Cyanophycinase

{{Short description|Class of enzymes}} {{more citations needed|date=May 2022}} {{Infobox enzyme | Name = Cyanophycinase | EC_number = 3.4.15.6 | CAS_number = 131554-16-0 | UniProt = P73832 | PDB = 3EN0 | Organism = Synechocystis sp. PCC 6803 | Symbol = cphB | GO_code = | image = Asymmetric Unit of Cyanophycinase.png | width = | caption = Asymmetric Unit of Cyanophycinase. PDB: 3EN0 Cyanophycinase consists of three identical chains. }} thumb|Biological assembly of cyanophycinase determined from the organism ''Synechocystis'' sp. PCC6803 thumb|The catalytic triad of Cyanophycinase is Ser 132, His 174, and Glu 201. Other conserved residues which form a pocket around the serine include: Gln101, Asp172, Gln173, Arg178, Arg180 and Arg183 '''Cyanophycinase''' ({{EC number|3.4.15.6}}, ''cyanophycin degrading enzyme'', ''beta-Asp-Arg hydrolysing enzyme'', ''CGPase'', ''CphB'', ''CphE'', ''cyanophycin granule polypeptidase'', ''extracellular CGPase'') is an enzyme.<ref>{{cite journal | vauthors = Obst M, Krug A, Luftmann H, Steinbüchel A | title = Degradation of cyanophycin by Sedimentibacter hongkongensis strain KI and Citrobacter amalonaticus strain G Isolated from an anaerobic bacterial consortium | journal = Applied and Environmental Microbiology | volume = 71 | issue = 7 | pages = 3642–52 | date = July 2005 | pmid = 16000772 | pmc = 1169039 | doi = 10.1128/aem.71.7.3642-3652.2005 | bibcode = 2005ApEnM..71.3642O }}</ref><ref>{{cite journal | vauthors = Obst M, Oppermann-Sanio FB, Luftmann H, Steinbüchel A | title = Isolation of cyanophycin-degrading bacteria, cloning and characterization of an extracellular cyanophycinase gene (cphE) from Pseudomonas anguilliseptica strain BI. The cphE gene from P. anguilliseptica BI encodes a cyanophycinhydrolyzing enzyme | journal = The Journal of Biological Chemistry | volume = 277 | issue = 28 | pages = 25096–105 | date = July 2002 | pmid = 11986309 | doi = 10.1074/jbc.m112267200 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Richter R, Hejazi M, Kraft R, Ziegler K, Lockau W | title = Cyanophycinase, a peptidase degrading the cyanobacterial reserve material multi-L-arginyl-poly-L-aspartic acid (cyanophycin): molecular cloning of the gene of Synechocystis sp. PCC 6803, expression in Escherichia coli, and biochemical characterization of the purified enzyme | journal = European Journal of Biochemistry | volume = 263 | issue = 1 | pages = 163–9 | date = July 1999 | pmid = 10429200 | doi = 10.1046/j.1432-1327.1999.00479.x | doi-access = free }}</ref> It catalyses the following chemical reaction

: [L-Asp(4-L-Arg)]n + H2O <math>\rightleftharpoons</math> [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 == 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.<ref name="auto">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</ref>

== Enzyme function == 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.<ref name="auto"/>

== Connection to nitrogen storage in Cyanobacteria == 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<ref>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</ref> 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 == {{Reflist}}

== External links == * {{MeshName|Cyanophycinase}}

Category:EC 3.4.15