Discovery and characterization of a putrescine oxidase from Rhodococcus erythropolis NCIMB 11540

A gene encoding a putrescine oxidase (PuO(Rh), EC 1.4.3.10) was identified from the genome of Rhodococcus erythropolis NCIMB 11540. The gene was cloned in the pBAD vector and overexpressed at high levels in Escherichia coli. The purified enzyme was shown to be a soluble dimeric flavoprotein consisting of subunits of 50 kDa and contains non-covalently bound flavin adenine dinucleotide as a cofactor. From all substrates, the highest catalytic efficiency was found with putrescine (K (M) = 8.2 μM, k (cat) = 26 s(−1)). PuO(Rh) accepts longer polyamines, while short diamines and monoamines strongly inhibit activity. PuO(Rh) is a reasonably thermostable enzyme with t (1/2) at 50°C of 2 h. Based on the crystal structure of human monoamine oxidase B, we constructed a model structure of PuO(Rh), which hinted to a crucial role of Glu324 for substrate binding. Mutation of this residue resulted in a drastic drop (five orders of magnitude) in catalytic efficiency. Interestingly, the mutant enzyme showed activity with monoamines, which are not accepted by wt-PuO(Rh). ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00253-007-1310-4) contains supplementary material, which is available to authorized users.