Tyrosine phenol-lyase (EC 4.1.99.2) from Citrobacter freundii has been cloned and the primary sequence deduced from the DNA sequence. From the BrCN digest of the NaBH4-reduced holoenzyme, five peptides were purified and sequenced. The amino acid sequences of the peptides agreed with the corresponding parts of the tyrosine phenol-lyase sequence obtained from the gene structure. K257 is the pyridoxal 5'-phosphate binding residue. Assisted by the sequence data, the crystal structure of apotyrosine phenol-lyase, a pyridoxal 5'-phosphate-dependent enzyme, has been refined to an R-factor of 16.2% at 2.3-%L resolution using synchrotron radiation diffraction data. The tetrameric molecule has 222 symmetry, with one of the axes coincident with the cr stallographic 2-fold symmetry axis of the crystal which belongs to the space group P2,212 with a = 76.0 l, b = 138.3 A, and c = 93.5 A. Each subunit comprises 14 a-helices and 16 &strands, which fold into a small and a large domain. The coenzyme-binding lysine residue is located at the interface between the large and small domains of one subunit and the large domain of a crystallographically related subunit. The fold of the large, pyridoxal 5'-phosphate binding domain and the location of the active site are similar to that found in aminotransferases. Most of the residues which participate in binding of pyridoxal 5'-phosphate in aminotransferases are conserved in the structure of tyrosine phenol-lyase. Two dimers of tyrosine phenol-lyase, each of which has a domain architecture similar to that found in aspartate aminotransferases, are bound together through a hydrophobic cluster in the center of the molecule and intertwined N-terminal arms.

We previously detected in salivary gland secretions of the medicinal leech (Hirudo medicinalis) a novel enzymatic activity, endo-epsilon(gamma-Glu)-Lys isopeptidase, which cleaves isopeptide bonds formed by transglutaminase (Factor XIIIa) between glutamine gamma-carboxamide and the epsilon-amino group of lysine. Such isopeptide bonds, either within or between protein polypeptide chains are formed in many biological processes. However, before we started our work no enzymes were known to be capable of specifically splitting isopeptide bonds in proteins. The isopeptidase activity we detected was specific for isopeptide bonds. The enzyme was termed destabilase. Here we report the first purification of destabilase, part of its amino acid sequence isolation and sequencing of two related cDNAs derived from the gene family that encodes destabilase proteins, and the detection of isopeptidase activity encoded by one of these cDNAs cloned in a baculovirus expression vector. The deduced mature protein products of these cDNAs contain 115 and 116 amino acid residues, including 14 highly conserved Cys residues, and are formed from precursors containing specific leader peptides. No homologous sequences were found in public databases.

Earlier we detected a novel enzymatic activity in salivary gland secretion of the medicinal leech, splitting isopeptide bonds between the glutamine T-carboxamide and lysine E-amino group. This activity is due to destabilase. We described its partial amino acid sequence and sequences of two closely related cDNAs, but none of them perfectly matched the protein isolated. Here we report the isolation and sequence peculiarities of the third cDNA of the family as well as the complete sequence of the destabilase protein. The inferred mature protein product of this cDNA matches the independently determined destabilase protein sequence. It contains 115 amino acid residues including 14 highly conserved Cys residues and is formed from a precursor containing specific leader peptide.

No abstract

Destabilase, endo-epsilon-(gamma-Glu)-Lys isopeptidase from the medicinal leech, inhibits arterial thrombus formation in rats. Inhibition of platelet aggregation was supposed to be one of the main mechanisms of this phenomenon. To elucidate this question highly purified destabilase preparations were used. Aggregation was monitored both by a turbidometric method and by a method based on real-time estimation of mean aggregate size. Spontaneous aggregation of human platelets was completely blocked by destabilase. At 5 microM ADP maximal inhibition was 63%. Aggregation induced by PAF (100 nM) and collagen (0.1 mg/ml) was inhibited in the presence of destabilase by 50 and 65%, respectively. This enzyme does not activate adenylate cyclase but inhibits it. We suggest that destabilase interacts with high-affinity binding sites on the platelet plasma membrane, thus providing an anti-aggregating effect. This idea coincides with the data that destabilase primary structure has high homology with some adhesive proteins.