Function
Sterol-4-alpha-carboxylate 3-dehydrogenase; part of the third module of ergosterol biosynthesis pathway that includes the late steps of the pathway (PubMed:15995173, PubMed:9811880). ERG26 is a catalytic component of the C-4 demethylation complex that catalyzes the oxidative decarboxylation that results in a reduction of the 3-beta-hydroxy group at the C-3 carbon to an oxo group (PubMed:9811880). The third module or late pathway involves the ergosterol synthesis itself through consecutive reactions that mainly occur in the endoplasmic reticulum (ER) membrane. Firstly, the squalene synthase ERG9 catalyzes the condensation of 2 farnesyl pyrophosphate moieties to form squalene, which is the precursor of all steroids. Squalene synthase is crucial for balancing the incorporation of farnesyl diphosphate (FPP) into sterol and nonsterol isoprene synthesis. Secondly, the squalene epoxidase ERG1 catalyzes the stereospecific oxidation of squalene to (S)-2,3-epoxysqualene, which is considered to be a rate-limiting enzyme in steroid biosynthesis. Then, the lanosterol synthase ERG7 catalyzes the cyclization of (S)-2,3 oxidosqualene to lanosterol, a reaction that forms the sterol core. In the next steps, lanosterol is transformed to zymosterol through a complex process involving various demethylation, reduction and desaturation reactions. The lanosterol 14-alpha-demethylase ERG11 (also known as CYP51) catalyzes C14-demethylation of lanosterol to produce 4,4'-dimethyl cholesta-8,14,24-triene-3-beta-ol, which is critical for ergosterol biosynthesis. The C-14 reductase ERG24 reduces the C14=C15 double bond of 4,4-dimethyl-cholesta-8,14,24-trienol to produce 4,4-dimethyl-cholesta-8,24-dienol. 4,4-dimethyl-cholesta-8,24-dienol is substrate of the C-4 demethylation complex ERG25-ERG26-ERG27 in which ERG25 catalyzes the three-step monooxygenation required for the demethylation of 4,4-dimethyl and 4alpha-methylsterols, ERG26 catalyzes the oxidative decarboxylation that results in a reduction of the 3-beta-hydroxy group at the C-3 carbon to an oxo group, and ERG27 is responsible for the reduction of the keto group on the C-3. ERG28 has a role as a scaffold to help anchor ERG25, ERG26 and ERG27 to the endoplasmic reticulum and ERG29 regulates the activity of the iron-containing C4-methylsterol oxidase ERG25. Then, the sterol 24-C-methyltransferase ERG6 catalyzes the methyl transfer from S-adenosyl-methionine to the C-24 of zymosterol to form fecosterol. The C-8 sterol isomerase ERG2 catalyzes the reaction which results in unsaturation at C-7 in the B ring of sterols and thus converts fecosterol to episterol. The sterol-C5-desaturase ERG3 then catalyzes the introduction of a C-5 double bond in the B ring to produce 5-dehydroepisterol. The C-22 sterol desaturase ERG5 further converts 5-dehydroepisterol into ergosta-5,7,22,24(28)-tetraen-3beta-ol by forming the C-22(23) double bond in the sterol side chain. Finally, ergosta-5,7,22,24(28)-tetraen-3beta-ol is substrate of the C-24(28) sterol reductase ERG4 to produce ergosterol (PubMed:32679672)
Subunit
Heterotetramer of ERG25, ERG26, ERG27 and ERG28. ERG28 acts as a scaffold to tether ERG27 and other 4,4-demethylation-related enzymes, forming a demethylation enzyme complex, in the endoplasmic reticulum
