Coenzyme relative specific activities

Table VIII. Relative Specific Activities of Vitamin B12 Coenzyme, Residual Substrate, and Product During the Conversion of D-1,2-Propanediol-1 - H to Propionaldehyde-2- H...

An enzyme consists of a polypeptide chain with a particular spatial configuration specific to that sequence of amino acids. The molecule twists and turns, forming structural features that are catalytically active, these being known as active sites. There may be more than one active site per enzyme molecule. Sometimes an auxiliary catalyst, known as a coenzyme, is also needed. Apparently, only the relevant active site of the enzyme comes into contact with the substrate and is directly involved in the catalysed reaction. The active site consists of only a few amino acid residues. These are not necessarily adjacent to one another in the peptide chain but may be brought into proximity by the characteristic folding of the enzyme structure. The active site may also include the coenzyme. The remainder of the enzyme molecule fulfils the essential function of holding the components of the active site in their appropriate relative positions and orientation. 

The principal limitation of these data is the lack of definition of the individual forms for the CYP2C subfamily. Analysis of this subfamily has remained problematic due to high cross-reactivities of all of the distinct forms with most antibody preparations. In addition, Western blot analysis does not distinguish between active and inactive forms of the protein. Furthermore, distinct enzymes may have different affinities for coenzymes necessary for catalytic activity, which will serve to unlink abundance of the protein and its catalytic activity. Therefore the assumptions must be made that the ratios of active to inactive protein are similar for all forms and that all forms have similar affinities for coenzymes. These assumptions may not be justified. However, even with these limitations, the study of Shimada et al. (1994) contributes greatly to our understanding of relative enzyme abundance in human liver. In addition, the relative abundance data, coupled with the absolute P450 content (per unit protein) and the turnover numbers for enzyme-specific substrates (per unit protein), can provide an estimate of the turnover number for individual enzymes in the human liver membrane environment. This provides an important benchmark for evaluation of turnover number data from cDNA-expressed enzymes. 

Various arylpropionic acids show similar specificity. For most, if not all, the (5) enantiomer is the pharmacologically active one, whereas the R) enantiomer is usually much less active, although the ratio of iS)/ R) activity varies from drug to drug (and species to species). Only one of these drugs, however, is administered as the separated (S) enantiomer (naproxen, Naprosyn ). Normally these drugs are considered safe, and one cannot readily differentiate between the relative activities of the (S) and (R) forms because the in vivo half-life is very short, typically one or two hours. In patients with impaired renal function, where clearance is much slower, however, problems can arise. From in vivo studies of ibuprofen, it was established that the (S)-(-l-) isomer was responsible for antiinflammatory activity. In vivo, however, the (/ )-(-) isomer may become active because there is stereoselective inversion from R) to (S) (but not from 5 to R) in vivo with a half-life of about two hours. This inversion apparently proceeds by stereoselective formation of the coenzyme A (CoA) ester of the (f )-(-)-arylpropionic acid, followed by epimerization and release of the (S)-(+)-enantiomer. This epimerization is observed in vivo before the oxidative metabolism. Such inversion from (R) to (S) in vivo is also known for fenoprofen and benoxa-profen, and is expected to occur for most of the drugs of this series. ... 

Nicotinamide mononucleotide is not a cofactor for bovine GDH 31 la). However, replacement of the adenine moiety of the coenzymes with either hypoxanthine, cytidine, or nicotinamide yields a derivative which will function as eoenzyme, although with diminished efficiencies of 91, 53, and 3% (relative to the activity with NAD), respectively. Nicotinamide mononucleotide phosphoriboside is 0.4% as effective as NAD. These data, which give further indication of the lack of specificity in the adenosine diphosphate site, show that although the adenosine moiety of the coenzyme may be altered in a number of ways, it must be present in some form in order for the analog to function as a cofactor. 

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