J. W. Cornforth

Asymmetric Methyl Groups: Asymmetric Methyl Groups, and the Mechanism of Malate Synthase

The following two articles present a way of determining the chirality of doubly labelled methyl groups, and reveal that the condensation reaction catalysed by malate synthase takes place with inversion of configuration.

Studies on the Biosynthesis of Cholesterol. XIX. Steric Course of Hydrogen Eliminations and of C-C bond Formations in Squalene Biosynthesis

1. Three molecules of isopentenyl pyrophosphate are used in the biosynthesis of one molecule of farnesyl pyrophosphate. In this process, one hydrogen atom is added and three are eliminated : these three were all originally attached to C-4 of an earlier precursor, mevalonate. It is demonstrated that these eliminations are stereospecific and that all three occur in the same stereochemical sense. The absolute configuration of the eliminated hydrogen is defined. 2. In the association of three isopentenyl pyrophosphate molecules to form farnesyl pyrophosphate, two new C—C bonds are formed with expulsion of two pyrophosphate ions. It is demonstrated that both these processes occur with inversion of configuration at the pyrophosphate-bearing carbon atom. 3. In the biosynthesis of squalene from two molecules of farnesyl pyrophosphate, one farnesyl residue suffers exchange of one hydrogen atom and one new C—C bond is formed. It is demonstrated that this bond is formed with inversion of configuration at that farnesyl residue which does not exchange hydrogen.

Studies on the biosynthesis of cholesterol—7 : Rearrangement of methyl groups during enzymic cyclisation of squalene

Abstract The present state of knowledge of the biosynthesis of cholesterol is summarised. An experiment is described wherein a special application of the carbon isotope 13 C permits a decision between possible modes of rearrangement during the enzymic cyclisation of squalene.

Preparation of antituberculous polyoxyethylene ethers of homogeneous structure

Abstract Polyoxyethylene ethers of homogeneous structure have been prepared from a macrocyclic, tetrahydric phenol (“HOC”), and from cholesterol by reaction with monofunctional derivatives of polyethylene glycols. Many practical difficulties had to be overcome in the preparation of these compounds. The effect on experimental tuberculosis of some of these products was compared with that of corresponding polyoxyethylene ethers produced by random polymerization. Several other surface-active compounds were prepared in an exploration of the structural requirements for chemotherapeutic activity.

The stereochemistry of rubber biosynthesis

The stereochemistry of the formation of natural rubber and trans-trans-farnesyl pyrophosphate in latex has been studied in vitro using [2-14C-(4R)-4-3H1] and [2-14C-(4S)-4-3H1]mevalonates as substrates. The proton eliminated from C-2 of isopentenyl pyrophosphate during the formation of farnesyl pyrophosphate in latex has the same steric position as that released in the liver system, and is epimeric with that eliminated during the biosynthesis of rubber. Complete stereospecificity is exhibited by the enzymes in the two cases. In rubber biosynthesis isoprene residues are incorporated directly in an all-cis configuration without the intervention of trans structures.

Stereochemistry of isopentenyl pyrophosphate isomerase

Mevalonic acid stereospecifically labelled with tritium in the 2-position was incubated, in a deuterium oxide medium containing adenosine triphosphate, with a soluble enzyme preparation from pig liver; and so converted into farnesyl pyrophosphate. This was hydrolysed enzymically to farnesol which was oxidized, by ozone followed by sodium hypoiodite, to acetic acid originating from the terminal isopropylidene group of farnesol. The tritium in this acetic acid was found, on analysis by a recently developed enzymic method, to be present largely in chiral methyl groups the chirality of which was R or S according to the chirality at C-2 of the parent mevalonic acid. It is deduced that these chiral methyl groups were formed on the enzyme isopentenyl pyrophosphate isomerase, by addition of a deuteron from the medium to the 3 re, 4 re face of the double bond in isopentenyl pyrophosphate. The stereochemical relationship between the added and abstracted hydrogen in the prototropic isomerization mediated by this enzyme is thereby established. A preliminary communication of these results has been made (Clifford, Cornforth, Mallaby & Phillips 1971).

Isolation of sycamore dormin and its identity with abscisin II

Abstract A growth-inhibitory substance, present in the acidic fraction of extracts of sycamore leaves ( Acer pseudoplatanus L.) and associated with the induction of dormancy, has been isolated in crystalline form and showed to be identical with abscisin II, previously obtained from young cotton fruits.

Synthesis of 5-[18O]mevalonolactone

Abstract A preparation of RS -mevalonolactone containing over 90 at.% of 18 O attached to C-5 is described. The isotope was introduced by acid-catalysed hydrolysis of the dimethyl acetal of methyl mevaldate with H 2 18 O, followed by reduction in situ with sodium borohydride.