Abstract

The vitamin K-dependent carboxylase carries out the posttranslational modification of specific glutamate residues in proteins to γ-carboxy glutamic acid (Gla) in the presence of reduced vitamin K, molecular oxygen, and carbon dioxide. In the process, reduced vitamin K is converted to vitamin K epoxide, which is subsequently reduced to vitamin K, by vitamin K epoxide reductase (VKOR) for use in the carboxylation reaction. The modification has a wide range of physiological implications, including hemostasis, bone calcification, and signal transduction. The enzyme interacts with a high affinity γ-carboxylation recognition sequence (γ-CRS) of the substrate and carries out multiple modifications of the substrate before the product is released. This mechanism ensures complete carboxylation of the Gla domain of the coagulation factors, which is essential for their biological activity. γ-Carboxylation, originally discovered in mammals, is widely distributed in the animal kingdom. It has been characterized in sea squirt (Ciona intestinalis), in flies (Drosophila melanogaster), and in marine snails (Conus textile), none of which have a blood coagulation system similar to mammals. The cone snails express a large array of γ-carboxylated peptides that modulate the activity of ion channels. These findings have led to the suggestion that γ-carboxylation is an extracellular posttranslational modification that antedates the divergence of molluscs, arthropods, and chordates. I will first summarize recent understanding of γ-carboxylase and γ-carboxylation gleaned from experiments using the mammalian enzyme, and then I will briefly describe the available information on γ-carboxylation in D. melanogaster and C. textile.

Article Outline

I. Introduction

II. Reviews

III. γ-Carboxylation Reaction

A. Processivity of catalysis

IV. Vitamin K Cycle

A. Epoxide reductase

B. Mutations in epoxide reductase

V. Mechanism of γ-Carboxylation

A. Amine rather than thiolate is the active base

VI. Proposed Topology of γ-Glutamyl Carboxylase

VII. Substrate Recognition (Propeptide)

A. Propeptide binding site

VIII. Structure-Function Relationship

IX. Expression of GGCX During Development

X. Gla-Containing Proteins and γ-Carboxylase in Urochordate

XI. Drosophila γ-Glutamyl Carboxylase

XII. γ-Carboxylated Peptides in Conus

XIII. Conus γ-Glutamyl Carboxylase

XIV. Future Prospects

Acknowledgements

References