Squalene/phytoene synthase family
Squalene synthase EC (farnesyl-diphosphate farnesyltransferase) (SQS) and Phytoene synthase EC (PSY) are enzymes that share a number of functional similarities. These similarities are also reflected in their primary structure.[1][2][3] In particular three well conserved regions are shared by SQS and PSY; they could be involved in substrate binding and/or the catalytic mechanism. SQS catalyzes the conversion of two molecules of farnesyl diphosphate (FPP) into squalene. It is the first committed step in the cholesterol biosynthetic pathway. The reaction carried out by SQS is catalyzed in two separate steps: the first is a head-to-head condensation of the two molecules of FPP to form presqualene diphosphate; this intermediate is then rearranged in a NADP-dependent reduction, to form squalene:
| SQS_PSY | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | SQS_PSY | ||||||||
| Pfam | PF00494 | ||||||||
| InterPro | IPR002060 | ||||||||
| PROSITE | PDOC00802 | ||||||||
| SCOPe | 1ezf / SUPFAM | ||||||||
| OPM superfamily | 297 | ||||||||
| OPM protein | 3v66 | ||||||||
| Membranome | 635 | ||||||||
| |||||||||
2 FPP -> presqualene diphosphate + NADP -> squalene
SQS is found in eukaryotes. In yeast it is encoded by the ERG9 gene, in mammals by the FDFT1 gene. SQS is membrane-bound.
PSY catalyzes the conversion of two molecules of geranylgeranyl diphosphate (GGPP) into phytoene. It is the second step in the biosynthesis of carotenoids from isopentenyl diphosphate. The reaction carried out by PSY is catalyzed in two separate steps: the first is a head-to-head condensation of the two molecules of GGPP to form prephytoene diphosphate; this intermediate is then rearranged to form phytoene.
2 GGPP -> prephytoene diphosphate -> phytoene
PSY is found in all organisms that synthesize carotenoids: plants and photosynthetic bacteria as well as some non-photosynthetic bacteria and fungi. In bacteria PSY is encoded by the gene crtB. In plants PSY is localized in the chloroplast.
References
- Summers C, Karst F, Charles AD (December 1993). "Cloning, expression and characterisation of the cDNA encoding human hepatic squalene synthase, and its relationship to phytoene synthase". Gene. 136 (1-2Che): 185–92. doi:10.1016/0378-1119(93)90462-c. PMID 8294001.
- Robinson GW, Tsay YH, Kienzle BK, Smith-Monroy CA, Bishop RW (May 1993). "Conservation between human and fungal squalene synthetases: similarities in structure, function, and regulation". Mol. Cell. Biol. 13 (5): 2706–17. PMC 359645. PMID 8474436.
- Ramer S, Hugueney P, Bouvier F, Camara B, Kuntz M (November 1993). "Expression of the genes encoding the early carotenoid biosynthetic enzymes in Capsicum annuum". Biochem. Biophys. Res. Commun. 196 (3): 1414–21. doi:10.1006/bbrc.1993.2410. PMID 8250898.