Coenzymes dissociation

Stone CL, Bosron WF, Dunn MF. Amino acid substitutions at position 47 of human beta 1 beta 1 and beta 2 beta 2 alcohol dehydrogenases affect hydride transfer and coenzyme dissociation rate constants. J Biol Chem 1993 268 892-899... 

Q. This finding eliminates a truly rapid equilibrium random mechanism, for which k and k must be much smaller than fc 4, k-i, k, and k-2, since the two exchange rates must then be equal. In fact, the differences between the two exchange rates show that the dissociation of A and/or P from the ternary complexes must be slow compared with that of B and/or Q, and also slow relative to the interconversions of the ternary complexes (32). This means that in at least one direction of reaction the dissociation of products in the overall reaction is essentially ordered for all these enzymes, the coenzymes dissociating last, as in the preferred pathway mechanism (Section I,B,4). With malate, lactate, and liver alcohol dehydrogenases, the NAD/NADH exchange rate increased to a... 

The rate of oxidation of ethanol can be increased by the simultaneous presence of an aldehyde (343). It was suggested that the rate increase was resulting from the absence of an enzyme-coenzyme dissociation step. Using a suitable aldehyde this reaction can be used as an assay method (34 ). 

Coenzyme dissociation is rate limiting for both enzymes [341,4 3). Furthermore, no kinetic evidence has been obtained for an isomerization of the binary YADH-NADH complex, in contrast to the corresponding LADH complex 303). Hydrogen-deuterium exchange studies (464), however, indicate a conformational change upon coenzyme binding. 

I have already mentioned that enzymes with prosthetic groups are more stable in the holoenzyme form than after dissociation of coenzyme. In addition, Litwack and Rosenfield (1974) reported a correlation between the rate of coenzyme dissociation of a number of liver enzymes and their degradation rate constants in vivo. Of course these experiments were carried out in vitro, often with purified enzymes, and it is not established whether apoenzymes are especially unstable in the intact animal. In fact, three biotin-dependent enzymes, acetyl-CoA carboxylase, propionyl-CoA carboxylase, and pyruvate carboxylase, are present in normal amounts during biotin deficiency but with very low catalytic activities. Catalytic activity is rapidly restored when biotin is administered to the enzymes (Chiang and Mistry, 1974 Jacobs et al., 1970). 

Litwack, G., and Rosenfield, S., 1973, Coenzyme dissociation, a possible determinant of short half-life of inducible enzymes in mammalian liver, Biochem. Biophys. Res. Com-mun. 52 181. 

As pointed out above, dehydrogenases are involved in numerous metabolic reactions in living cells balancing carbon flux and free energy in a complicated network mediated by the coenzymes involved. NAD(P)/NAD(P)H are usually not very tightly bound values are found in the range M. Therefore, coenzymes dissociate freely and are... 

These dehydrogenases use nicotinamide adenine dinucleotide (NAD ) or nicotinamide adenine dinucleotide phosphate (NADP )—or both—and are formed in the body from the vitamin niacin (Chapter 45). The coenzymes are reduced by the specific substrate of the dehydrogenase and reoxidized by a suitable electron acceptor (Figure 11-4). They may freely and reversibly dissociate from their respective apoenzymes. 

Such internal thermodynamic equilibria where A is a protein are found for non-metal components, including free coenzymes and substrates where B is a small molecule, or where free M is an ion of either a non-metal, e.g. Cl" or HCOj, or a metal, e.g. K+ or Mg2+, or is H+, and they are involved in, even necessary for, catalysis, pumping and cooperative controls of many metabolic paths. All such combinations reach equilibrium, as long as exchange is fast, where a fast rate can be taken as, say, 10-3 s for dissociation in cells. Note that equilibria with defined binding constants for AB or AM formation in any system reduce the number of variables and hence AB and AM concentrations are defined by those of free A, B and M, leaving two independent variables for each equilibrium. In some cases, the... 

Many enzymes require the participation of dissociable coenzymes such as NAD+, NADP+ or ATP for their catalytic activities. The use of coenzymes to activate immobilized enzymes on a large scale is hampered by their relatively low stability and high cost. Attempts are therefore being made to stabilize the coenzymes and to find suitable means for their continuous regeneration. The principal approach has been to covalently attach a co-enzyme to a polymeric water-soluble matrix, thus making the co-enzyme, like the enzyme, potentially reusable (9,10). 

O. Miyawaki and T. Yano, Dynamic affinity between dissociable coenzyme and immobilized enzyme in an affinity chromatographic reactor with single enzyme, Biotechnol. Bioeng., 39, 314-319 (1992). 

A dissociable activator (or coenzyme) present in the enzyme solution may result in upward curvature as the enzyme concentration increases, so also does the concentration of the activator. 

Cloning Coenzyme Process of making genetically identical copies. A coenzyme is a dissociable, low-molecular weight, non-proteinaceous organic compound (often... 

Some enzymes associate with a nonprotein cofactor that is needed for enzymic activity. Commonly encountered cofactors include metal ions such as Zn2+ or Fe2+, and organic molecules, known as coenzymes, that are often derivatives of vitamins. For example, the coenzyme NAD+contains niacin, FAD contains riboflavin, and coenzyme A contains pantothenic acid. (See pp. 371-379 for the role of vitamins as precursors of coenzymes.) Holoenzyme refers to the enzyme with its cofactor. Apoenzyme refers to the protein portion of the holoenzyme. In the absence of the appropriate cofactor, the apoenzyme typically does not show biologic activity. A prosthetic group is a tightly bound coenzyme that does not dissociate from the enzyme (for example, the biotin bound to carboxylases, see p. 379). 

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