Mechanistic inferences from stereochemistry.

I am still impressed that the textbook language of organic chemistry is so successful in explaining the reactions that we find in nature. Thismight not have been anticipated because we find that biological reactions are invariably stereospecific whereas nonbiological reactions are invariably not. This realization came to light about 150 years ago when Pasteur found that only half of chemically prepared tartarate was fermentable contrary to chemically identical tartarate from grapes. Only the natural product rotated the plane of polarized light (1). A new geometry had to be found for carbon to explain this. It could not be planar. As understood by Ogston in 1948 (2) the stereospecificity of biological reactions derives from the chiral properties of the active sites of the enzymes that catalyze them. (Pasteur realized that only a chiral reagentwould be able to distinguish between substrates that were not superimposable on theirmirror images. He spent much effort searching for a natural force that might have caused chirality in the first place. It remains an unsolved problem.) When I began studying simple enzymatic reactions at carbon centers in 1955, I was not at all sure what I might run into. Organic chemists were acquiring evidence for stable ion pair intermediates in carbonium and carbanion rearrangements in solution that were completely stereospecific. Would the stereospecificity of enzymatic reactions turn out to be explained in terms of a physical or chemical role of the enzyme? If this seems like an overstatement it may be recalled that in 1955 there were no examples to cite in which an enzyme could be analyzed to be acting as a base to abstract a proton from–CH to a carbonyl. However, therewerewell known enzymes available such as the aldolases and aldose-ketose isomerases, the mechanisms of which had not been analyzed. My hope then was to use a stereochemical approach to the study of enzyme reaction chemistry as my first research problem. Fortunately scintillation counterswere becoming available in 1955. I was somewhat ahead of the game because of the hobby of Seymour Lipsky, an M.D. in the Department of Medicine who consulted for the New Haven-based Technical Measurement Co. and had one of their first commercial counters. Tritium, especially T-water, was also becoming available, making it unnecessary for me to repair the Rittenberg model mass spectrometer that Henry Hoberman had left to the Yale Department of Biochemistry. The first demonstration of an enzyme acting as a basewas probably our observation in 1955 that muscle aldolase catalyzed the stereospecific exchange of one of the C-1 hydroxymethyl protons of dihydroxyacetone-P with TOH in the absence of an aldehyde partner (3). From the stereochemistry of the T-exchanged product (4) compared with that of C-4 of the condensation products one THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 281, NO. 10, pp. 6117–6119, March 10, 2006 © 2006 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in the U.S.A.