A 30 angstrom long U-shaped catalytic tunnel in the crystal structure of polyamine oxidase

Background: Polyamines are essential for cell growth and differentiation; c ompounds interfering with their metabolism are potential anticancer agents. Polyamine oxidase (PAO) plays a central role in polyamine homeostasis. The enzyme utilises an FAD cofactor to catalyse the oxidation of the secondary amino groups of spermine and spermidine. Results: The first crystal structure of a polyamine oxidase has been determ ined to a resolution of 1.9 Angstrom. PAO from Zea mays contains two domain s, which define a remarkable 30 Angstrom long U-shaped catalytic tunnel at their interface. The structure of PAO in complex with the inhibitor MDL7252 7 reveals the residues forming the catalytic machinery and unusual enzyme-i nhibitor CH ... O hydrogen bonds. A ring of glutamate and aspartate residue s surrounding one of the two tunnel openings contributes to the steering of the substrate towards the inside of the tunnel. Conclusions: PAO specifically oxidises substrates that have both primary an d secondary amino groups. The complex with MDL72527 shows that the primary amino groups are essential for the proper alignment of the substrate with r espect to the flavin. Conservation of an N-terminal sequence motif indicate s that PAO is a member of a novel family of flavoenzymes. Among these, mono amine oxidase displays significant sequence homology with PAO, suggesting a similar overall folding topology.