catalyzes the aid like addition of acetyl CoA to oxaloacetate to give citrate (Section 23.1 This reaction is the first step in the so-called citric a ld cycle, in which ace groups produced by degradation of food molecules are metabolic burned to yield CO2 and H2O. We ll look at the details of the citric cycle in Section 29.5. 

Recurring decarboxylations. Which reaction in the citric cycle is most analogous to the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate What kind oi enzvme-bound intermediate is formed in both reactions ... 

Physicochemical properties and purification of malate dehydrogenase MDase, as an enzyme of the citric cycle, is ubiqui-... 

If one acetyl -CoA involved in TCA cycle gives = 10 ATPs ATPs due 8 acetyl-CoA = 8 X 10 = 80 ATPs due to 7 FADH2 = 1.5 x 7= 10.5 ATPS due 7 (NADH + H ) = 2.5 x 7.5 = 17.5. The total of number ATPs produced 108. During the initiation of the y - oxidation pathway a 2 ATPs converts into a 2 AMP and 2 Pi for the activation of fatty acid. So, net ATPs produced by palmitic acid are 106. These calculations assume that mitoehondrial oxidative phosphorlation produces 1.5 ATPS/FADH2 oxidized and 2.5 ATP/NADH2 oxidized. The Guanosine-5 -triphosphate (GTP) produced directly in the acid citric cycle yields ATP in the reaetion eatalyzed by nucleoside diphosphate kinase [99, 130]. 

The Citric Cycle (Krebs and Johnson, 1937).—Citric acid, like succinic acid, may stimulate carbohydrate oxidation in muscle, and can arise from any one of the four acids in the oxaloacetic system. Krebs and Johnson believe that the oxaloacetic system is part of a cycle in which the order is —... 

There are a total of eight reactions and eight enzymes in the citric acid cycle. Initially, an acetyl gronp (2C) from acetyl-CoA bonds with oxaloacetate (4C) to yield citrate (6C) (see Figure 18.11). Then, two decarboxylation reactions remove carbon atoms as CO2 molecules to give succinyl-CoA (4C). Finally, a series of reactions converts four-carbon succinyl-CoA to oxaloacetate, which combines with another acetyl-CoA, and the citric cycle starts aU over, hi one turn of the citric acid cycle, four oxidation reactions provide hydrogen ions and electrons, which are used to reduce FAD and NAD+ coenzymes (see Figure 18.12). 

---