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Alcohol Dehydrogenase of Class IV (σσ-ADH) from Human Stomach

cDNA Sequence and Structure/Function Relationships

Authors


  • Note. The novel nucleotide sequence data reported here have been submitted to the EMBL/GenBank Data Banks and is available under the accession number X76342.

X. Parés, Department of Biochemistry and Molecular Biology, Faculty of Sciences, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain
Fax: +34 3 581 1264.

Abstract

Human stomach mucosa contains a characteristic alcohol dehydrogenase (ADH) enzyme, σσ-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.

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