Structure-function relationships in 3-phosphoglycerate kinase: role of the carboxy-terminal peptide.

Yeast 3-phosphoglycerate kinase (PGK) is a monomeric enzyme (Mr approximately 45,000) composed of two globular domains. Each domain corresponds approximately to the amino- and carboxy-terminal halves of the polypeptide chain. The carboxy-terminal end extends over the interdomain "hinge" region and packs against the amino-terminal domain. It has been proposed that domain movement, resulting in closure of the active site cleft, is essential for the catalytic function of PGK. Large-scale conformational changes have also been postulated to explain activation of the enzyme by sulfate ions. Using site-specific mutagenesis, we have removed a 15-amino-acid carboxy-terminal fragment, in order to probe its role in the substrate- and sulfate-induced conformational changes. The truncated enzyme exhibited approximately 1% of the activity of native PGK and lost the ability to undergo sulfate-induced activation. The Km for ATP was essentially unchanged (Km = 0.23 mM) in comparison to the native enzyme (Km = 0.30 mM), whereas the Km value for 3-phosphoglycerate was increased about eightfold (Km = 3.85 mM and 0.50 mM, respectively). These results suggest that the carboxy-terminal segment is important for the mechanism of the substrate- and sulfate-induced conformational transitions. CD spectra and sedimentation velocity measurements indicate that the carboxy-terminal peptide is essential for structural integrity of PGK. The increased susceptibility of the truncated enzyme to thermal inactivation implies that the carboxy-terminal peptide also contributes to the stability of PGK.