Structures of Human Alcohol and Aldehyde Dehydrogenases

Abstract

Human alcohol dehydrogenase is a dimeric zinc metalloenzyme for which forms of three classes, I, II and III, have been distinguished. Subunits hybridize within but not between classes. There are three types of subunit, a, ß and y, in class I. The primary structures of all three forms have been established, as well as the overall properties and the effects of the amino acid substitutions between the various forms. Each subunit has 374 residues, of which 35 exhibit differences among the a, ß and y chains. Corresponding cDNA structures are also known, as are the genetic organization and details of the gene structures. Allelic variants occur at the ß and y loci. Corresponding amino acid substitutions have been characterized, and enzymatic differences between the allelic forms are explained by defined residue exchanges. The results also illustrate recent and repeated isozyme evolution, a subject where alcohol dehydrogenases exceptionally well offer detailed examples. Human aldehyde dehydrogenase occurs of two types, a mitochondrial and a cytosolic form. The enzymes are tetramers, do not contain functional metals, and have subunits which do not form inter-type hybrids. The primary structures have been determined, revealing a positional identity of 68% (in 500 residues) between the mitochondrial and cytosolic forms. The N-terminus is heterogeneous and is not blocked in the subunit of the mitochondrial enzyme, in contrast to that of the cytosolic enzyme or those of all the alcohol dehydrogenases (also cytosolic). A reactive cysteine residue at position 302 has been ascribed functional importance at or close to the active site, is conserved in the two aldehyde dehydrogenases, and is associated with the action of disulfiram on the enzyme. In Oriental populations, a mutant allelic variant of the mitochondrial protein with impaired enzyme function has also been characterized.

Rat liver alcohol dehydrogenase of class III

“…3). These structural results support the classification of the class 111 enzymes as a different type of alcohol dehydrogenase derived from an early duplication [8], in agreement cLass1 133 differences ~(~~~ 164% identity) and y z subunits are also known [7] but not included in the figure since they only affect values marginally) Fig. 3), suggesting more strict structure/function relationships for the class I11 enzyme type.…”

“…strands in the coenzyme-binding domain (PA-BF) [4, 51 present few and highly conservative exchanges in the same manner as found in the class 1/11 comparison [8]. A similar distribution of conservation between the human class I isozymes has also been reported [7]. All main residues implicated in the coenzyme binding [4, 5, 301 are highly conserved, including Asp-223, Ile-224 and Lys-228 (Table 4).…”

“…With the present structure added to those known before (rat class I [19,201 and human class I [7] and 111 [9] enzymes) all four pairs of alcohol dehydrogenase forms of two species (rat/human) and two classes (I/III) can be compared (Fig. 3).…”

Comparison of three classes of human liver alcohol dehydrogenase

“…Subunit types a, ,/I and y [l] in dimeric combinations constitute the isozymes of the human class I enzyme [2] and are all homologous to the E subunit of the horse EE isozyme [3], the only alcohol dehydrogenase crystallographically analyzed [4]. The class I1 and I11 enzymes differ considerably in primary structure [5 -81, constituting essentially separate and distinct enzymes [9] with different evolutionary rates [S].…”

Cloning and high-level expression of the glutathione-independent formaldehyde dehydrogenase gene from Pseudomonas putida

“…These include aldehyde dehydrogenases (16,39), formaldehyde dismutase (21), glutathione-or factordependent FDHs (class III ADHs) (38,40), and glutathioneindependent FDH from P. putida. Aldehyde dehydrogenases are nonspecific enzymes; class III ADHs require a helper substrate (glutathione or factor), producing a formyl ester.…”