278 2. ANALOGS OF ENZYME REACTION COMPONENTS 



of the inhibitors with the variously protonated forms of the enzyme. It 

 was found that meso-tartrate is the most potent inhibitor of this group 

 and it was concluded that this configuration of OH groups allows the 

 formation of two hydrogen bonds with the Rl ^^^ ^d enzyme groups 

 (Fig. 2-2). The singly protonated form of the enzyme, EH, binds the 



C-—C FUMARATE 



/ \ 



Rp-H COO 



© 



ooc 



Rjj-H 



L-MALATE 



MESO- TARTRATE 



Fig. 2-2. Simplified scheme of the fumarase active site described by Wigler and 



Alberty (1960). The cationic, R^, and R^ groups occur on different levels and are so 



located they can interact with certain isomers of substrates and inhibitors. 



meso-tartrate most tightly. The binding energies contributed by the hy- 

 drogen bonds can be estimated from the relative interactions of succinate 

 and the tartrates. Weak hydrogen bonds are indicated for the doubly pro- 

 tonated form of the enzyme, EHgl, with D- and L-tartrates, while stronger 

 bonds (— 2.8 to — 3.3 kcal/mole) are formed with meso-tartrate and the 

 less protonated enzyme (see tabulation). 



