Inventory of interactions that participate in the proper positioning of the catalytic histidine next to the catalytic nucleophile in 40 ABH fold enzyme families.

The main-chain oxygen atom of residue XIV interacts with both the catalytic histidine and the catalytic nucleophile (column CA/Nucleophile–O/XIV), orienting the imidazole ring of the catalytic histidine with respect to the side-chain of the catalytic nucleophile. Residue XIV+1 also interacts with the catalytic nucleophile (column O/Nucleophile–CA/XIV+1) and thus, has an indirect influence on the optimal arrangement of the catalytic histidine-nucleophile pair. Unrelated to the coordination of the catalytic residues, an additional conserved contact (column Sc/(Nucleophile+4)–O/XIV+2) is formed between the residue that is located four positions after the catalytic nucleophile and the residue that is located two positions after residue XIV; this conserved contact is formed close to the catalytic nucleophile, residue XIV and residue XIV+1. In the second row of the table, the interactions from the carboxylesterase SshEstI (SCOP family #2, PDB ID:3WJ1_A) are listed, corresponding to the interactions that are illustrated in Fig 2. In four ABH fold families [SCOP families #12 (PDB ID:1MJ5_A), #23, #27 and #29], residue XIV is the catalytic acid residue. SCOP family #12 is represented by two structures in order to properly reflect the local structural variations in ABH fold enzymes that have their catalytic acid at the canonical position (i.e. at the turn that follows strand β7, e.g. structure PDB ID:1B6G_A) or at the alternate position (i.e. at the position of residue XIV, e.g. structure PDB ID:1MJ5_A). Alternative types of contacts to those described as conserved occur in a few representative structures, for example in ABH fold enzyme families of Epoxide hydrolase (SCOP family #18), Atu1826-like (SCOP family #36) and PHB depolymerase-like (SCOP family #37). Values in parentheses correspond to the distances of the hydrogen bond to the hydrogen atom.

残基XIV的主链氧原子同时与催化组氨酸（catalytic histidine）和催化亲核体（catalytic nucleophile）发生相互作用（对应列CA/Nucleophile–O/XIV），可使催化组氨酸的咪唑环相对于催化亲核体的侧链实现定向排列。残基XIV+1同样与催化亲核体发生相互作用（对应列O/Nucleophile–CA/XIV+1），因此可间接影响催化组氨酸-亲核体配对的最佳排布方式。与催化残基的配位作用无关的是，在催化亲核体下游第4位的残基与残基XIV下游第2位的残基之间，会形成一处额外的保守相互作用（对应列Sc/(Nucleophile+4)–O/XIV+2）；该保守相互作用紧邻催化亲核体、残基XIV以及残基XIV+1形成。表格第二行列出了羧酸酯酶SshEstI（SCOP家族（SCOP family）#2，PDB编号（PDB ID）：3WJ1_A）的相互作用模式，对应图2中展示的相互作用。在4个ABH折叠（ABH fold）家族[SCOP家族#12（PDB编号：1MJ5_A）、#23、#27及#29]中，残基XIV即为催化酸性残基（catalytic acid residue）。SCOP家族#12采用两个结构进行代表，以充分反映ABH折叠酶的局部结构变异：这类酶的催化酸性残基可位于经典位置（即β7链下游的转角处，例如PDB编号1B6G_A对应的结构），或是位于替代位置（即残基XIV所在的位置，例如PDB编号1MJ5_A对应的结构）。在少数代表性结构中，存在与上述保守相互作用类型不同的其他相互作用模式，例如环氧化物水解酶（Epoxide hydrolase）（SCOP家族#18）、Atu1826样（Atu1826-like）（SCOP家族#36）以及PHB解聚酶样（PHB depolymerase-like）（SCOP家族#37）对应的ABH折叠酶家族。括号内的数值为氢键（hydrogen bond）与其氢原子之间的距离。