Structural replacement of active site monovalent cations by the epsilon-amino group of lysine in the ATPase fragment of bovine Hsc70

We have assessed the ability of the epsilon-amino group of a non-native lysine chain to substitute for a monovalent cation in an enzyme active site. In the bovine Hsc70 ATPase fragment, mutation of cysteine 17 or aspartic acid 206 to lysine potentially allows the replacement of an active site potassium ion with the epsilon-amino nitrogen. We examined the ATP hydrolysis kinetics and crystal structures of isolated mutant ATPase domains. The introduced epsilon-amino nitrogen in the C17K mutant occupies a significantly different position than the potassium ion. The introduced epsilon-amino nitrogen in the D206K mutant occupies a position indistinguishable from that of the potassium in the wild-type structure. Each mutant retains <5% ATPase activity when compared to the wild type under physiological conditions (potassium buffer) although substrate binding is tighter, probably as a consequence of slower release. It is possible to construct a very good structural mimic of bound cation which suffices for substrate binding but not for catalytic activity.