Squalene synthase <db_xref db="EC" dbkey="2.5.1.21"/> (farnesyl-diphosphate farnesyltransferase) (SQS) and Phytoene synthase <db_xref db="EC" dbkey="2.5.1.32"/> (PSY) share a number of functional similarities. These similarities are also reflected at the level of their primary structure [<cite idref="PUB00001836"/>, <cite idref="PUB00003686"/>, <cite idref="PUB00000230"/>]. In particular three well conserved regions are shared bySQS and PSY; they could be involved in substrate binding and/or the catalyticmechanism. <p>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:<reaction>2 FPP -> presqualene diphosphate + NADP -> squalene</reaction>SQS is found in eukaryotes. In yeast it is encoded by the ERG9 gene, in mammals by the FDFT1 gene. SQS seems to be membrane-bound. </p><p>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.<reaction>2 GGPP -> prephytoene diphosphate -> phytoene</reaction>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.</p><p>Both SQS and PSY share a number of functional similarities which are also reflected at the level of their primary structure. In particular, three well conserved regions are shared by SQS and PSY, which could be involved in substrate binding and/or the catalytic mechanism. This entry contains the second and third conserved regions located in the central part of these enzymes.</p>
Squalene/phytoene synthase, conserved site