A study of formaldehyde dismutation by liver alcohol dehydrogenase with NAD plus-analogs

Novel pathway for assimilation of dimethylsulphoniopropionate widespread in marine bacteria

Dimethylsulphoniopropionate (DMSP) accounts for up to 10% of carbon fixed by marine phytoplankton in ocean surface waters, producing an estimated 11.7-103 Tmol S per year, most of which is processed by marine bacteria through the demethylation/demethiolation pathway. This pathway releases methanethiol (MeSH) instead of the climatically active gas dimethylsulphide (DMS) and enables marine microorganisms to assimilate the reduced sulphur. Despite recognition of this critical microbial transformation for over two decades, the biochemical pathway and enzymes responsible have remained unidentified. Here we show that three new enzymes related to fatty acid β-oxidation constitute the pathway that assimilates methylmercaptopropionate (MMPA), the first product of DMSP demethylation/demethiolation, and that two previously unknown coenzyme A (CoA) derivatives, 3-methylmercaptopropionyl-CoA (MMPA-CoA) and methylthioacryloyl-CoA (MTA-CoA), are formed as novel intermediates. A member of the marine roseobacters, Ruegeria pomeroyi DSS-3, requires the MMPA-CoA pathway for MMPA assimilation and MeSH production. This pathway and the ability to produce MeSH from MMPA are present in diverse bacteria, and the ubiquitous SAR11 clade bacterium Pelagibacter ubique possesses enzymes for at least the first two steps. Analysis of marine metagenomic data indicates that the pathway is widespread among bacterioplankton in the ocean surface waters, making it one of the most important known routes for acquisition of reduced carbon and sulphur by surface ocean heterotrophs.

Formaldehyde dismutase, a novel NAD-binding oxidoreductase from Pseudomonas putida F61

A novel enzyme, formaldehyde dismutase, was purified and crystallized from the cell extract of an isolated bacterium, Pseudomonas putida F61. The enzyme catalyzes the dismutation of aldehydes and alcohol:aldehyde oxidoreduction in the absence of an exogenous electron acceptor. The enzyme is composed of four identical subunits with a Mr of 44 000. Each subunit contains 1 mol NAD(H) and 2 mol zinc/mol. The ratio of NAD+ and NADH in a crystalline preparation of the enzyme was about 7:3. The enzyme-bound coenzyme was completely reduced and oxidized on the addition of a large amount of an alcohol and an aldehyde respectively. Both the oxidized and reduced enzymes catalyzed the dismutation reaction to the same extent. Steady-state kinetics of the enzyme were investigated using an oxidoreduction reaction between an alcohol and p-nitroso-N, N-dimethylaniline. The enzyme obeys a ping-pong mechanism and is competitively inhibited by an alcoholic substrate analogue, pyrazole, but not coenzyme analogues, such as AMP, N-methylnicotinamide. These results indicate that NAD(H) binds firmly (but not covalently) at each active site. The enzyme-bound NAD(H) was reduced and oxidized only by the added second substrates, alcohol and aldehyde respectively, and not by exogenous electron acceptors [including NAD(H)].

The specificity of dehydrogenases

The specificity of dehydrogenases for coenzyme (and coenzyme analogues), and substrate (and substrate analogues) is discussed in relation to structure, function, and evolution. Examples concern compounds that have very different structures, reactions that play widely differing roles in the life of the organism, and organisms of greatly differing types. The examples illustrate general points of interest and importance.