The structure of the complex between the 2,3-diphosphoglycerate-independent phosphoglycerate mutase (iPGM) from Bacillus stearothermophilus and its 3-phosphoglycerate substrate has recently been solved, and analysis of this structure allowed formulation of a mechanism for iPGM catalysis. In order to obtain further evidence for this mechanism, we have solved the structure of this iPGM complexed with 2-phosphoglycerate and two Mn 2؉ ions at 1.7-Å resolution. The structure consists of two different domains connected by two loops and interacting through a network of hydrogen bonds. This structure is consistent with the proposed mechanism for iPGM catalysis, with the two main steps in catalysis being a phosphatase reaction removing the phosphate from 2-or 3-phosphoglycerate, generating an enzyme-bound phosphoserine intermediate, followed by a phosphotransferase reaction as the phosphate is transferred from the enzyme back to the glycerate moiety. The structure also allowed the assignment of the function of the two domains of the enzyme, one of which participates in the phosphatase reaction and formation of the phosphoserine enzyme intermediate, with the other involved in the phosphotransferase reaction regenerating phosphoglycerate. Significant structural similarity has also been found between the active site of the iPGM domain catalyzing the phosphatase reaction and Escherichia coli alkaline phosphatase.
Phosphoglycerate mutases (PGMs)1 catalyze three types of reactions including interconversion of 1,3-phosphoglycerate and 2,3-phosphoglycerate (23PGA) and of 3-phosphoglycerate (3PGA) and 2-phosphoglycerate (2PGA) as well as synthesis of 3PGA from 23PGA (1). There are two distinct types of PGMs, bisphosphoglycerate mutase and monophosphoglycerate mutase, and only the first type has the ability to perform all of the reactions listed above, while monophosphoglycerate mutases catalyze primarily the interconversion of 3PGA and 2PGA in both glycolysis and gluconeogenesis. There are also two classes of monophosphoglycerate mutases that are distinguished by their requirement for 23PGA for catalysis (1, 2). The PGMs requiring 23PGA for catalysis are termed 23PGA-dependent and are the predominant PGM in mammals, yeast, and some bacteria. They also have the ability to perform all reactions noted above but at significantly different rates. The monophosphoglycerate mutases that do not require DPG for catalysis are termed 23PGA-independent (iPGMs) and are the predominant PGM in plants and some other bacteria, including endosporeforming Gram-positive bacteria and their close relatives; iPGMs, unlike cofactor-dependent phosphoglycerate mutases and bisphosphoglycerate mutases, can only carry out the interconversion of 2PGA and 3PGA (1, 3, 4). The two classes of monophosphoglycerate mutases are extremely different in amino acid sequence, catalytic mechanism, and structure, both tertiary and quaternary. Despite the differences between these two types of PGMs, the iPGMs all have very conserved amino acid sequences, as do the cofactor-d...