The following text describes a protein:
Function:
Squalene synthase; part of the third module of ergosterol biosynthesis pathway that includes the late steps of the pathway (Probable). Erg9 produces squalene from 2 farnesyl pyrophosphate moieties (By similarity). 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, squalene is converted into lanosterol by the consecutive action of the squalene epoxidase erg1 and the lanosterol synthase erg7. Then, the delta(24)-sterol C-methyltransferase erg6 methylates lanosterol at C-24 to produce eburicol. Eburicol is the substrate of the sterol 14-alpha demethylase encoded by cyp51A and cyp51B, to yield 4,4,24-trimethyl ergosta-8,14,24(28)-trienol. The C-14 reductase erg24 then reduces the C14=C15 double bond which leads to 4,4-dimethylfecosterol. A sequence of further demethylations at C-4, involving the C-4 demethylation complex containing the C-4 methylsterol oxidases erg25A or erg25B, the sterol-4-alpha-carboxylate 3-dehydrogenase erg26 and the 3-keto-steroid reductase erg27, leads to the production of fecosterol via 4-methylfecosterol. The C-8 sterol isomerase erg2 then 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 erg3B then catalyzes the introduction of a C-5 double bond in the B ring to produce 5-dehydroepisterol. The 2 other sterol-C5-desaturases, erg3A and erg3C, seem to be less important in ergosterol biosynthesis. 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 reductases erg4A and erg4B to produce ergosterol. Possible alternative sterol biosynthetic pathways might exist from fecosterol to ergosterol, depending on the activities of the erg3 isoforms (Probable).
Miscellaneous:
MISCELLANEOUS: In Aspergillus, the biosynthesis pathway of the sterol precursors leading to the prevalent sterol ergosterol differs from yeast. The ring system of lanosterol in S.cerevisiae is firstly demethylised in three enzymatic steps leading to the intermediate zymosterol and secondly a methyl group is added to zymosterol by the sterol 24-C-methyltransferase to form fecosterol. In Aspergillus, lanosterol is firstly transmethylated by the sterol 24-C-methyltransferase leading to the intermediate eburicol and secondly demethylated in three steps to form fecosterol.
Subcellular Localization:
Endoplasmic reticulum membrane; Single-pass membrane protein. Microsome.
Sequence:
MGVAANAFYYIFHPSELRSILQWKIWHNPVHERDESNESETAKTCFKFLDLTSRSFSAVIKELHPELLLPMCIFYLTLRGLDTIEDDTSIPLETKEPLLRGFKDVLEQDGWNFTGNRPEEKDRELLVQFHNVITEFKRLKPAYKAVIKDITEKMGNGMADYCRKAALDDASVKTVEEYDLYCWYVAGLVGEGSTRLFVEAEFGNPALLKRTELYKSMGLFLQKTNIIRDVREDFDDGRQFWPKEIWSKHVTNFEDLFKPENREAALNCSSEMVLNALRHAEECLFYLAGLREQSVFNFCAIPQSMAIATLSLCFRNPAIFERNIKIKKGEACQLMMESTQNLRILYEAFRRYAREIHKKNTPKDPNFLEIGIACAKIEKFIETIFPSQSAEEANRRVLGQKSEAEQEKARLEAETRKDVLFMMALMGVIVVFVSIVMIGFAWYFGARFDLAWQELRKGNFRPPKHLRDREL