Alanine mutations of the proposed catalytically essential residues in his-toaspartic protease (HAP) (H34A, S37A and D214A) were generated to investigate whether: (a) HAP is a serine protease with a catalytic triad of His34, Ser37 and Asp214 [Andreeva N, Bogdanovich P, Kashparov I, Popov M & Stengach M (2004)Proteins55, 705–710]; or (b) HAP is a novel protease with Asp214 acting as both the acid and the base during substrate catalysis with His34 providing critical stabilization [Bjelic S & Aqvist J (2004)Biochemistry43, 14521–14528]
