Amino acid residues associated with enzymatic activities of the isomerizing phycoviolobilin-lyase PecE/F

PecE and PecF jointly catalyze the covalent attachment of phycocyanobilin to Cys-alpha 84 of PecA and its concomitant isomerization to phycoviolobilin. (a) An Eschertchia coli supernatant expressing pecF has a residual activity of 6%; compared to the holoenzyme, this activity is lost upon purification. (b) Functional domains of both subunits from the cyanobacterium Mastigocladus laminosus were evaluated by mutageneses and chemical modification of amino acids. When in PecE the two motifs Y29YAAWWL and D263DLL were deleted, the holoenzyme lost its activity; it is also inactivated upon deletion of a central part (R111 to A122). The three conserved cysteines C48, C91, and Cl 61 have only minor effects on catalysis. When in PecF the 20 C-terminal and 56 N-terminal amino acids were truncated, the lyase-isomerase activity in combination with PecE decreased to 12% and 15%, respectively, compared to the native enzyme. The catalytic efficiency (k(cat)/K-m) decreased 16-fold when the unique four histidine residues in PecF beginning at H53 were deleted. H121 and C122 of PecF are essential for the enzyme activity; they are part of a unique stretch extending from A 104 to NI 25 which is absent in the beta-subunit of related but nonisomerizing lyases. A single histidine and a single tryptophan are required for activity in both PecE and PecF, as judged from diethyl pyrocarbonate and N-bromosuccinimide modification and statistical analyses. Inactivation of PecE and PecF is also possible by arginine-specific reagents, while modifications of lysine, glutamate, and aspartate retained activity. (c) PecE and PecF, as well as most of the mutants, bind PCB covalently in substoichiometric amounts, as assayed by Zn2+-induced fluorescence on denaturing gels.