Biochemical significance of the non-common amino acid sequences specifying the common functions of enzymes, polysaccharide lyases

多糖裂解酶是一种催化海藻酸盐、黄原胶、结冷胶和透明质酸等多糖解聚的酶，这些多糖都是由细菌产生的，并作为反应产物产生寡糖或单糖。该酶严格识别分散在多糖分子上的糖醛酸，并通过β消除反应在酸性位置裂解多糖。然而，这些酶在一级结构上没有相似之处，因此表明这些酶的功能是由不同的氨基酸序列决定的。为了阐明不同一级结构的酶具有相同的底物识别和β消除反应的原因，对鞘氨醇单胞菌的X射线晶体结构进行了研究。测定了A1藻酸盐裂解酶(A1-III)、铜绿假单胞菌PAO1海藻酸裂解酶(PA1167)和芽孢杆菌GL1黄原胶裂解酶，并与猪肾N-酰基-D-葡萄糖胺2-异构酶和芽孢杆菌…进行了比较GL1不饱和葡萄糖醛酸水解酶含量较高。这些酶的结构比较表明：(1)A1-III通过LID-环模块表达β-ELIZATION活性。这种结构也在XL中发现，尽管PA1167没有。(2)A1-III具有α/α-Barell结构。尼龙1167呈反平行β-Sheet结构。XL表现为α/α-Barrell结构和反平行β-Sheet结构的组合结构。(3)在所研究的所有多糖裂解酶中，Tyr是必需的催化氨基酸残基，His和Arg参与底物的识别和稳定。从这些结果可以得出结论，尽管一级结构不同，但通过蛋白质模块的运动，可以产生相似的催化部位和底物识别结构。这种观点表明，根据蛋白质的一级结构来估计蛋白质的功能并不总是可靠的，蛋白质的最终功能应该结合蛋白质的三维结构来确定。较少

Polysaccharide lyase is an enzyme catalyzing the depolymerization of polysaccharides such as alginate, xanthan, gellan, and hyarunonate, all of which are produced by bacteria, and gives rise oligosaccharides or monosaccharides as reaction products. The enzyme strictly recognizes uronic acids scattered on the polysaccharide molecules and splits the polysaccharides at the acid site through β-elimination reaction. However, these enzymes have no similarities in their primary structures, thus suggesting that the function of these enzymes are written by different amino acid sequences. To elucidate the reason why the enzymes with the different primary structures could show the same substrate recognition and β-elimination reaction, the X-ray crystal structures of Sphingomonas sp. A1 alginate lyase (A1-III), Pseudomonas aeruginosa PAO1 alginate lyase (PA1167), and Bacillus sp.GL1 xanthan lyase were determined and compared with those of porcine kidney N-acyl-D-glucoamine 2-epimerase and Bacillus … More sp.GL1 unsaturated glucuronylhydrolase. The structural comparison of these enzymes indicated that (1)A1-III expresses β-eliination activity by using a lid-loop module. Such the structure is also found in XL, although PA1167 has not. (2) A1-III has α/α -barell structure. PA1167 shows anti-parallel β-sheet structure. XL shows a combined structure of α/α -barrell structure and anti-paralell β-sheet structure. (3)In all the polysaccharide lyases examined, Tyr is an essential catalytic amino acid residue, and His and Arg are involved in the recognition and stabilization of substrate. From these results, it was concluded that, even though the primary structure is different, similar structure for catalytic site and substrate recognition are created through the movement of protein modules. This notion indicates that the estimation of protein function on the basis of primary structure is not always reliable and definitive function of proteins should be determined in combination with the three-dimensional structures of them. Less