Cyanophycinase
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| Cyanophycinase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Asymmetric Unit of Cyanophycinase. PDB: 3EN0 Cyanophycinase consists of three identical chains. | |||||||||
| Identifiers | |||||||||
| EC no. | 3.4.15.6 | ||||||||
| CAS no. | 131554-16-0 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| |||||||||
Cyanophycinase (EC 3.4.15.6, cyanophycin degrading enzyme, beta-Asp-Arg hydrolysing enzyme, CGPase, CphB, CphE, cyanophycin granule polypeptidase, extracellular CGPase) is an enzyme.[1][2][3] It catalyses the following chemical reaction
- [L-Asp(4-L-Arg)]n + H2O [L-Asp(4-L-Arg)]n-1 + L-Asp(4-L-Arg)
The enzyme is highly specific for the branched polypeptide cyanophycin. It is similar to Dipeptidase E, another S51 family serine protease.
Structure
[edit | edit source]The asymmetric unit of cyanophycinase consists of three identical chains, each containing 291 residues. The structure of cyanophycinase was determined from the freshwater cyanobacterium Synechocystis sp. PCC 6803 at 1.5-A resolution, which showed that the structure is dimeric.[4]
Enzyme function
[edit | edit source]Cyanophycinase is a carboxy terminal specific exopeptidase, meaning it catalyzes the cleavage of the carboxy terminal peptide bond of cyanophycin. It was hypothesized that cyanophycinase was a serine protease due to extreme inhibition of the enzyme when used with known serine protease inhibitors, such as DMSO. Site directed mutagenesis experiments confirmed that the enzyme is a serine protease and suggested that Ser 132 is the primary catalytic residue. Other key residues for specificity include Gln101, Asp172, Gln173, Arg178, Arg180 and Arg183 which form a conserved pocket adjacent to Ser 132. Kinetic characterization of the enzyme demonstrates that the enzyme displays Michaelis–Menten kinetics with a kcat of 16.5 s−1 and a kcat/KM of 7.5 × 106 M−1 s−1.[4]
Connection to nitrogen storage in Cyanobacteria
[edit | edit source]Cyanophycin is highly resistant to degradation by all conventional proteases, and the only enzyme known to be capable of hydrolyzing it is cyanophycinase. Cyanophycin is a non-ribosomally synthesized peptidyl polymer that is used for nitrogen storage by cyanobacteria and other select eubacteria. Approximately 90% of cyanobacteria are diazotrophic, meaning that they can grow without an external source of fixed nitrogen. Diazotrophic growth[5] was severely impaired in bacteria with a mutated cyanophycinase gene, indicating that the inability to degrade cyanophycin is detrimental for the diazotrophic growth of the cyanobacterium, due to an excess of nitrogen storage.
References
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- ^ a b A.M. Law et al. "The structural basis of beta-peptide-specific cleavage by the serine protease cyanophycinase". J. Mol. Biol. (2009) https://doi.org/10.1016/j.jmb.2009.07.001
- ^ Picossi S, Valladares A, Flores E, Herrero A. "Nitrogen-regulated genes for the metabolism of cyanophycin, a bacterial nitrogen reserve polymer: expression and mutational analysis of two cyanophycin synthetase and cyanophycinase gene clusters in heterocyst-forming cyanobacterium Anabaena sp. PCC 7120" J Biol Chem. 2004 Mar 19;279(12):11582-92. doi: https://doi.org/10.1074/jbc.m311518200
External links
[edit | edit source]- Cyanophycinase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
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