Succinyl phosphate acetylation

Oxoacyl-CoA transferase (see fig. 18.8) is involved in the transfer of a CoASH from succinyl-CoA to acetoacetate to produce succinate and aceto-acetyl-CoA. A cursory examination of this reaction suggests a simple transfer of the CoA moiety. However, it is soon realized that the loss of an oxygen by the acetoacetate and the gain of an oxygen by the succinyl group present a dilemma. Produce a rational mechanism that explains the preservation of the thio-ester energy and solves this dilemma. Hint Consider a succinyl phosphate intermediate. 

The importance of the phosphoenzyme in the mechanism of action of succinyl-CoA synthetase in reactions (27a)-(27c) is also unknown. The mechanisms of action of aminoacyl-tRNA synthetases and of acyl-CoA synthetases do not include covalent enzymic intermediates. The fact that succinyl-CoA synthetase involves succinyl phosphate as the activated substrate, whereas the others involve acyl adenylates, does not explain the difference. There is no chemical catalytic basis for the mechanisms of the formation of these intermediates to vary in this way. Moreover, acetate kinase produces acetyl phosphate without the intermediate formation of a phosphoenzyme, so that at least acetate kinase has the capacity to catalyze direct phosphorylation of a carboxylate group. 

Oxidation of 2 molecules of glyceraldehyde-3-phosphate yields 2 NADH Pyruvate conversion to acetyl-CoA (mitochondria) 2 NADH Citric acid cycle (mitochondria) 2 molecules of GTP from 2 molecules of succinyl-CoA + 2 + 2... 

Vitamin B1 (thiamine) has the active form, thiamine pyrophosphate. It is a cofactor of enzymes catalyzing the conversion of pyruvate to acetyl CoA, a-ketoglutarate to succinyl CoA, and the transketolase reactions in the pentose phosphate pathway. A deficiency of thiamine causes beriberi, with symptoms of tachycardia, vomiting, and convulsions. In Wernicke-Korsakoff syndrome (most common in alcoholics), individuals suffer from apa thy, loss of memory, and eye movements. There is no known toxicity for this vitamin. 

The further biosynthetic pathways from 3-phos-phoglycerate to the myriad amino acids, nucleotides, lipids, and miscellaneous compounds found in cells are complex and numerous. However, the basic features are relatively simple. Figure 17-11 indicates the origins of many substances including the 20 amino acids present in proteins, nucleotides, and lipids. Among the additional key biosynthetic precursors that can be identified from this chart are glucose 6-phosphate, pyruvate, oxaloacetate, acetyl-CoA, 2-oxoglutarate, and succinyl-CoA. 

In the synthesis of fatty acids the acetyl irnits are condensed and then are reduced to form straight hydrocarbon chains. In the oxo-acid chain elongation mechanism, the acetyl unit is introduced but is later decarboxylated. Tlius, the chain is increased in length by one carbon atom at a time. These two mechanisms account for a great deal of the biosynthesis by chain extension. However, there are other variations. For example, glycine (a carboxylated methylamine), under the influence of pyridoxal phosphate and with accompanying decarboxylation, condenses with succinyl-CoA (Eq. 14-32) to extend the carbon chain and at the same time to introduce an amino group. Likewise, serine (a carboxylated ethanolamine) condenses with... 

The tronsfarmation of a>]ietQgtuUiratc to succinyl OsvA in step 4 is a multistep praeens catalyzed by an cncyme entaplex, analogous to the trails formatton of pyruvate to acetyl CoA that we saw in the previous section. In both cases, an p knto acid loses COj in a step catalytced by thiamine pyrp phosphate. ... 

Acetyl-CoA, pyruvate, PEP, a-ketoglutarate, succinyl-CoA, oxaloace-tate, and several sugar phosphates, such as glucose-6-phosphate and fructose-6-phosphate. 

The carbon skeleton of homocysteine is derived from the corresponding hydroxy amino acid, homoserine. The hydroxyl of homoserine is acylated with either a succinyl (bacteria) or acetyl (yeast, fungi, plants) group derived from the corresponding coenzyme A derivative. The O-acyl substituent is then displaced by the thiol group of cysteine producing a mixed thioether, cystathionine. This in turn undergoes a pyridoxal phosphate dependent -elimination to homocysteine, pyruvate and... 

Thiamin pyrophosphate (or thiamin diphosphate) is a coenzyme involved in (1) the oxidative decarboxylation of pyruvate to acetyl coenzyme A (enzyme pyruvate dehydrogenase), (2) the oxidative decarboxylation of a-ketoglutarate to succinyl coenzyme A (a-ketoglutarate dehydrogenase) in the tricarboxylic add cycle, (3) the pentose phosphate pathway (transketolase) and (4) the synthesis of branched-chain amino acids such as valine (branched-chain ketoacid dehydrogenase) in bacteria, yeasts and plants. 

Although the schemes presented in Fig. 4 represent one important mechanism of coupling substrate oxidation with phosphorylation in bacteria, other mechanisms may occur also. In higher animals, phosphotransacetylase and phosphotranssuc-cinylase appear to be absent. In these organisms, therefore, the acyl-SCoA derivatives formed in the oxidation of pyruvate and a-ketoglutarate are used to synthesize ATP by mechanisms that do not involve the intermediate formation of acyl phosphate. ATP formation from acetyl-SCoA (11) appears to occur by reactions (1), (2), and (3), whereas the synthesis of ATP from succinyl-SCoA occurs by an as yet undetermined pathway (15). 

Thiamine was the first vitamin to have its precise biochemical functions determined. In the form of its pyrophosphate, thiamine participates in several very important enzyme systems namely (1) pyruvate dehydrogenase (page 232) which converts pyruvate to acetyl-CoA and carbon dioxide in the course of carbohydrate breakdown (2) the reaction of the citrate cycle in which oxoglutarate is oxidatively decarboxylated to succinyl-CoA (page 242) (3) the transketolase reaction of the pentose phosphate pathway of glucose breakdown (page 233). 

X indicates knocked-out gene. PEP, phos-phoenolpyruvate OAA, oxaloacetate MAL, malate FUM, fumarate Suc-CoA, succinyl-CoA cr-KG, cr-ketoglutarate ICT, isocitrate CIT, citrate PYR, pyruvate AcCoA, acetyl-CoA IdhA, lactate dehydrogenase pfiB, pyruvate formatelyase pta, phosphate... 

Fig. 15. Effect of chemically modified cytochrome c on the reduction of cytochrome a. (A) Acetylated cytochrome c. (B) Succinylated cytochrome c. The sample cuvette contained 9.1 x 10" M phosphate buffer at pH 7.4, 1% Emasol 1130, 3.3 X I0 MKCN, 2.5 x I O M cytochrome a, 1.0 x 10 M hydroqui-none, and 4.5 x I0" M cytochrome c or 4.8 x 10 M modified cytochrome c. The total volume was 3.0 ml. The reaction was initiated by the addition of hydro-quinone. The reduction was determined by measuring the increase in optical density of the

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