Roles of Ala-149 in the catalytic activity of diadenosine tetraphosphate phosphorylase from Mycobacterium tuberculosis H37Rv

Diadenosine 5′,5′′′-P¹,P⁴-tetraphosphate (Ap₄A) phosphorylase from Mycobacterium tuberculosis H37Rv (MtAPA) belongs to the histidine triad motif (HIT) superfamily, but is the only member with an alanine residue at position 149 (Ala-149). Enzymatic analysis revealed that the Ala-149 deletion mutant displayed substrate specificity for diadenosine 5′,5′′′-P¹,P⁵-pentaphosphate and was inactive on Ap₄A and other substrates that are utilized by the wild-type enzyme.