The propeptide of the vitamin K-dependent carboxylase substrate accelerates formation of the gamma-glutamyl carbanion intermediate

Effect of vitamin K-dependent protein precursor propeptide, vitamin K hydroquinone, and glutamate substrate binding on the structure and function of {gamma}-glutamyl carboxylase

The γ-glutamyl carboxylase utilizes four substrates to catalyze carboxylation of certain glutamic acid residues in vitamin K-dependent proteins. How the enzyme brings the substrates together to promote catalysis is an important question in understanding the structure and function of this enzyme. The propeptide is the primary binding site of the vitamin K-dependent proteins to carboxylase. It is also an effector of carboxylase activity. We tested the hypothesis that binding of substrates causes changes to the carboxylase and in turn to the substrate-enzyme interactions. In addition we investigated how the sequences of the propeptides affected the substrate-enzyme interaction. To study these questions we employed fluorescently labeled propeptides to measure affinity for the carboxylase. We also measured the ability of several propeptides to increase carboxylase catalytic activity. Finally we determined the effect of substrates: vitamin K hydroquinone, the pentapeptide FLEEL, and NaHCO(3), on the stability of the propeptide-carboxylase complexes. We found a wide variation in the propeptide affinities for carboxylase. In contrast, the propeptides tested had similar effects on carboxylase catalytic activity. FLEEL and vitamin K hydroquinone both stabilized the propeptide-carboxylase complex. The two together had a greater effect than either alone. We conclude that the effect of propeptide and substrates on carboxylase controls the order of substrate binding in such a way as to ensure efficient, specific carboxylation.

Vitamin K-dependent proteins

Vitamin K is required for the synthesis of gamma-carboxyglutamate (Gla) during postribosomal protein modification. Substrates include blood clotting proteins, bone proteins, cell signaling, and receptor proteins. In addition, Gla is a component of short toxin peptides from the marine snail Conus. Studies of structure-function relationships are the most advanced for the blood coagulation proteins. Reviews of vitamin K action and blood coagulation are presented. Special focus is on the structure-function role of Gla in blood coagulation and the impact of this amino acid on enzyme reaction kinetics. This amino acid forms calcium and membrane binding sites for these proteins. Two proposed mechanisms of protein-membrane attachment are reviewed. One involves membrane attachment by protein insertion into the hydrocarbon region of the membrane, while another considers attachment by specific interactions with phospholipid head groups. Membrane attachment generates the potential for several forms of nonclassical enzyme kinetic behaviors, all of which have been observed in vitro. For example, the reaction may be limited by properties of the enzyme active site, a condition that allows use of classic steady-state enzyme kinetic parameters. However, the reaction may be limited by substrate binding to the membrane, by substrate flux through solution, and/or by solvent flow rates across the membrane surface. These states provide special mechanisms that are not anticipated by classical steady-state kinetic derivations. They may be used to regulate coagulation in vivo. Overall, vitamin K research spans the spectrum of biological research and experience. Exciting new ideas and findings continue to emanate from vitamin K-related research.