Farrés J, Moreno A, Crosas B, Peralba J M, Allali-Hassani A, Hjelmqvist L, Jörnvall H, Parés X
Department of Biochemistry and Molecular Biology, Faculty of Sciences, Universitat Autònoma de Barcelona, Spain.
Eur J Biochem. 1994 Sep 1;224(2):549-57. doi: 10.1111/j.1432-1033.1994.00549.x.
Human stomach mucosa contains a characteristic alcohol dehydrogenase (ADH) enzyme, sigma sigma-ADH. Its cDNA has been cloned from a human stomach library and sequenced. The deduced amino acid sequence shows 59-70% identities with the other human ADH classes, demonstrating that the stomach enzyme represents a distinct structure, constituting class IV, coded by a separate gene, ADH7. The amino acid identity with the rat stomach class IV ADH is 88%, which is intermediate between constant and variable dehydrogenases. This value reflects higher conservation than for the classical liver enzymes of class I, compatible with a separate functional significance of the class IV enzyme. Its enzymic features can be correlated with its structural characteristics. The residues lining the substrate-binding cleft are bulky and hydrophobic, similar to those of the class I enzyme; this explains the similar specificity of both classes, compatible with the origin of class IV from class I. Position 47 has Arg, in contrast to Gly in the rat class IV enzyme, but this Arg is still associated with an extremely high activity (kcat = 1510 min-1) and weak coenzyme binding (KiaNAD+ = 1.6 mM). Thus, the strong interaction with coenzyme imposed by Arg47 in class I is probably compensated for in class IV by changes that may negatively affect coenzyme binding: Glu230, His271, Asn260, Asn261, Asn363. The still higher activity and weaker coenzyme binding of rat class IV (kcat = 2600 min-1, KiaNAD = 4 mM) can be correlated to the exchanges to Gly47, Gln230 and Tyr363. An important change at position 294, with Val in human and Ala in rat class IV, is probably responsible for the dramatic difference in Km values for ethanol between human (37 mM) and rat (2.4 M) class IV enzymes.
人类胃黏膜含有一种独特的酒精脱氢酶（ADH），即σσ-ADH。其cDNA已从人类胃文库中克隆并测序。推导的氨基酸序列与其他人类ADH类别具有59 - 70%的同源性，表明胃酶代表一种独特的结构，构成IV类，由一个单独的基因ADH7编码。与大鼠胃IV类ADH的氨基酸同源性为88%，介于恒定和可变脱氢酶之间。该值反映出比I类经典肝脏酶更高的保守性，这与IV类酶的单独功能意义相符。其酶学特征与其结构特征相关。底物结合裂隙内衬的残基体积大且疏水，与I类酶相似；这解释了两类酶相似的特异性，与IV类起源于I类相符。位置47处为精氨酸，与大鼠IV类酶中的甘氨酸不同，但该精氨酸仍与极高的活性（kcat = 1510 min-1）和较弱的辅酶结合（KiaNAD+ = 1.6 mM）相关。因此，I类中由精氨酸47施加的与辅酶的强相互作用在IV类中可能通过可能对辅酶结合产生负面影响的变化得到补偿：谷氨酸230、组氨酸271、天冬酰胺260、天冬酰胺261、天冬酰胺363。大鼠IV类更高的活性和更弱的辅酶结合（kcat = 2600 min-1，KiaNAD = 4 mM）可能与47位的甘氨酸、230位的谷氨酰胺和363位的酪氨酸交换有关。人类IV类（37 mM）和大鼠IV类（2.4 M）酶之间乙醇Km值的巨大差异可能归因于294位的重要变化，人类为缬氨酸，大鼠为丙氨酸。
