Osteocalcin binds tightly to the gamma-glutamylcarboxylase at a site distinct from that of the other known vitamin K-dependent proteins

Vitamin K-dependent proteins contain a propeptide that is required for recognition by the enzyme gamma-glutamylcarboxylase. Substrates used in vitro for carboxylation studies lacking a prosequence are characterized by Km values in the millimolar range, whereas the Km for peptides containing a prosequence is three or four orders of magnitude smaller. Here we report that descarboxy-osteocalcin is an exception in this respect. With descarboxy-osteocalcin in purified propeptide-free recombinant carboxylase, the Km was 1.8 microM. Furthermore, osteocalcin was an inhibitor of descarboxy-osteocalcin carboxylation with a Ki of 76 microM. In contrast with the other vitamin K-dependent proteins, free propeptides do not inhibit descarboxy-osteocalcin carboxylation. Moreover, propeptide-containing substrates were inhibited neither by osteocalcin nor by its propeptide. From our studies we conclude that descarboxy-osteocalcin must have an internal recognition sequence that binds to gamma-glutamylcarboxylase at a site different from the propeptide-recognition site.

Role of the propeptide and gamma-glutamic acid domain of factor IX for in vitro carboxylation by the vitamin K-dependent carboxylase

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the processive carboxylation of specific glutamates in a number of proteins related to blood coagulation and bone. To address the independent importance of the propeptide, gamma-carboxyglutamic acid (Gla) domain and elements beyond the Gla domain of factor IX in vitamin K-dependent carboxylation, we have examined the kinetics of carboxylation of peptides containing (1) propeptide and Gla domain, (2) the Gla domain alone, (3) uncarboxylated bone Gla protein, (4) propeptide followed by the entire uncarboxylated factor IX molecule, and (5) the factor IX propeptide followed by a non-Gla domain sequence. Our studies indicate that peptides with a covalently linked propeptide have Km values similar to the physiological substrate of the carboxylase. In contrast, the Gla domain of factor IX has a >/=230-fold higher Km for the carboxylase than the corresponding peptide with a covalently linked propeptide. This contrasts with bone Gla protein, another vitamin K-dependent protein, which appears not to require a covalently linked propeptide for high-affinity binding to the carboxylase. Analysis of the carboxylation products of a propeptide/non-Gla domain substrate indicate that it is carboxylated multiple times in a processive manner. These studies show that the perceived binding affinity of the carboxylase substrate and processivity is conferred by the propeptide without requiring the conserved Gla domain sequences and that factor IX and bone Gla protein may have distinct mechanisms of interacting with the carboxylase.