Metal-free dehydrogenase enzyme

The final member of the yeast medium-chain alcohol dehydrogenase superfamily, encoded by the ZTA1 gene, differs in several key aspects from the six putative yeast reductases shown in Fig. 3A. Most important, this enzyme lacks the residues that ligate the catalytic Zn(II) ion, in common with other crystallin homologs. Related metal-free proteins are capable of reducing quinones [53] however, the catalytic activity and substrate specificity of the protein encoded by ZTA1 is currently unknown. 

More reeently, Mesecar and Koshland [32], following an investigation of the enzyme isoeitrate dehydrogenase, have found that the three-point model does not always hold. Examination of metal-free crystals of the enzyme strueture reveals that only the 25,37 -(L)-isocitrate binds, whereas in the presenee of magnesium ions only the 22 ,3iS -(D)-enantiomer binds. Examination of x-ray structures of the two enzyme-substrate complexes reveals three common binding sites for both enantiomeric substrates that differ at a fourth site. Based on their observations, the authors proposed that the three-point model is only applicable if the assumption is made that the substrate can approach a planar surface from one direction. Thus a fourth location, either a direction requirement or an additional binding site, is essential to distinguish between a pair of enantiomers (Fig. 5). 

Recently a Cl NMR line width study of another metal-free enzyme, pig heart lactate dehydrogenase, has been presented by Ward [48Z]. Two types of Cl binding sites were inferred, one of which is... 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

The NAD+-dependent alcohol dehydrogenase from horse liver contains one catalytically essential zinc ion at each of its two active sites. An essential feature of the enzymic catalysis appears to involve direct coordination of the enzyme-bound zinc by the carbonyl and hydroxyl groups of the aldehyde and alcohol substrates. Polarization of the carbonyl group by the metal ion should assist nucleophilic attack by hydride ion. A number of studies have confirmed this view. Zinc(II) catalyzes the reduction of l,10-phenanthroline-2-carbaldehyde by lV-propyl-l,4-dihy-dronicotinamide in acetonitrile,526 and provides an interesting model reaction for alcohol dehydrogenase (Scheme 45). The model reaction proceeds by direct hydrogen transfer and is absolutely dependent on the presence of zinc(II). The zinc(II) ion also catalyzes the reduction of 2- and 4-pyridinecarbaldehyde by Et4N BH4-.526 The zinc complex of the 2-aldehyde is reduced at least 7 x 105 times faster than the free aldehyde, whereas the zinc complex of the 4-aldehyde is reduced only 102 times faster than the free aldehyde. A direct interaction of zinc(II) with the carbonyl function is clearly required for marked catalytic effects to be observed. 

Three human redox enzymes, and a variety of bacterial enzymes, contain molybdenum chelated by two sulfur atoms in a modified pterin molybdopterin (see Figure 10.1). In sulfite oxidase, the other two chelation sites of the molybdenum are occupied by oxygen in xanthine oxidase / dehydrogenase (Section 7.3.7) and aldehyde oxidase, one site is occupied by oxygen and one by sulfur. In some bacterial enzymes, molybdopterin occurs as a guanine dinucleotide rather than free. In others, tungsten rather than molybdopterin is the chelated metal there is no evidence that any mammalian enzymes contain tungsten. 

Heavy metals frequently act as enzyme inhibitors particularly when free -SH groups participate in the reaction. Papain, urease, myosin, triose phosphate dehydrogenase and many other enzymes fall into this category and are readily inactivated by Ca, Hg " " etc. The inhibition may sometimes be overcome by removing the metal ion, e.g. with H2S or with a chelating agent such as EDTA. 

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