NONCODED AMINO-ACID REPLACEMENT PROBES OF THE ASPARTATE-AMINOTRANSFERASE MECHANISM

The primary role of Tyr225 in the aspartate aminotransferase mechanism is to provide a hydrogen bond to stabilize the 3'O- functionality of bound pyridoxal phosphate. The strength of this hydrogen bond is pertu rbed by replacement of Tyr225 with 3-fluoro-L-tyrosine (FlTyr) by in v itro transcription/translation. This mutant enzyme exhibits k(cat)/K-M (Asp) values that are near to those of wild type enzyme; however, the k(cat)/K-M(Asp) vs pH profile is much sharper with similar pK(a)s of s imilar to 7.5 for both the ascending and descending limbs. The pK(a)s are assigned to the endocyclic proton of the internal aldimine and to the bridging hydrogen bond, respectively. The pK(a)s in the k(cat) vs pH profile of 7.2 and 8.7 are assigned to the epsilon-NH3+ Of lysine 2 58 and to the endocyclic protons of the ketimine complex, respectively . Arginine 292, forms a salt bridge with the B-COOH of the substrate, aspartate. An improvement on the earlier attempt to invert the substra te charge specificity via R292D mutation-induced arginine transaminase activity [Cronin, C. N., & Kirsch, J. F. (1988) Biochemistry 27, 4572 -4579] is described. Here Arg292 is replaced with homoglutamate (R292h oGlu). This construct exhibits 6.8 x 10(4)-fold greater activity for t he cationic substrate -[C-alpha-H-3]-alpha-amino-beta-guanidinopropion ic acid (D,L-[C-alpha-H-3]AGPA) than does wild type enzyme. The gain i n selectivity for this substrate is at least 4500-fold greater than th at achieved in the 1988 experiment, i.e., {(k(cat)/K-m)(R292hoGlu)/(k( cat)/K-M)(WT) (D,L-[C-alpha-H-3]AGPA)} greater than or equal to 4500 x {(k(cat)/K-M)(R292D)/(k(cat)/K-M)(WT) (L-arginine)}. The value of (k( cat)/K-M)(R292D) is 0.43 M-1 s(-1) with L-Arg while (k(cat)/K-M)(R292h oGlu) is 29 M-1 s(-1) with D,L-[C-alpha-H-3]AGPA (it is assumed that t he D-enantiomer is unreactive). The latter value is the lower limit be cause of the uncertain value of H-3 kinetic isotope effect.