Brief history of glyoxalase I and what we have learned about metal ion-dependent, enzyme-catalyzed isomerizations.
Glyoxalase I, a member of the metalloglutathione (GSH) transferase superfamily, plays a critical detoxification role in cells by catalyzing the conversion of cytotoxic methylglyoxal (as the diastereomeric GSH-thiohemiacetals) to S-D-lactoylglutathione via a 1,2-hydrogen transfer. The mechanism-of-action of this Zn2+-metalloenzyme has been the subject of considerable controversy over the past 50 years. Key issues relate to the role of the active-site metal ion in catalysis and how the enzyme is able to use directly both diastereomeric thiohemiacetals as substrates. The results of recent X-ray crystallographic measurements on the enzyme in complex with a transition state analogue and site-directed mutagenesis studies now strongly support a base-mediated, proton-transfer mechanism in which the bound diastereomeric substrates undergo catalytic interconversion before the 1S-diastereomer goes to product via a Zn2+-coordinated, cis-enediolate intermediate. Comparisons with chemical model systems suggest that Zn2+-coordination of thiohemiacetal substrate will dramatically increase the thermodynamic and kinetic acidity of the C1-H bond of substrate. In the human enzyme, the carboxyl group of Glu (172) is well positioned to catalyze a suprafacial proton transfer between the adjacent carbons of substrate. The Zn2+-coordinated carboxyl group of Glu(99) is a reasonable candidate to catalyze proton transfer between the Zn2+-coordinated oxygen atoms of the enediolate intermediate. Other Zn2+ metalloenzymes appear to use similar reaction mechanisms to facilitate proton transfers.
Creighton
D J
DJ
Department of Chemistry and Biochemistry,  University of Maryland, Baltimore 21250, USA. creighto@umbc.edu
Hamilton
D S
DS
D020134:Catalytic Domain; D007791:Lactoylglutathione Lyase / Q000737:chemistry / Q000378:metabolism*; D008667:Metalloproteins / Q000737:chemistry / Q000378:metabolism*; D008956:Models, Chemical; D013237:Stereoisomerism; D013379:Substrate Specificity; D015032:Zinc
D008667:Metalloproteins; D007791:Lactoylglutathione Lyase; D015032:Zinc