Biosynthesis of pectin, a plant cell wall polysaccharide

果胶是植物细胞壁多糖，主要由α-半乳糖醛酸（GalA）连接而成，具有调节细胞强度、细胞壁孔隙度、细胞间粘附和通讯等多种生理功能。以赤豆（Vigna angularis）7 d龄幼苗为材料，制备了一种含有半乳糖醛酸转移酶（GTase）的膜，该酶能<14>将UDP-[^C] GalA<14>转化为聚半乳糖醛酸（polyGalA）作为外源受体。该酶在pH 6.8-7.5和25-35℃、5 mM Mn^（2+）和0.5%（w/v）TritonX-100存在下具有最大活性。酸溶性低分子量聚合物（平均分子量为10，000）是一种比酸不溶性聚合物（分子量为70，000）更有效的受体底物。UDP-GalA和低分子量polyGalA的表观米氏常数分别为0.11mM和0.02mg/ml。不同甲酯化度的果胶均为弱受体，酶活性随甲酯化度的降低而降低。UDP-[^<14>C]GalA和低M_r polyGalA孵育<14>的转移产物在用内切多聚半乳糖醛酸酶消化后产生[^ C]GalA_2作为主要产物，证实GalA通过连续的α-1，4-键并入polyGalA。

Pectins are plant cell wall polysaccharides consisting mainly of contiguous unbranched (1→4)-linked α-galacturonic acid (GalA) residues, and participate in various physiological functions including regulation of cell strength, wall porocity, and intercellular adhesion and communication. A membrane preparation of 7-d-old seedlings from azuki bean (Vigna angularis) contained galacturonosyltransferase (GTase) capable of transferring [^<14>C] GalA from UDP-[^<14>C]GalA into polygalacturonic acid (polyGalA) as an exogenous acceptor. The enzyme was maximally active at pH 6.8-7.5 and 25-35℃ in the presence of 5 mM Mn^<2÷> and 0.5% (w/v) TritonX-100. Acid-soluble low-M_r (average M_r 10,000) polyGalA was a more efficient acceptor substrate than acid-insoluble polymer (M_r 70,000). The apparent Michaelis constants for UDP-GalA and low-M_r polyGalA were 0.11 mM and 0.02 mg/ml, respectively. Various pectins with different degrees of methyl-esterification (DE) were poor acceptors, and the enzyme activity tended to decrease with decreasing DE of the pectins. The transfer products from incubation of the enzyme with UDP-[^<14>C]GalA and the low-M_r polyGalA yielded [^<14>C]GalA_2 as the major product upon digestion with an endopolygalacturonase, confirming the incorporation of GalA into polyGalA through contiguous α-1,4-linkages.