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

William R. Nes 1, P. A. Govinda Malya 1, Frank B. Mallory 2, Karen A. Ferguson 2, Josephine R. Landrey 2, and Robert L. Conner 2

From the 1 From the Department of Biological Sciences, Drexel University, Philadelphia, Pennsylvania 19104
2 From the Departments of Chemistry and Biology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010

Both ergosta-5,24(28)-dien-3ß-ol and stigmasta-5,trans-24(28)-dien-3ß-ol were metabolized in Tetrahymena pyriformis to give the respective Dgr5,7,22,24(28)-tetraenes, vis. ergosta-5,7,trans-22,24(28)-tetraen-3ß-ol and stigmasta-5,7,-trans-22,trans-24(28)-tetraen-3ß-ol. While stigmasta-5, trans-24(28)-dien-3ß-ol yielded some triene (stigmasta-5,7, trans-24(28)-trien-3ß-ol), stigmasta-5,cis-24(28)-dien-3ß-ol was converted almost entirely to the triene (stigmasta-5,7,-cis-24(28)-trien-3ß-o1). No dealkylation at C-24 occurred. The results are interpreted to mean that, in the enzyme-substrate complex for the introduction of a Dgr22 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 Dgr7 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 Dgr7-oxidoreductase.

Submitted on July 27, 1970


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