Lactaldehyde dehydrogenase

Related Reading

The aldehyde dehydrogenases (ALDH) represent a superfamily of NAD+- or NADP+-dependent enzymes that catalyze the oxidation of aldehydes to the corresponding carboxylic acids.These enzymes are ancient and widespread in all living systems, from archea to eukaryotes, where they metabolize endogenous and exogenous aldehydes. In E. coli, 17 aldehyde dehydrogenases have been identified and categorized to date based on their substrate specificity or sequence similarity. Two isozymes sharing 33% sequence identity have been identified as products of the genes aldA and aldB.

Functions

Lactaldehyde dehydrogenase is a zinc-containing enzyme. Its molecule consists of two subunits, one of which is located in the active center of the enzyme and the other acts as a stabilizer of the quaternary structure. Lactaldehyde dehydrogenase activity in serum is a diagnostic indicator of acute hepatic parenchymal cell injury. Serum enzyme activity is negative in normal subjects or in patients without secondary liver disease. These are of great importance for humans and aldehyde dehydrogenase will be widely used.

L-Lactaldehyde

L-Lactaldehyde is generated as an intermediate in the metabolism of L-fucose and L-rhamnose in bacteria that utilize these carbohydrates as a carbon source. Each of these carbohydrates is metabolized through a similar sequence of permease, isomerase, kinase, and aldolase enzyme-catalyzed reactions. The 6-deoxyhexoses generated in the last step of each pathway, L-fuculose and L-rhamnulose-1-phosphate, are cleaved to yield identical products, dihydroxyacetone phosphate and lactaldehyde.Lactaldehyde is then oxidized to pyruvate in two consecutive steps.

Structure of unliganded lactaldehyde dehydrogenase

The asymmetric unit of the unit cell contains one monomer and the symmetry operations corresponding to space group P6422 result in the assembly of an isologous homotetramer in which the four monomers are related by 222 point group symmetry (D2 symmetry). Tetrameric quaternary structure is consistent with the results of gel filtration chromatography and recent ultracentrifugation experiments. Each monomer is composed of a catalytic domain (residues from G254 - Q438), a cofactor binding domain (D21 - K96, F442 - E468, and residues in the Rossmann fold, G144 - G253), and an oligomerization domain (residues from Y122 - A142 and Y469 - S479). The Rossmann fold domain comprises part of the N-terminal region and creates one wall of the active site. As expected by Weiner and colleagues based on a homology model of lactaldehyde dehydrogenase, large patches of hydrophobic surface in addition to the oligomerization domain comprise the subunit interface.

Mechanism

First, lactaldehyde dehydrogenase oxidizes lactaldehyde to lactate using NAD+ as cofactor. Lactate dehydrogenase then catalyzes the oxidation of lactate to form pyruvate, which subsequently undergoes decarboxylation and conversion to acetyl CoA for entry into the Krebs cycle in a series of reactions catalyzed by the pyruvate dehydrogenase complex. Alternatively, under anaerobic conditions lactaldehyde is reduced by NADH and L-1,2-propanediol oxidoreductase.

Purification of lactaldehyde dehydrogenase

The preparation and purification of E. coli lactaldehyde dehydrogenase was serendipitous. During the heterologous expression in E. coli and purification of taxadiene synthase from the Pacific yew, lactaldehyde dehydrogenase was identified as a ∼5% side product. Lactaldehyde dehydrogenase was co-expressed and co-purified with taxadiene synthase using the method described by Williams and colleagues.