Electrostatic compared with hydrophobic interactions between bovine serum amine oxidase and its substrates

A steady-state kinetic study of bovine serum amine oxidase activity was performed, over a wide range of pH values (5.4–10.2) and ionic strength (10–200mM), using various (physiological and analogue) substrates as specific probes of the active-site binding region. Relatively small changes in k_cat values (approx. one order of magnitude) accompanied by marked changes in K_m and k_cat/K_m values (approx. six orders of magnitude) were observed. This behaviour was correlated with the presence of positively charged groups or apolar chains in the substrates. In particular, it was found that the docking of the physiological polyamines, i.e. spermidine and spermine, appears to be modulated by three amino acid residues of the active site, which we have named L^-H⁺, G^-H⁺ and IH⁺, characterized by pK_a values of 6.2±0.2 [Di Paolo, Scarpa, Corazza, Stevanato and Rigo (2002) Biophys. J. 83, 2231–2239], 8.2±0.3 and 7.8±0.4 respectively. The electrostatic interaction between the protonated substrates and the enzyme containing the residues L^-H⁺, G^-H⁺ and IH⁺ in the deprotonated form, the on/off role of the IH⁺ residue and the role of hydrophobic interactions with substrates characterized by apolar chains are discussed.