The physiological function of fructosamine-3-kinase (FN3K) is relatively well understood. As shown in several studies, most conclusively by data on the FN3K-KO mouse, this enzyme breaks down compounds produced by the non-enzymatic glycation of proteins by D-glucose. In contrast with FN3K, very little is known about the function of the fructosamine-3-kinase-related-protein (FN3KRP) even though it has a 65% amino-acid sequence identity with FN3K. We do know that this enzyme is a kinase as evidenced by its ability to phosphorylate non-physiological compounds such a psicosamines, ribulosamines, erythrulosamines, and glucitolamines. However, FN3KRP does not phosphorylate any of the numerous Amadori products that are the physiological substrates of FN3K. The fact that FN3KRP is highly conserved in all vertebrates and present throughout nature suggests that it plays an important role in cellular metabolism and makes identification of its physiological substrates an important objective. In this paper, we propose that FN3KRP phosphorylates products resulting from a non-enzymatic glycation of amines by ketoses (fructation) that involves a 2,3-enolization and produces the stable Amadori intermediate, 2-amino-2-deoxy-D-ribo-hex-3-ulose (ADRH). This ketosamine is then phosphorylated to 2-amino-2-deoxy-D-ribo-hex-3-ulose-4-phosphate (ADRH-4-P). Since phosphates are much better leaving groups than hydroxyls, this destabilizes the C-2 amine bond and results in a spontaneous β-elimination of the phosphate to regenerate an unmodified amine with the concomitant production of 4-deoxy-2,3-diulose. Consequently, we postulate that the principal physiological function of FN3KRP is the breakdown of nonenzymatic fructation products. If confirmed in future studies, this hypothesis opens up new perspectives for an improved understanding of biological Maillard reactions and mechanisms for their control and/or reversal.
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