稍作改动：
the primary structure of XG contains pentameric repeating units which are formed by two D-glucose,two D-mannose and one D-glucuronic acid
或者
the primary structure of XG contains pentameric repeating units formed by two D-glucose,two D-mannose and one D-glucuronic acid
 
repeat units 是专业术语吗?如果不是,建议改成上面建议的形式
units which are formed 中的时态无需过去时.which are 可以省略