Structural insight into bicarbonate-mediated carboxylation by human vitamin K-dependent carboxylase.

Vitamin K-dependent (VKD) carboxylation, mediated by γ-glutamyl carboxylase (GGCX), is essential for the maturation of VKD proteins involved in critical physiological processes such as blood clotting, vascular calcification and bone metabolism. Here, we present cryo-electron microscopic structures of human GGCX alone and in complex with VKD proteins, vitamin K, and inhibitor anisindione. GGCX specifically recognizes diverse VKD substrates through high-affinity propeptide binding, while substrates like osteocalcin utilize a secondary exosite to enhance interaction. GGCX employs a conserved dipeptide anchoring mechanism that ensures processive carboxylation of glutamate residues. GGCX undergoes allosteric conformational changes that enable coordinated binding of vitamin K and glutamate substrates, facilitating the catalytic process. Additionally, we reveal a bicarbonate-mediated CO₂ capture mechanism that is conserved across bacterial and eukaryotic species, suggesting that this strategy for CO₂ utilization is both ancient and universal. Our findings lay the foundation for developing targeted anticoagulant drugs and innovative enzymatic CO₂ fixation strategies.