Coenzyme binding site

Catalysis by flavoenzymes has been reviewed and various analogues of FAD have been prepared e.g. P -adenosine-P -riboflavin triphosphate and flavin-nicotinamide dinucleotide ) which show little enzymic activity. The kinetic constants of the interaction between nicotinamide-4-methyl-5-acetylimidazole dinucleotide (39) and lactic dehydrogenase suggest the presence of an anionic group near the adenine residue at the coenzyme binding site of the enzyme. ... 

Fig. 2.2.4.S Sequence alignment of LK-ADH with LB-ADH. The typical coenzyme binding site, GXXXGXG, of short-chain ADHs is shaded in gray.

In zinc metalloenzymes. zinc is a selective stoichiometric constituent and is essential for catalytic activity. It is frequently present in numerical correspondence with the number of active enzymatic sites, coenzyme binding sites, or enzyme subunits Removal of zinc results in loss of activity. Inhibition by metal complexing agents is a characteristic feature of zinc metalloenzymes. However, no direct relationship holds between the inhibitory effectiveness of these agents and their affinity for ionic zinc. Although zinc is the only constituent of zinc metalloenzymes in vivo, it can be replaced by other metals m vitro, such as cobalt, nickel, iron, manganese, cadmium, mercury, and lead, as m the case of carboxy-peprida.ses. 

N. Bernard, K. Johnson, J. J. Holbrook, and J. Delcour, D175 discriminates between NADH and NADPH in the coenzyme binding site of Lactobacillus delbrueckii subsp. bulgaricus D-lactate dehydrogenase, Biochem. Biophys. Res. Commun. 1995, 208, 895-900. 

Within the sequence of the first 40 amino acids of the N-terminus, which is generally regarded as the coenzyme-binding site, six amino acid differ from each other in the L. brevis and L. kefir ADHs. Three of them are responsible for differing ionic properties of this region, Asn-3 (L. brevis) changed into Asp (L. kefir), Asp-6 into Lys, and Thr-25 into Asp. The coenzyme-binding sequence G-G-T-L-G-I-G found at the positions 14—20 of L. brevis ADH is identical in both enzymes. 

The primary structure of the ADH from L. brevis contains several structures which are typical for short-chain ADHs. The N-terminus, with a length of approximately 30 amino acids, is widely regarded as the coenzyme binding site with the conserved motif G-X-X-X-G-X-G, which is G-G-T-L-G-I-G for Lactobacillus brevis. A second conserved domain found in the L. brems-ADH sequence is a hydrophobic region comprising 10 or 11 residues, respectively. It contains two highly conserved glycines (G82 and G92), separated by nine amino acids. Such structures seem to be located inside the protein and determine the conformation of the enzyme. 

Marginal inadequacy, affecting amino acid metabolism and possibly also steroid hormone responsiveness, may be relatively common. A number of vitamin Be dependency syndromes have been reported - inborn errors of metabolism in which the defect is in the coenzyme binding site of the affected enzyme. 

Tri- and tetracyclic adenine analogues as dimensional probes of enzyme coenzyme binding sites 82ACR128. 

NAD coenzyme and will be referred to as holo and apo subunits. In the crystal structure of dimeric LADH the two subunits are related by strict 2-fold symmetry and do not contain NAD+. The coenzyme binding site was determined from the complex of LADH with ADP-ribose. 

The preliminary X-ray data on coenzyme-containing complexes are not inconsistent with coenzyme-induced asymmetry of the two subunits, since these are not related by crystallographic symmetry in such complexes (see Section II,C,1 and Table II). Considerable nonfunctional differences in conformation between chemically identical protein molecules have previously been observed 361). It is theoretically possible that the coenzyme-induced conformational change in LADH produces functionally similar coenzyme binding sites but nonequivalent substrate binding sites. 

Subsequently, Cross et al. (SOI) demonstrated that formation of the E-NADPH binary complex, and the abortive ternary complexes E-NADPH-L-glutamate and E-NADP-a-ketoglutarate are all characterized by a red shift in the tryptophan absorption spectrum. It appears likely, therefore, that a tryptophan residue is located in or near the coenzyme binding site. 

II) complexes inhibit pig heart m-MDH at low metal concentrations (1-4 moles of ions per mole of enzyme) ( ). Similar platinum compounds inhibit pig heart s-MDH (91). In the case of s-MDH, the inhibition is essentially irreversible but the rate of inhibition is reduced in the presence of coenzyme (91). The steric relationships of these metal binding sites relative to the coenzyme binding sites were determined by X-ray crystallographic studies (91). Phenols and substituted phenols are inhibitors of pig heart m-MDH and appear to be competitive with NAD+ (98). 

Although some discrepancies exist, it appears that s-MDH is essentially insensitive to thiol modification whereas m-MDH possesses a thiol group on each subunit that is located near the coenzyme binding site. Its integrity appears to be essential for enzymic activity. 

---