Different action patterns for apple pectin methylesterase at pH 7.0 and 4.5

The mechanism of action of purified apple pectin methylesterase on pectin ( degree of methoxylation: DM 75) and methoxylated homogalacturonans (DM 70 a nd 90) was studied at pH 7.0 (optimal pH of the enzyme) and at pH 4.5 (clos e to the pH of apple juice). Different interchain distributions of the free carboxyl groups were obtained at pH 7.0 and 4.5: high-performance ion exch ange chromatography indicated a typical single chain mechanism at pH 7.0, b ut a mechanism differing from the single and multiple chain ones at pH 4.5. However, the same intrachain distribution of the newly demethoxylated gala cturonic acid residues was observed for both pHs by H-1 NMR. The high conte nt of consecutive de-esterified or consecutive esterified galacturonic acid residues suggested that apple PME acted with a multiple attack mechanism o n the pectic substrate. The degree of multiple attack of the enzyme was gre ater than or equal to 10-11. (C) 2000 Elsevier Science Ltd. All rights rese rved.