Structure, Function and Regulation of a Second Pyruvate Kinase Isozyme in .

(PA) depends on the Entner-Doudoroff pathway (EDP) for glycolysis. The main enzymatic regulator in the lower half of the EDP is pyruvate kinase. PA contains genes that encode two isoforms of pyruvate kinase, denoted PykA and PykF. In other well-characterized organisms containing two pyruvate kinase isoforms (such as ) each isozyme is differentially regulated. The structure, function and regulation of PykA has been previously characterized in detail, so in this work, we set out to assess the biochemical and structural properties of the PykF isozyme. We show that expression is induced in the presence of the diureide, allantoin. In spite of their relatively low amino acid sequence identity, PykA and PykF display broadly comparable kinetic parameters, and are allosterically regulated by a very similar set of metabolites. However, the x-ray crystal structure of PykF revealed significant differences compared with PykA. Notably, although the main allosteric regulator binding-site of PykF was empty, the "ring loop" covering the site adopted a partially closed conformation. Site-directed mutation of the proline residues flanking the ring loop yielded apparent "locked on" and "locked off" allosteric activation phenotypes, depending on the residue mutated. Analysis of PykF inter-protomer interactions supports a model in which the conformational transition(s) accompanying allosteric activation involve re-orientation of the A and B domains of the enzyme and subsequent closure of the active site.