Therefore, it should be concluded that, based on our calculations, any H/D exchange between Cys493 and Gly829 of the apoenzyme would be highly enantioselective and would not easily exchange the H atom at Gly along the depicted mechanism. The predicted kinetic constants (7.3*10 -7 vs. 1.6*10 -18 s -1 ) show that the favored H/D exchange pathway in apo-BSS proceeds through R-GlyHD, regardless whether the barriers of the proS process need to be increased by 86.8 kJ/mol because the higher energy level of this conformant. However, even the favored H/D exchange pathway in apo-BSS would be 2.67*10 6 -fold slower than that in the E:S or E:P complex. This equals to one predicted exchange per 378 h, practically precluding H/D exchange in the absence of bound substrate or product.