Insights into the serine protease mechanism from atomic resolution structures of trypsin reaction intermediates.

Atomic resolution structures of trypsin acyl-enzymes and a tetrahedral intermediate analog, along with previously solved structures representing the Michaelis complex, are used to reconstruct events in the catalytic cycle of this classic serine protease. Structural comparisons provide insight into active site adjustments involved in catalysis. Subtle motions of the catalytic serine and histidine residues coordinated with translation of the substrate reaction center are seen to favor the forward progress of the acylation reaction. The structures also clarify the attack trajectory of the hydrolytic water in the deacylation reaction.

Role of the intramolecular hydrogen bond network in the inhibitory power of chymotrypsin inhibitor 2.

A series of mutants of chymotrypsin inhibitor 2 (CI2), at residues involved in intramolecular interactions that shape and constrain the binding loop, were studied to determine their relative importance for inhibition of the serine protease subtilisin BPN', and for resistance of the inhibitor to proteolysis. These functional properties were investigated in tandem with the crystal structures of the mutant inhibitor-enzyme complexes. A dense hydrogen bonding network that supports the binding loop in the vicinity of the scissile bond was found to be important both for enzyme affinity and for stability to proteolysis. Structural analysis, in combination with biochemical measurements, allows differentiation of the structural components most important for resistance to proteolysis and/or binding. The most critical participating residues in the network were found to be Thr-58, Glu-60, Arg-65, and Gly-83. Glu-60 is more important for resistance to proteolysis than for binding, while Arg-65 and two other Arg residues play a greater role in binding than in resistance to proteolysis. Structural comparisons reveal a wide variety of subtle conformational changes in response to mutation, with built-in robustness in the hydrogen bond network, such that loss of one contact is compensated by other new contacts.