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In enzymology, a Δ-sterol 5(6)-desaturase (EC 1.14.19.20) is an enzyme that catalyzes the chemical reaction

Δ-sterol + 2 ferrocytochrome b₅ + O₂ + 2 H = Δ-sterol + 2 ferricytochrome b₅ + 2 H₂O

The four substrates of this enzyme are Δ-sterol, ferrocytochrome b₅, H, and O₂. Its three products are Δ-sterol, ferricytochrome b₅, and H₂O.

Classification

This enzyme is one of C-5 sterol desaturases, belongs to the family of oxidoreductases, specifically those acting on paired donors, with O₂ as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O₂. With oxidation of a pair of donors resulting in the reduction of molecular oxygen to two molecules of water.

Nomenclature

The systematic name of this enzyme class is Δ-sterol,ferrocytochrome b5:oxygen oxidoreductase 5,6-dehydrogenating. Other names in common use include:

• lathosterol oxidase
• Δ-sterol Δ-dehydrogenase
• Δ-sterol 5-desaturase
• Δ-sterol-C5(6)-desaturase
• 5-DES

Gene names:

• SC5D (vertebrates)
• ERG3 (yeast)

Biological role

This enzyme participates in biosynthesis of steroids.

History

Previously, this enzyme was known under the name lathosterol oxidase and (EC 1.14.21.6), and the entry is now transferred. The following incorrect reaction was suggested:

5α-cholest-7-en-3β-ol + NAD(P)H + H + O₂ ${\displaystyle \rightleftharpoons }$ cholesta-5,7-dien-3β-ol + NAD(P)+ + 2 H₂O

The substrates of this enzyme were listed as 5α-cholest-7-en-3β-ol, NAD(P)H, H, and O₂. Its products were listed as cholesta-5,7-dien-3β-ol (provitamin D₃), NAD or NADP, and H₂O. It was stated that the enzyme has two cofactors: FAD, and FMN.

See also

• Fatty acid desaturase
• C-5 sterol desaturase

References

• Dempsey ME, Seaton JD, Schroepfer GJ, Trockman RW (1964). "The Intermediary Role of Δ-cholestadien-3-β-ol in Cholesterol Biosynthesis". J. Biol. Chem. 239: 1381–7. PMID 14189869.
• Nishino H, Nakaya J, Nishi S, Kurosawa T, Ishibashi T (1997). "Temperature-induced differential kinetic properties between an initial burst and the following steady state in membrane-bound enzymes: studies on lathosterol 5-desaturase". Arch. Biochem. Biophys. 339 (2): 298–304. doi:10.1006/abbi.1996.9871. PMID 9056262.
• Taton M, Rahier A (1996). "Plant sterol biosynthesis: identification and characterization of higher plant Δ-sterol C5(6)-desaturase". Arch. Biochem. Biophys. 325 (2): 279–88. doi:10.1006/abbi.1996.0035. PMID 8561508.
• Taton M, Husselstein T, Benveniste P, Rahier A (2000). "Role of highly conserved residues in the reaction catalyzed by recombinant Δ-sterol-C5(6)-desaturase studied by site-directed mutagenesis". Biochemistry. 39 (4): 701–11. doi:10.1021/bi991467t. PMID 10651635.

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