Glyoxalase I is part of the glyoxalase system present in the cytosol of cells. The glyoxalase system catalyses the conversion of reactive, acyclic α-oxoaldehydes into the corresponding α-hydroxyacids. Glyoxalase I catalyses the isomerization of the hemithioacetal, formed spontaneously from α-oxoaldehyde and GSH, to S-2-hydroxyacylglutathione derivatives [RCOCH(OH)-SG→RCH(OH)CO-SG], and in so doing decreases the steady-state concentrations of physiological α-oxoaldehydes and associated glycation reactions. Physiological substrates of glyoxalase I are methylglyoxal, glyoxal and other acyclic α-oxoaldehydes. Human glyoxalase I is a dimeric Zn2+ metalloenzyme of molecular mass 42 kDa. Glyoxalase I from Escherichia coli is a Ni2+ metalloenzyme. The crystal structures of human and E. coli glyoxalase I have been determined to 1.7 and 1.5 Å resolution. The Zn2+ site comprises two structurally equivalent residues from each domain – Gln-33A, Glu-99A, His-126B, Glu-172B and two water molecules. The Ni2+ binding site comprises His-5A, Glu-56A, His-74B, Glu-122B and two water molecules. The catalytic reaction involves base-catalysed shielded-proton transfer from C-1 to C-2 of the hemithioacetal to form an ene-diol intermediate and rapid ketonization to the thioester product. R- and S-enantiomers of the hemithioacetal are bound in the active site, displacing the water molecules in the metal ion primary co-ordination shell. It has been proposed that Glu-172 is the catalytic base for the S-substrate enantiomer and Glu-99 the catalytic base for the R-substrate enantiomer; Glu-172 then reprotonates the ene-diol stereospecifically to form the R-2-hydroxyacylglutathione product. By analogy with the human enzyme, Glu-56 and Glu-122 may be the bases involved in the catalytic mechanism of E. coli glyoxalase I. The suppression of α-oxoaldehyde-mediated glycation by glyoxalase I is particularly important in diabetes and uraemia, where α-oxoaldehyde concentrations are increased. Decreased glyoxalase I activity in situ due to the aging process and oxidative stress results in increased glycation and tissue damage. Inhibition of glyoxalase I pharmacologically with specific inhibitors leads to the accumulation of α-oxoaldehydes to cytotoxic levels; cell-permeable glyoxalase I inhibitors are antitumour and antimalarial agents. Glyoxalase I has a critical role in the prevention of glycation reactions mediated by methylglyoxal, glyoxal and other α-oxoaldehydes in vivo.
Abbreviations used: AGE, advanced glycation end-product; CDNB, 1-chloro-2,4-dinitrobenzene; SpBrBzGSH, S-p-bromobenzylglutathione; SpBrBzGSHCp2, S-p-bromobenzylglutathione cyclopentyl diester; SpBrBzGSHEt2, S-p-bromobenzylglutathione ethyl diester; CEdG, N2-(1-carboxyethyl)deoxyguanosine; CEL, N∊-(1-carboxyethyl)lysine; dG-MG, 6,7-dihydro-6,7-dihydroxy-6-methylimidazo-[2,3-b]purine-9(8)one; dG-MG2, N2,7-bis-(1-hydroxy-2-oxopropyl)deoxyguanosine; MG-H1, NΔ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine; MOLD, methylglyoxal-derived lysine dimer [1,3-di(N∊-lysino)-4-methyl-imidazolium salt].
679th Meeting of the Biochemical Society held at the University of Essex, Colchester, 2–4 July 2003