Osteocalcin binds tightly to the γ-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 K-m values in the millimolar range, whereas the K-m 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 K-m was 1.8 mu M. Furthermore, osteocalcin was an inhibitor of descarboxy-osteocalcin carboxylation with a K-i of 76 mu M. 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.