Editing Tacrolimus (section)

==Biosynthesis==

[[File:Tacrolimus biosynthesis part 1.tif|class=skin-invert-image|300px]]
[[File:Tacrolimus biosynthesis part 2.tif|class=skin-invert-image|300px]]
[[File:Tacrolimus biosynthesis part 3..tif|class=skin-invert-image|300px]]

The biosynthesis of tacrolimus is hybrid synthesis of both type 1 [[polyketide synthases]] (PKS 1) and [[nonribosomal peptide]] syntheses (NRPS). The research shows the hybrid synthesis consists of ten modules of type 1 polyketide synthase and one module of nonribosomal peptide synthase. The synthetic enzymes for tacrolimus are found in 19 gene clusters named fkb. The 19 genes are fkbQ, fkbN, fkbM, fkbD, fkbA, fkbP, fkbO, fkbB, fkbC, fkbL, fkbK, fkbJ, fkbI, fkbH, fkbG, allD, allR, allK and allA.<ref name = "Ordóñez-Robles_2018">{{cite journal | vauthors = Ordóñez-Robles M, Santos-Beneit F, Martín JF | title = Unraveling Nutritional Regulation of Tacrolimus Biosynthesis in ''Streptomyces tsukubaensis'' through ''omic'' Approaches | journal = Antibiotics | volume = 7 | issue = 2 | pages = 39 | date = May 2018 | pmid = 29724001 | pmc = 6022917 | doi = 10.3390/antibiotics7020039 | doi-access = free }}</ref>

There are several possible ways of biosynthesis of tacrolimus. The fundamental units for biosynthesis are following: one molecule of 4,5-dihydroxycyclohex-1-enecarboxylic acid (DHCHC) as a starter unit, four molecules of malonyl-CoA, five molecules of methylmalonyl-CoA, one molecule of allylmalonyl-CoA as elongation units. However, two molecules of malonyl-CoA are able to be replaced by two molecules of methoxymalonyl CoA. Once two malonyl-CoA molecules are replaced, post-synthase tailoring steps are no longer required where two methoxymalonyl CoA molecules are substituted. The biosynthesis of methoxymalonyl CoA to Acyl Carrier protein is proceeded by five enzymes (fkbG, fkbH, fkbI, fkbJ, and fkbK). Allylmalonyl-CoA is also able to be replaced by propionylmalonyl-CoA.<ref name = "Ordóñez-Robles_2018" />

The starter unit, DHCHC from the [[chorismic acid]] is formed by fkbO enzyme and loaded onto CoA-ligase domain (CoL). Then, it proceeds to NADPH dependent reduction(ER). Three enzymes, fkbA,B,C enforce processes from the loading module to the module 10, the last step of PKS 1. fkbB enzyme is responsible of allylmalonyl-CoA synthesis or possibly propionylmalonyl-CoA at C21, which it is an unusual step of general PKS 1. As mentioned, if two methoxymalonyl CoA molecules are substituted for two malonyl-CoA molecules, they will take place in module 7 and 8 (C13 and C15), and fkbA enzyme will enforce this process. After the last step (module 10) of PKS 1, one molecule of <small>L</small>-[[pipecolic acid]] formed from <small>L</small>-[[lysine]] and catalyzed through fkbL enzyme synthesizes with the molecule from the module 10. The process of <small>L</small>-pipecolic acid synthesis is NRPS enforced by fkbP enzyme. After synthesizing the entire subunits, the molecule is cyclized. After the cyclization, the pre-tacrolimus molecule goes through the post-synthase tailoring steps such as oxidation and [[S-Adenosyl methionine|''S''-adenosyl methionine]]. Particularly fkbM enzyme is responsible of alcohol methylation targeting the alcohol of DHCHC starter unit (Carbon number 31 depicted in brown), and fkbD enzyme is responsible of C9 (depicted in green). After these tailoring steps, the tacrolimus molecule becomes biologically active.<ref name = "Ordóñez-Robles_2018" /><ref name="Chen_2013">{{cite journal | vauthors = Chen D, Zhang L, Pang B, Chen J, Xu Z, Abe I, Liu W | title = FK506 maturation involves a cytochrome p450 protein-catalyzed four-electron C-9 oxidation in parallel with a C-31 O-methylation | journal = Journal of Bacteriology | volume = 195 | issue = 9 | pages = 1931–1939 | date = May 2013 | pmid = 23435975 | pmc = 3624582 | doi = 10.1128/JB.00033-13 }}</ref><ref name="Mo_2009">{{cite journal | vauthors = Mo S, Ban YH, Park JW, Yoo YJ, Yoon YJ | title = Enhanced FK506 production in Streptomyces clavuligerus CKD1119 by engineering the supply of methylmalonyl-CoA precursor | journal = Journal of Industrial Microbiology & Biotechnology | volume = 36 | issue = 12 | pages = 1473–1482 | date = December 2009 | pmid = 19756799 | doi = 10.1007/s10295-009-0635-7 | s2cid = 32967249 | doi-access = free }}</ref>