Methionine residues dehydrogenases

Methemoglobin reductase, cytochrome bt reductase and, 164-165 Methionine residues glyceraldehyde-3-phosphate dehydrogenase, 11... 

Figure 6b shows the modeled a helix F interface in human 17P-hydroxysteroid dehydrogenase type 2. Alanine-237 is 3 A from the hydrophobic part of the side chain of methionine-241 on the other subunit. Methionine-241 is 3.2 A from serine-234. Alanine-230 is 3.7 A from phenylalanine-242 and 4.5 A from valine-245. Alanine-238, the other anchoring residue, is 4.1 A from alanine-238 on the other subunit. 

A question of particular interest concerns the catalytic advantage that a selenol in the active site of an enzyme has in comparison to a thiol group. It has been approached by replacing the selenocysteine in the formate dehydrogenase Ft by a cysteine residue and by determining the catalytic parameters of the Se-wild-type species with the mutant S-species. Table 1 presents the data obtained. They show that koit of the Se enzyme is more than 300-fold higher than that of the S variant and that the affinity to the substrate is only marginally affected. Similar replacements of selenocysteine by cysteine in other enzymes like the deiodinase type 1 or methionine sulfoxide reductase B corroborated these results. 

In Escherichia coli K-12, glyceraldehyde-3-phos-phate dehydrogenase (EC 1.2.1.12) showed the greatest susceptibility to ozone (Komanapalli et al. 1997). When the active site sulphydryl (cysteine) is reversibly blocked by tetrathionate during ozone treatment, enzyme activity is completely retained despite the oxidation of tryptophan, methionine, and histidine residues (Knight and Mudd 1984). 

There are several points of attack on the protein molecule, and in glutamate dehydrogenase and RNase, exposure to UV in the presence of psoralen extensively modified histidine, methionine, tryptophan, phenylalanine and tyrosine residues by photo-oxidation. ... 

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