Membrane Protein Structure in the Vitamin K Cycle pp. 157-172
Authors: (Matthew C. Johnson, Ingeborg Schmidt-Krey, Georgia Institute of Technology, School of Biology, Atlanta, Georgia)
Abstract: Vitamin K-dependent -glutamyl carboxylase (GGCX) and vitamin K epoxide reductase (VKOR) are integral membrane proteins central to the vitamin K cycle. The post-translational modification of glutamate residues in vitamin K-dependent proteins to -carboxyglutamate by GGCX is coupled to the oxidation of vitamin K hydroquinone to vitamin K epoxide, which is subsequently reduced by VKOR. Structural data on these enzymes are of key importance in understanding the catalytic mechanisms involved, and are therefore applicable to GGCX and VKOR related genetic disorders, potential drug design, as well as understanding some of the difficulties in current coumarin-based therapy. To date, the greater part of structural information has been obtained by biochemical means, and additional data is emerging by crystallographic techniques. GGCX has been identified as an 87-kDa protein located within the ER membrane and in vitro translation/cotranslation determined that it consists of five predicted transmembrane domains, with the N-terminus on the cytoplasmic face, and the C-terminus in the lumen along with a large hydrophilic domain. The enzyme works in a processional manner, modifying multiple glutamate residues before releasing its substrate. Two-dimensional crystallization and electron crystallography has produced projection maps that identify GGCX as a monomer. VKOR is an 18 kDa ER membrane protein that is proposed to consist of three to four transmembrane domains, with a lumenal N-terminus, a large cytoplasmic loop, a small lumenal loop, and a cytoplasmic C-terminus. Multiple sequence alignments and site-directed mutagenesis have identified several well-conserved residues that are critical to VKOR activity, particularly a CXXC motif in the third putative transmembrane segment, which is believed to be the active site for the reduction of vitamin K epoxide. Combined, these findings contribute highly valuable information to the understanding of the GGCX and VKOR structures.