Towards Understanding the Role of Inorganic Phosphate in Diabetes Mellitus.

The nonenzymatic glycation of proteins is a ubiquitous process related to pathophysiology in diabetes mellitus and normal aging. The most studied of the reactions involves glucose binding with human hemoglobin, HbA. Five interconverting glucose structures generated upon mutarotation within erythrocytes undergo non-covalent and reversible binding. Over 99.99% of the initial non-covalently bound glucose isomers are ring-closed species that must ring open while bound in order to proceed in the glycation process. The bound transient ring-opened glucose then can ultimately yield structurally-modified proteins. Computational modeling and NMR spectroscopy were used to assess the role(s) that the inorganic phosphate (Pi) plays in glycation. Pi can transiently bind in an HbA cavity with the bound cyclic glucose isomers, and can enhance glucose ring opening and the formation of Schiff base. Multiple mechanisms exist because of variations in the identity and the number of bound Pi molecules, and the geometry between the bound reagents relative to each other and to proximate amino acid residues. Mechanisms can also differ based upon the identity of the initially-bound isomer. The parameters that may dictate site specificity and overall rate of HbA glycation in the non-covalent stages are the relative and absolute concentrations of Pi and the partitioning between available ring-closed isomers