THE GTP EFFECTOR SITE OF ORNITHINE DECARBOXYLASE FROM LACTOBACILLUS 30A - KINETIC AND STRUCTURAL CHARACTERIZATION

A nucleotide effector site of the biodegradative form of ornithine dec arboxylase from Lactobacillus 30a (OrnDC L30a) has been identified. Or nDC L30a activity at pH 8.0, where the enzyme is normally inactive, is stimulated by GTP and dGTP and to a lesser extent by GDP but not by A TP, CTP, or UTP. The pH profile indicates that activation by GTP is re flected by an increase in k(cat)/K-M,K-orn (above pH 6.8), while V-max remains constant over the pH range 4.0-9.0. Scatchard plot analysis s hows that GTP binds to OrnDC L30a at both pH 5.8 (K-D = 0.11 mu M) and pH 8.0 (K-D = 1.6 mu M), but unexpectedly, half-site binding is obser ved at the higher pH. The OrnDC L30a dodecamer dissociates into dimers at high pH in the presence or absence of GTP. The GTP binding site wa s located in difference electron density maps using low-resolution X-r ay data. This represents a new type of GTP binding site. A model expla ining the activation of OrnDC L30a by GTP is presented.