Mevalonic acid is the primary precursor of an the terpenoids and steroids biosynthesised by plants. It is derived from acetyl CoA through the intermediate formation of acetoacetyl CoA and 3-hydroxy-3-methylglutaryl CoA (HMG CoA), these reactions being catalysed by acetyl CoA acetyltransferase and HMG CoA synthase, respectively. Reduction of HMG CoA, catalysed by HMG CoA reductase, gives mevalonic acid.
Before the pathway can continue, mevalonic acid must be catalytically phosphorylated by ATP and mevalonic acid kinase. Mevalonic acid kinase activity has been detected in many plants and has been found to be inhibited by such products of the acetate-mevalonate pathway as geranyl, farnesyl, geranylgeranyl and phytyl pyrophosphates. Thus, phosphorylation of mevalonic acid is a primary point at which control of terpenoid and steroid biosynthesis operates. Phosphorylation leads first to the mono- and then to the pyrophosphate. The second phosphorylation is catalysed by phosphomevalonate kinase.
