β-HYDROXY ASPARTIC ACID AND ft-HYDROXYASPARAGINE IN THEEGF-HOMOLOGY REGIONS OF PROTEIN C AND PROTEINS
Authors: Stenflo J , öhlin A-K , Lundvall Å , Dahlback B 
Published: August 2018

The amino acid sequence has been determined for all of the vitamin K-dependent proteins and the gene structure is known for some of them. These findings have shown the proteins to consist of four clearly discernible domains, except protein S which has six domains. The protein domains seem to be coded on separate exons (Foster, D. C. et. al. 1985 Proc. Natl. Acad. Sci. USA 82,4673).
The vitamin K-dependent γ-carboxyglutamic acid (Gla) containing domain isthe common structural denominator of the members of this protein family. In addition, all of these proteins except prothrombin contain domains that are homologous to the precursor of the epidermal growth factor (EGF). Such domains arealso found in proteins that are not vitamin K-dependent, such as the low density lipoprotein receptor, thrombomodulin, factor XII, plasminogen, the tissue type plasminogen activator, urokinase and the complement protein Clr. The vitamin K-dependent proteins can be dividedinto three groups. Factors VII, IX, X, protein C and protein Z form one group, which in addition to the Gla-region have two EGF-homology regions and one domain that is homologous to the serine proteases. Prothrombin has two 'kringle' structures and a serine protease domain and constitutes a group of its own. Protein S is also unique in that it has four EGF-homology regions and a COOH-terminal region that is homologous to the sexual hormone binding globulin (see poster by Edenbrand et. al.).

Recently a posttranslationally modified amino acid, B-hydroxyaspatic acid (Hya), was identified in position 71 in the NH2-terminal EGF-homology region ofbovine protein C. The amino acid is formed by hydroxylation of aspartic acid. It has also been identified in the corresponding positions in factors VII, IX,X and protein Z (i. e. proteins which like protein C have two EGF-homology regions each). In protein S the N2-terminal of four EGF-homology regions has hydroxy lated aspartic acid .whereas the following three EGF-like domains have B-hydroxyasparagine. The nucleotide sequence codes for asparagine in the three latter positions. Neither vitamin K nor vitamin C seem to be involvedin the formation of the two hydroxylated amino acids. Recently, Hya was identified in acid hydrolysates of the complement protein Clr. Hya and Hyn have onlybeen found in domains that are homologous to the EGF precursor. In an attempt to identify the structural requirement of the hydroxylating enzyme, we have compared the sequences of EGF-homology regions that contain Hya or Hyn with the corresponding sequences that have been shown not to contain the modified amino acids. The domains that have Hya or Hyn have the consensus sequence Cx xxxx xCxC. This sequence has been found in three EGF-like domains in the EGF-precursor, in two in the LDL-receptor and in two in thrombomodulin. Furthermore, the neurogenic Notch locus in Drosophila melanogaster codes for 36 EGF-homolgy regions, 22 of which contain the consensussequence, whereas the Lin-12 locus in Caenorhabditis elegans codes for at least 11 EGF-like repeats, two of which comply with the consensus sequence. Whether any of these proteins contain Hya orHyn is not yet known with certainty.

It has been hypothesized that Hya isinvolved in the Gla independent Ca2+binding of factors IX, X and protein C. In an attempt to resolve this issue, we have isolated the EGF-homology region from human protein C and been able to demonstrate that it binds Ca2+ (see poster by öhlin and Stenflo). However, we do not yet know whether Hya is directly involved in the Ca2+binding.