Citrate-cleavage enzyme

This enzyme [EC 4.1.3.8], also known as ATP-citrate (pro-S-)-lyase and citrate cleavage enzyme, catalyzes the reaction of citrate with ATP and coenzyme A to yield oxaloacetate, acetyl-CoA, ADP, and orthophosphate. 

A three-substrate, three-product enzyme-catalyzed reaction scheme in which one particular substrate (A) is the only substrate that can bind to the free enzyme. After the EA binary complex has formed, the other two substrates (B and C) can bind in any order. Following the catalytic event, two of the products (P and Q) can be released in a random sequence, but the third product (R) has to be the last product released. Citrate cleavage enzyme and y-glutamylcysteine synthetase are reported to operate by this mechanism. See Multisubstrate Mechanisms... 

Kling D, Gamble W. 1980. In vivo inhibition of citrate cleavage enzyme by polychlorinated biphenyls. Experientia 37 1258-1259. 

N3-Phosphohistidine has been isolated from an alkaline hydrolysate of liver acid phosphatase (73) while N phosphohistidine has been isolated from prostate, placenta, and wheat germ acid phosphatases (74, 75). ]V3-Phosphohistidine is found in histone H4 (IV, F2al) (76). Phosphohistidines, substituted on either ring nitrogen atom, and c-N-phospholysine were isolated from a purified citrate cleavage enzyme (77). There is a good possibility that certain acidic nuclear proteins may contain e-iV-phospholysine and w-N-phosphoarginine (78). e-N-Phospholysine is present in histone I (FI) (78a). Phosphothreonine is found in certain proteins of which phosvitin is an example (79,80). 

Only one single concentration term present, but all three Michaelis constants have finite values. The term present is the A term, and the mechanism involves steady-state addition of A, followed by random addition of B and C. Citrate cleavage enzyme [ATP citrate (pro-35)-lyase] has this mechanism 16). 

Fig. 33.7. Fate of citrate in the cytosol. Citrate lyase is also called citrate cleavage enzyme. OAA = oxaloacetate circled t = inducible enzyme.

Adenosine triphosphate conservation in metabolic regulation. Rat liver citrate cleavage enzyme. J. Biol Chent, 242, 3239-3241. 

Fig. 1. Pathway of fatty acid synthesis from glucose in animal tissues. The key enzymes or enzyme systems involved are (1) pyruvate dehydrogenase, (2) pyruvate carboxylase, (3) citrate synthase, (4) citrate translocation system, (5) citrate cleavage enzyme, (6) acetyl-CoA carboxylase, (7) fatty acid synthetase, (8) 3-phosphoglyceraldehyde dehydrogenase, (9) malate dehydrogenase, (10) malic enzyme, (11) hexose monophosphate shunt.

Schmidt and Katz [45] suggested an alternative cyclic process involving malate, which would bypass the citrate cleavage enzyme and transfer intramitochondrial reducing equivalents to the cytosol without producing acetyl units. Were this short-circuit of the malate transhydrogenation cycle found to play a major role in adipose tissue, it could supply more than 50% of the NADPH required by the synthetase. Another potential source of extramitochrondial NADPH is isocitrate dehydrogenase however, it does not appear to be of major importance in fatty acid synthesis, as will be discussed later. 

Following the administration of insulin to diabetic rats, fatty acid synthesis increases [90,100,102-104] as would be predicted, increases are observed in the citrate cleavage enzyme, fatty acid synthetase, and... 

Citrate cleavage enzyme is inhibited by ADP, and this inhibition has been shown to be competitive with respect to ATP [70-72]. Atkinson (see his chapter, this volume) proposes that cleavage enzyme activity is a sensitive function of energy charge ((ATP - - i ADP)/(AMP + ADP + ATP)). This is an attractive possibility because it relates the production of acetyl-CoA, the precursor of fatty acids, to energy charge. ... 

Suggestive evidence also exists for the activation of citrate cleavage enzyme by citrate [73,141]. Citrate has been found to activate the cleavage enzyme-catalyzed hydrolysis of citryl-CoA [141]. 

It has been shown that the carboxylation of acetyl-CoA is effectivelyf the rate-determining reaction of fatty acid synthesis in animal tissues [94] and therefore has regulatory potential. In the absence of tricarboxylic acid activator, acetyl-CoA carboxylase activity is lower by nearly two orders of magnitude than in the fully activated state where its catalytic capacity is nearly kinetically matched to those of the citrate cleavage enzyme and fatty acid synthetase [76,77,94,150]. Therefore, changes in the level of tricarboxylic acid effector presumably could control the rate of the carboxylase reaction, and thus regulate fatty acid synthesis. 

D. R. Nelson and R. W. Rinne, Citrate cleavage enzyme from developing soybean cotyledons. Incorporation of citrate carbon into fatty acids. Plant Physiol. 55 69 (1975). 

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