Conformational Analysis of the Enzyme-Substrate Complex in the Dehydrogenation of Sterols by Tetrahymena pyriformis

• Published in: The Journal of biological chemistry 1971-02, Vol.246 (3), p.561-568
• Main Authors: Nes, William R., Malya, P.A. Govinda, Mallory, Frank B., Ferguson, Karen A., Landrey, Josephine R., Conner, Robert L.
• Format: Article
• Language: English
• Subjects: Animals; Chemical Phenomena; Chemistry; Chromatography, Gas; Chromatography, Thin Layer
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)62451-2

Both ergosta-5,24(28)-dien-3β-ol and stigmasta-5,trans-24(28)-dien-3β-ol were metabolized in Tetrahymena pyriformis to give the respective Δ5,7,22,24(28)-tetraenes, vis. ergosta-5,7,trans-22,24(28)-tetraen-3β-ol and stigmasta-5,7,-trans-22,trans-24(28)-tetraen-3β-ol. While stigmasta-5, trans-24(28)-dien-3β-ol yielded some triene (stigmasta-5,7, trans-24(28)-trien-3β-ol), stigmasta-5,cis-24(28)-dien-3β-ol was converted almost entirely to the triene (stigmasta-5,7,-cis-24(28)-trien-3β-o1). No dealkylation at C-24 occurred. The results are interpreted to mean that, in the enzyme-substrate complex for the introduction of a Δ22 bond, the side chain assumes a conformation in which C-20, C-22, C-23, C-24, C-25, C-28, and (if present) C-29 lie in a plane, and one side of the plane is exposed to the enzyme with cis-elimination of 2 hydrogen atoms. Such a conformation is thermodynamically favorable with the 24-methylene and trans-24-ethylidene sterols, allowing 22,23-dehydrogenation to occur, but unfavorable with the cis-24-ethylidene sterol, thereby restricting 22,23-dehydrogenation. Since in all three cases the Δ7 bond was introduced, the capacity of the side chain to assume one or the other of these conformations appears to be irrelevant in the formation of a complex with the Δ7-oxidoreductase.