Medium chain dehydrogenases/reductases : alcohol dehydrogenases of novel types

Abstract

The medium chain dehydrogenase/reductase (MDR) superfamily is separated into at least eight families, of which the dimeric alcohol dehydrogenase (ADH) is one. Including species variants, close to 1000 MDR forms have been characterized. The dimeric ADHs are Zn-metalloenzymes that catalyze the reversible oxidation of alcohol to aldehyde/ketone using NAD+/NADH as electron acceptor/donor. ADHs are involved in many important functions, such as protection against a variety of toxic compounds, regulation of hormone and growth factor effects, and participation in the intermediary metabolism. Differences in isozyme distribution among populations have been correlated with diseases. The non-MDR aldehyde dehydrogenase (ALDH) family is more diverse than the ADHs, consisting of 17 known functional genes in the human. ALDH catalyzes the oxidation of aldehydes to carboxylic acids using NAD(P)+/NAD(P)H as electron acceptor/donor.

This thesis concerns structural, functional and evolutionary relationships of the MDR enzymes. Comparisons with the ALDH family have also been included regarding certain patterns recognized through the extensive studies of ADH. ADH3 is of ancient origin, slowly evolving, has a well-defined function and has two segments of variability located in nonfunctional areas. A conserved pattern has now been shown to occur also in the constant ALDH2 and ALDH9 forms. Thus, although ADH and ALDH are of completely different origins, they appear to have certain similarities in overall evolutionary pathways.

A new form of formaldehyde dehydrogenase, from the Gram-positive bacterium Amycolatopsis methanolica, was shown to be a distant member of the MDR superfamily. This enzyme (MD-FDH) utilizes mycothiol instead of glutathione when forming the actual substrate, the thiohemiacetal with formaldehyde. The primary structure determined was aligned and modeled into the human ADH1beta crystal structure. By docking the mycothiolformaldehyde adduct into the active site pocket of the model, it was found that three polar residues are able to provide the interactions necessary for binding. MD-FDH was found to have an evolutionary position intermediate between those of the dimeric ADHs and other families of the MDR superfamily.

Nicotinoprotein (Np-ADH) from A. methanolica was also characterized. It has a tightly bound redoxactive cofactor (NAD+). The reducing equivalents do not physically exchange with those of the cytosolic NAD(P)-pool. They are either transferred to the respiratory chain, temporarily stored and subsequently returned to the substrate, or donated to a second substrate. The Np-ADH primary structure determined was modeled into the crystal structure of human ADH1beta, allowing establishment of the overall relationship to the MDR superfamily and identification of additional interactions with the cofactor, compatible with the nicotinoprotein properties of Np-ADH.

We also characterized mushroom ADH3 from Agaricus bisporus and yeast ADH3 from Saccharomyces cerevisiae. The studies showed a strong evolutionary restriction on the catalytic properties and emphasized the central role of ADH3 as an enzyme in formaldehyde detoxification.