Hydrolysis of bismetallic complexes of linalyldiphosphate and their participation in the biosynthesis of cyclic monoterpenes

Abstract

The allylic diphosphate ester linalyldiphosphate forms mono- and bis-metallic complexes with Mg²⁺ and Mn²⁺. Complexes of the trianion of the ligand are 50 to 100 times more stable than those of the dianion. Mg²⁺ and Mn²⁺ are bound in the monometallic complex 10³ and 10⁴ times more strongly than in the bismetallic species.

The rate constant of the uncatalysed hydrolysis of linalyldiphosphate at pH 7.0 is 1.2 × 10⁴s^–1, which is higher than reported values for the primary diphosphates. Mg²⁺ and Mn²⁺ affect its rate of hydrolysis and it is shown that the rate enhancement observed is a function of the concentration of the bismetallic complexes.

The rate of cyclic terpene hydrocarbon biosynthesis catalysed by carbocyclase from Citrus limonum also correlates with the concentration of the bismetallic complexes.

Linalyl monophosphate, which is not a substrate for carbocyclase hydrolyses at a rate proportional to the concentration of the monometallic complex of Mn²⁺. The neutral charge of this complex, as opposed to the positively charged bismetallic complex of linalyl diphosphate may be an explanation of the absence of utilization of this complex as a substrate.

It appears from the data presented and from previous evidence that bismetallic complexes of allylic diphosphates are the most reactive species in their hydrolysis and the only reactive species in the enzymic cyclization of these precursors of monoterpenes.