Identification, expression, and characterization of Escherichia coli guanine deaminase.

Using the human cDNA sequence corresponding to guanine deaminase, the Escherichia coli genome was scanned using the Basic Local Alignment Search Tool (BLAST), and a corresponding 439-residue open reading frame of unknown function was identified as having 36% identity to the human protein. The putative gene was amplified, subcloned into the pMAL-c2 vector, expressed, purified, and characterized enzymatically. The 50.2-kDa protein catalyzed the conversion of guanine to xanthine, having a K(m) of 15 microM with guanine and a k(cat) of 3.2 s(-1). The bacterial enzyme shares a nine-residue heavy metal binding site with human guanine deaminase, PG[FL]VDTHIH, and was found to contain approximately 1 mol of zinc per mol of subunit of protein. The E. coli guanine deaminase locus is 3' from an open reading frame which shows homology to a bacterial purine base permease.

Role of conformational change in the fumarase reaction cycle.

Activation of fumarase by high concentrations of either malate or fumarate, often referred to as negative cooperativity, can be explained without assuming additional sites of substrate action or subunit-subunit interactions. The following observations support a model based on a rate-dependent recycling of free enzyme through a sequence of conformational states that differ in substrate specificity and catalytic activity: (1) Displacement from equilibrium of a radiolabeled malate/fumarate probe is readily induced by moderate concentrations of either substrate. This phenomenon, called substrate-induced countertransport, indicates that the steady-state ratio of free enzyme forms is very dependent on substrate concentration. (2) Related to this, the back-labeling that can be observed with either 14C product with either substrate in the steady state is more rapid than expected for a single free enzyme state model. (3) Fumarate, more strongly than malate, shows competitive effects as a product. This may reflect a higher affinity of fumarate for an isoform that also reacts with malate. (4) P(i), an activator of fumarase at midrange substrate concentration, overcomes strong competitive inhibition by fumarate of the M-->F reaction and increases recycling as shown by its effect on counterflow. To the extent that these effects are due to buffer activation, they suggest that proton transfer between solvent and the enzyme site is important in determining the recycling rate. (5) Transaconitate, a competitive inhibitor, overcomes counterflow induced by either substrate, indicating that recycling events occur in the enzyme-transaconitate complex.