Chem*4520 Metabolic Processes Fall Semester 2000 Modified August 2000
schematic view of the enzyme citrate synthase, with bound acetyl-CoA analog in green	Department of Chemistry and Biochemistry Home Page Back to Chem*4520 Home Page

Cytoplasmic hydroxymethylglutaryl-CoA is reduced to mevalonate.

This enzyme carries out a two step reduction involving two moles of NADPH, yielding the C₆compound, mevalonate. The initial step goes from carboxylate to aldehyde, and would normally be energetically unfavourable. However, the reduction is likely to pass through a thiohemiacetal intermediate, either to CoA itself or to an enzyme thiol (-SEnz ?), by analogy to the partly similar interconversion of 1,3-bisphosphoglycerate and glyceraldehyde-3-phosphate in the glycolysis/gluconeogenesis sequence.

The second step reduction from aldehyde to primary alcohol should be energetically favourable. The intermediate mevaldate is tightly bound. The enzyme accepts exogenous mevaldate as a substrate for a one-step reduction to mevalonate, but won't release the mevaldate produced internally.

Mevalonate is the key precursor for synthesis of cholesterol and related isoprenoid compounds.

Isoprenoids are compounds which appear to be formed by polymerization of multiple units of isoprene, C5H8. Monoterpenoids (C10) such as limonene, C10H16 are made from two isoprene units, and sesquiterpenoids (C15) from three units. These compounds are relatively volatile plant products which are important flavour and aroma components of food. Cyclic diterpenoids (C20) and triterpenoids (C10) are also widely distributed. Triterpenoids are the basis for compounds such as cholesterol or diosgenin although the loss of three methyl groups makes the isoprene origin less obvious. While higher plants have the greatest variety of isoprenoids, cholesterol and related sterols are important components of biological membranes. Cholesterol is also the precursor of the steroid hormones in animals.

Although isoprenoids have been known chemically for over a century, the biosynthetic origin was a mystery until the discovery of mevalonate as an essential growth factor for certain strains of lactobacilli.

Mevalonate is activated by a series of phosphorylation steps:

(1) mevalonate kinase

(2) 5-phosphomevalonate kinase.

(3) 5-pyrophosphomevalonate decarboxylase.

(4) isopentenyl pyrophosphate isomerase.

Prenyl transferase links up isoprenyl units by condensation-elimination of pyrophosphate

Prenyl transferase starts with one unit of dimethylallyl pyrophosphate, and aligns the pyrophosphate (head end) with the terminal double bond of isopentenyl pyrophosphate (tail end). Elimination of the pyrophosphate ( a good leaving group) creates a carbocation, which then reacts with isopentenyl double bond; a basic group in the enzyme then removes a proton from C-2 of the isopentyl.

The first condensation generates the C₁₀ product geranyl pyrophosphate, which is the precursor of monoterpenoids in plants. Additional isopentenyl pyrophosphate units can join by this head to tailcondensation, elongating the chain by one C₅ prenyl unit each time. Sterols such as cholesterol are derived from the C₁₅ product farnesyl pyrophosphate.

However longer polyprenyl chains may have special functions in cells, e.g. the membrane anchoring side chains of ubiquinone and the transmembrane oligosaccharide transfer factor dolichol pyrophosphate.

Sterols and most triterpenoids are made by head to head condensation of two units of farnesyl pyrophosphate.

Two units of farnesyl pyrophosphate are linked together in head to head mode condensation in a two step reaction catalysed by squalene synthase.

In the first step, elimination of PPi produces a carbocation. The carbocation accepts electrons from the double bond in the adjacent farnesyl pyrophosphate, producing a cyclopropane intermediate, presqualene pyrophosphate.

The second step begins by eliminating the second PPi, causing the cyclopropane ring to shift to the terminal CH₂. The ring is then opened by hydride transfer from NADPH to give the end producct, squalene.

Squalene is formed by head to tail condensation of three isoprene units, follwed by head to head linking to yield the C₃₀ product.

Department of Chemistry and Biochemistry Home Page

Web-related comments can be sent to Uwe Oehler at DrIguana@chembio.uoguelph.ca