The stereochemistry of hydrogen elimination at C-7,C-22 and C-23 during the conversion of cholesterol (cholest-5-en-3β-ol) into cholesta-5,7,22-trien-3β-ol by Tetrahymena pyriformis

The stereochemistry of hydrogen elimination from C-7 during biosynthesis of ecdysones in insects and plants

“…CA, Table 6. 3H/14C ratios and specific radioactivities of ecdysone, isolated from P. vulgare administered with [4-14C,7f_-3H]cholesterol, and its subsequent chemical transformation products Substance Ecdysone 2,3-Acetonidecdysone 2, ,3#-Acetonide-14a-hydroxy-5fl-pregn-7-ene-6,20-dione formis (Zander & Caspi, 1970;Wilton & Akhtar, 1970;Bimpson et al, 1969) and during conversion of cholestanol into cholest-Sa-7-en-3f,-ol in a cockroach (Clayton & Edwards, 1963). Although formation of all these Al bonds involves removal of the cis orientated 7fl,8fl-hydrogens, the process does not neces-sarily have to be a cis dehydrogenation.…”

Identification of essential amino acid residues in a sterol 8,7-isomerase from Zea mays reveals functional homology and diversity with the isomerases of animal and fungal origin

“…Copying is not permitted, except with prior permission and as allowed by law. specificity [5,7]. Alanine scanning mutagenesis was used to identify residues of human EBP required for in vivo sterol 8,7-isomerization [24].…”

“…The mutated isomerases were assayed both in vivo by sterol analysis and quantification of 5,7 -sterols, and directly in vitro by examinating the activities of the recombinant mutated Zm8,7SI in the corresponding yeast microsomes extracts. It allowed particularly the identification of a number of amino acid residues essential for the plant 8,7SI activity.…”

“…It follows elimination of a proton at C7 accepted by a general base residue ( Figure 1) [4]. Loss of the 7 -H occurs in animals [5,6] and plants [7], while in fungi the enzyme proceeds with elimination of the 7 -H [7,8] ( Figure 1).…”

Regulation of the sequencing in sterol biosynthesis

“…On the assumption that the dehydrogenation proper, i.e., removal of the H-atoms, takes place faster than do conformational changes, we can infer from the configurational data that the side chain must assume the s-trans-conformation about the C-22,23 bond. From other data available on the stereochemistry of elimination (38), it becomes apparent that the two H-atoms leaving C-22 and C-23 must lie on the same side of the plane passing through C-20, C-22, C-23 and C-24. If this is true, substituents on C-24 (C-25 and C-28) must also lie in this plane if minimal interference is to occur in formation of the enzyme-substrate complex.…”