Ishii T, Matsunaga T, Pellerin P, O'Neill M A, Darvill A, Albersheim P
Forestry and Forest Products Research Institute, Tsukuba Norin Kenkyu, Danchi-Nai, Ibaraki 305-8687, Japan.
J Biol Chem. 1999 May 7;274(19):13098-104. doi: 10.1074/jbc.274.19.13098.
The location of the 1:2 borate-diol ester cross-link in the dimer of the plant cell wall polysaccharide rhamnogalacturonan II (RG-II) has been determined. The ester cross-links the apiofuranosyl residue of the 2-O-methyl-D-xylose-containing side chains in each of the subunits of the dimer. The apiofuranosyl residue in each of the two aceric acid-containing side chains is not esterified. The site of borate esterification is identical in naturally occurring and in in vitro synthesized dimer. Pb2+, La3+, and Ca2+ increase dimer formation in vitro in a concentration- and pH-dependent manner. Pb2+ is the most effective cation. The dimer accounts for 55% of the RG-II when the monomer (0.5 mM) is treated for 5 min at pH 3.5 with boric acid (1 mM) and Pb2+ (0.5 mM); at pH 5 the rate of conversion is somewhat slower. Hg2+ does not increase the rate of dimer formation. A cation's charge density and its ability to form a coordination complex with RG-II, in addition to steric factors, may regulate the rate and stability of dimer formation in vitro. Our data provide evidence that the structure of RG-II itself determines which apiofuranosyl residues are esterified with borate and that in the presence of boric acid and certain cations, two RG-II monomers self-assemble to form a dimer.
植物细胞壁多糖鼠李糖半乳糖醛酸聚糖II（RG-II）二聚体中1:2硼酸 - 二醇酯交联的位置已被确定。该酯使二聚体各亚基中含2 - O - 甲基 - D - 木糖侧链的阿吡呋喃糖基残基交联。两条含乙酰基酸侧链中的每个阿吡呋喃糖基残基未被酯化。硼酸酯化位点在天然存在的和体外合成的二聚体中是相同的。Pb2 +、La3 +和Ca2 +在体外以浓度和pH依赖性方式增加二聚体形成。Pb2 +是最有效的阳离子。当单体（0.5 mM）在pH 3.5下用硼酸（1 mM）和Pb2 +（0.5 mM）处理5分钟时，二聚体占RG-II的55%；在pH 5时转化速率稍慢。Hg2 +不会增加二聚体形成速率。除空间因素外，阳离子的电荷密度及其与RG-II形成配位络合物的能力可能调节体外二聚体形成的速率和稳定性。我们的数据提供了证据，证明RG-II自身的结构决定了哪些阿吡呋喃糖基残基被硼酸酯化，并且在硼酸和某些阳离子存在下，两个RG-II单体自组装形成二聚体。
