Stereochemistry is a venerable science, dating back to the work of Louis Pasteur on stereoisomeric tartrates and that of Emil Fischer on monosaccharides. One of the most telling methods for detecting covalent intermediates is the determination of the stereochemical course of an enzyme-catalyzed reaction. In particular, stereochemical analysis of reactant and product provides a highly reliable tool for differentiating double-displacement mechanisms from in-line single displacement mechanisms. The method employs a variety of analytical methods that unambiguously specify reactant stereochemistry. Moreover, as polymers of L-amino acids (or nucleotide units in the case of catalytic RNA), enzymes lack points and planes of symmetry, and their active-site clefts and cavities are inevitably asymmetric.9 Enzyme asymmetry has far-reaching implications for the chemical transformation of chiral and prochiral (see below) substrates into products. Stereochemistry is thus of utmost importance in understanding an enzyme's substrate specificity.
The tetrahedral carbon atom also gives rise to stereo-electronic interactions culminating in optical activity - the ability of electrons within a substance to interact with polarized light, measured as the change in the angle a of the plane of polarization when light of a specific wavelength passes through an optically active substance. (The specific optical rotation, symbolized by [a], is the angle that the plane of polarized light has changed upon passing through a solution of one gram of an optically active substance per milliliter of solution having a path length of one decimeter, using light of wavelength equal to the D-line in the sodium emission spectrum.) Nowadays, the absolute
9 To illustrate the extent to which stereospecificity is the essence of biochemistry, consider the fact that any failure to perpetuate the asymmetry of amino acids would prevent a protein from adopting a unique folded structure. For example, a 400 residue polypeptide with D- and L-amino acids equally abundant at each position would have 20 (or ~ 10 ) unique sequences!
stereochemistry of a compound can be established by X-ray crystallography or through its degradation by enzymes whose stereospecificity has been previously established.
Because nucleophilic substitution reactions have predictable stereochemical outcomes (see Section 2.7.3: General Types of Enzyme-Catalyzed Reaction Mechanisms), one may safely infer the stereochemical properties of an enzyme as well as the likely configuration of atoms in the transition state.
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