EXTENT OF PROTON-TRANSFER IN THE TRANSITION-STATES OF THE REACTION CATALYZED BY THE DELTA(5)-3-KETOSTEROID ISOMERASE OF COMAMONAS (PSEUDOMONAS) TESTOSTERONI - SITE-SPECIFIC REPLACEMENT OF THE ACTIVE-SITE BASE,ASPARTATE-38, BY THE WEAKER BASE ALANINE-3-SULFINATE

Previous studies of the mechanism of the steroid isomerase of Comamona s (Pseudomonas) testosteroni have identified aspartate 38 as the proto n porter which transfers the substrate's 4 beta proton to the 6 beta p osition of the product. Consequently, aspartate 38 functions as a base in the deprotonation of the substrate to form a dienol or dienolate i ntermediate, which then undergoes reprotonation from protonated aspart ate 38 at C-6 beta to give the product. We have tried to characterize the transition states for the proton transfers by altering the pK(a)' of aspartate 38 and then determining the effect of the alteration on t he kinetics of the enzyme. Alteration of the pK(a)' was accomplished b y replacement of the carboxyl carbon of aspartate 38 by sulfur, a chan ge which converts the carboxylate group to the much less basic sulfina te group. Employing Bronsted catalysis theory as applied to the indivi dual steps of the isomerase mechanism, we find that in the enolization step of the reaction proton transfer to aspartate 38 is well advanced in the transition state. In the subsequent ketonization step, proton transfer from aspartate 38 has barely started when that transition sta te is reached. A series of mutant KSIs with alternative bases at posit ion 38 have been constructed using a combination of site-directed muta genesis and chemical modification: Asp-38 to Glu (D38E), His (D38H), a nd S-(carboxymethyl)cysteine (D38CMC). While the D38H and D38E mutants both retain significant isomerase activity, D38CMC is essentially ine rt. From the results of kinetic experiments it is possible to get a qu alitative idea of the sensitivity of the enzyme's catalytic ability to the location of the base responsible for proton transfer.