Conversion of Reductases to Dehydrogenases by Regulatory Mutations

Abstract

Nicotinamide adenine dinucleotide (NAD⁺)-linked oxidoreductases catalyze reactions that are generally in favor o f NAD⁺ f ormation at neutral pH. However, in vivo enzymes of this kind can function as either dehydrogenases or reductases. We have a case in which an enzyme that normally acts to reduce L-lactaldehyde is converted by a series of mutations that affect gene expression to an enzyme that acts to oxidize L-l, 2-propanediol. In Escherichia coli both aerobic and anaerobic utilization of L-fucose requires the expression of an inducible trunk pathway mediated by fucose permease (1), fucose isomerase (2), fuculose kinase (3), and fuculose 1-phosphate aldolase (4). The aldolase cleaves the six carbon substrate into dihydroxy-acetone phosphate and lactaldehyde (Figure 1). Anaerobically, lactaldehyde is completely reduced to propanediol by L-l, 2-propanediol: NAD⁺ 1-oxidoreductase (propanediol oxidoredutase), an enzyme with a molecular weight of 76,000 consisting of two electro-phoretically indistinguishable subunits (5). For each mole of fucose fermented, one mole of propanediol is secreted into the medium (6). The sacrifice of one half of the carbon skeleton of fucose in this way permits the further metabolism of dihydroxyacetone phosphate without an exogenous hydrogen acceptor.