Dephospho CoA

This enzyme [EC 2.7.1.24] catalyzes the reaction of ATP with dephospho-CoA to produce ADP and coenzyme A. 

CARNOSINE SYNTHETASE CHAPERONES CHOLINE KINASE CHOLOYL-CoA SYNTHETASE COBALAMIN ADENOSYLTRANSFERASE 4-COUMAROYL-CoA SYNTHETASE CREATINE KINASE CTP SYNTHETASE CYTIDYLATE KINASE 2-DEHYDRO-3-DEOXYGLUCONOKINASE DEHYDROGLUCONOKINASE DEOXYADENOSINE KINASE DEOXYADENYLATE KINASE DEOXYCYTIDINE KINASE (DEOXYjNUCLEOSIDE MONOPHOSPHATE KINASE DEOXYTHYMIDINE KINASE DEPHOSPHO-CoA KINASE DETHIOBIOTIN SYNTHASE DIACYLGLYCEROL KINASE DIHYDROFOLATE SYNTHETASE DNA GYRASES DNA REVERSE GYRASE ETHANOLAMINE KINASE EXONUCLEASE V... 

DIETHYL PYROCARBONATE DEPHOSPHO-CoA KINASE Dephospho-CoA pyrophosphorylase, PANTETHEINE-RHOSRHATE ADENYLYL-TRANSFERASE DEPOLARIZATION ACTION ROTENTIAL HYPERROLARIZATION DEPOLYMERIZATION PROCESSIVITY... 

Figure 12.2. Biosynthesis of coenzyme A. Pantothenate kinase, EC 2.7.1.33 phosphopantothenylcysteine synthase, EC 6.3.2.5 phosphopantothenylcysteine decarboxylase, EC 4.1.1.36 phosphopantetheine adenyltransferase, EC 2.7.7.S and dephospho-CoA kinase, EC 2.7.1.24. CoASH, free coenzyme A.

Phosphopantetheine undergoes adenylyl transfer from ATP to yield de-phospho-CoA, which is then phosphorylated at the 3 position of the ribose moiety to yield CoA. Phosphopantetheine adenylyltransferase and dephos-pho- CoA kinase activities occur in a single bifunctional enzyme, which is found in both cytosol and mitochondria. However, in addition to the bifunctional protein, human tissues also contain a separate dephospho-CoA kinase (Begley et al., 2001 Zhyvoloup et al., 2002). 

CoA undergoes dephosphorylation, catalyzed by lysosomal acid phosphatase, to dephospho-CoA, followed by pyrophosphatase action to release 4 -phosphopantetheine and 5 -AMP - the reverse of the final stages of CoA synthesis shown in Figure 12.2. CoA is also a substrate for direct pyrophosphatase action, at about 10% of the rate of action on dephospho-CoA. The pyrophosphatase seems to be a general nucleotide pyrophosphatase of plasma membrane rather than an enzyme specific for the degradation of CoA. 

Pantothenic acid is taken in as dietary CoA compounds and dCphosphopantetheine and hydrolyzed by pyrophosphatase and phosphatase in the intestinal lumen to dephospho-CoA, phosphopantetheine, and pantetheine. This is further hydrolyzed to pantethenic acid. The vitamin is primarily absorbed as pantothenic acid by a saturable process at low concentrations and by simple diffusion at higher ones. The saturable process is facilitated by a sodium-dependent multivitamin transporter, for which biotin and lipoate compete. After absorption, pantothenic acid enters the circulation and is taken up by cells in a manner similar to its intestinal adsorption. The synthesis of CoA from pantothenate is regulated by pantothenate kinase, which itself is subject to negative feedback from the products CoA and acyi-CoA. The steps involved were outlined above. Pantothenic acid is excreted in the urine after hydrolysis of CoA compounds by enzymes that cleave phosphate and the cys-teamine moieties. Only a small fraction of pantothenate is secreted into milk and even less into colostrum. 

Dephospho-CoA kinase (DPCK EC 2.7.1.24) catalyzes the selective MgATP-dependent phosphorylation of the 3 -hydroxyl of the ribose moiety dephospho-CoA 11 to form CoA 1, the final product of the biosynthetic pathway (Equation (7)). As pointed out in the previous section, most eukaryotic organisms have the DPCK activity located with PPAT on a bifunctional protein referred to as CoA synthase. However, unlike PPAT, the DPCK domain of this protein exhibits good sequence homology with its bacterial counterparts. In A. thaliana, the protein is monofunctional. ... 

The first bacterial DPCK to be cloned and characterized was the E. coli CoaE protein coaE gene product, previously yacE), which was identified by a basic local alignment search tool (BLAST) using the N-terminal sequence of the wild-type enzyme purified from C. ammoniagenes The enzyme is a 22.6-kDa monomer in solution and exhibits apparent values of 140 pmol 1 and 740 pmol for ATP and dephospho-CoA, respectively. It displays poor activity with adenosine, AMP, and adenosine phosphosulfate (APS) as alternate phosphoryl acceptors, which all shows less than 8% activity compared with the natural substrate. Studies on the bifiinctional PPAT/ DPCK protein show that the normally reversible PPAT activity becomes irreversible when it is coupled to DPCK, most probably because the latter activity demonstrates a low K- for dephospho-CoA of 5.2 L5 pmol 1 (its for ATP was found to be 192 14 pmol 1 ). ° Similar values were determined in an independent study. ... 

Malonate decarboxylase (EC 4.1.1.88) and citrate lyase (EC 4.1.3.6) are both large enzyme complexes that consist of multiple subunits, the smallest of which acts as an ACP. These two complexes catalyze the decarboxylation of malonate to acetate and CO2 and the Mg -dependent cleavage of citrate to acetate and oxaloacetate, respectively. Both have been shown to require a thiol-containing prosthetic group for activity. However, unlike the carrier proteins described in the previous section, the ACP subunits of these proteins are not phosphopantetheinylated by a reaction with CoA. Instead, they rely on a unique cofactor, 2 -(5"-triphosphoribosyl)-3 -dephospho-CoA (24, dePCoA-RibPPP), as source of a 2 -(5"-phosphoribosyl)-3 -dephospho-CoA prosthetic group, which is bound to a conserved serine residue of the ACP. ° ° A similar prosthetic group has been identified in citramalate lyase (EC 4.1.3.22). The proposed biosynthesis and subsequent transfer reactions of the cofactor 24 to the ACPs of these complexes are shown in Scheme 5. 

Scheme 5 Malonate decarboxylase and citrate lyase both contain a bo/o-ACP (MdcC and CitD, respectively) that has a 2 -(5"-phosphoribosyl)-3 -dephospho-CoA prosthetic group which is attached by a phosphodiester linkage to a conserved serine residue. The prosthetic group originates from the cofactor 2 -(5"-triphosphoribosyl)-3 -dephospho-CoA 24, which is biosynthesized from dephospho-CoA 11 and ATP by either MdcB or CitG. The posttranslational modification of theapo-ACP proteins is catalyzed by a complex of the enzyme MdcG with 24, or CitX, depending on the system.

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