Chap 8. Stereochemistry of Enzymatic Reactions Enzyme catalysis: stereospecific A knowledge of the basic principle of stereochemistry: essential for understanding enzyme mechanisms Comparison of the stereochemistry of the substrates and products: a clue of intermediates

A. Optical Activity and Chirality Chiral: lack of a plane or center of symmetry Prochiral: potentially chiral, CR2R’R”

1. Notation RS convention The higher priority: The higher atomic number The higher mass number The atomic number of the next atoms out -SH > -OR > -OH > -NHCOR > -NH2 > -CO2R > -CO2H > -CHO > -CH2OH > -C6H5 > -CH3 > -T > -D > -H Re- and Si-faces for a compound containing a trigonal carbon atom

2. Differences between the Stereochemistries of Enzymatic and Nonenzymatic Reactions Enzymatic reactions: on the surface of an asymmetric protein Nonenzymatic reactions: in a homogeneous solution J. Am. Chem. Soc., 2005, 127, 2104 -2113.

3. Conformation and Configuration Conformation: any one of a molecule’s instantaneous orientations in space caused by free rotation about its single bonds Configuration: the geometry about a rigid or dissymmetric part of a molecule

B. Examples of Stereospecific Enzymatic Reactions NAD+- and NADP+-dependent oxidation and reduction Stereochemistry of the fumarase-catalyzed hydration of fumarate Demonstration that the enediol intermediate in aldose-ketose isomerase reactions is syn Use of locked substrates to determine the anomeric specificity of phosphofructokinase

C. Detection of Intermediates from Retention or Inversion of Configuration at Chiral Centers Stereochemistry of nucleophilic reactions: SN2: inversion SN1: a racemic product Two successive inversion: retention The validity of stereochemical arguments: Stereochemical evidence can only rule out alternative pathways Intermediates in reactions of lysozyme and b-galactosidase: both retention of configuration at the C-1 carbon lysozyme: a carbonium ion fromed in an SN1 reaction b-galactosidase: two successive SN2 displacements

D. The Chiral Methyl Group E. Chiral Phosphate

F. Stereoelectronic control of enzymatic reactions There is a relationship between the energitics of the electronic changes that occur in bond making and breaking and the conformation or configuration of the reactants An optimal conformation for a particular reaction could be important in enzymatic reactions for controlling product formation and minimizing side reactions

1. Pyridoxal phosphate reactivity A Schiff base intermediate formed: 8.44 Three possible bond cleavage to give different products: 8.45 2. Stereoelectronic effects in reactions of proteases A tetrahedral intermediate formed: 8.47 The lowest-energy transition state for breakdown is anti to the leaving group: 8.48 Perhaps inversion on N is an essential step