Affinity and hydrophobic chromatography of Escherichia coli aspartate transcarbamoylase

Inactivation of wheat-germ aspartate transcarbamoylase by the triazinyl dye, procion red HE3B

Aspartate transcarbamoylase from wheat germ is irreversibly inactivated by the triazinyl dye Procion Red HE3B. Since triazinyl dyes may mimic nucleotides, and UMP is a known allosteric modifier of this enzyme, the reaction was studied to elucidate whether the dye is an 'affinity label' for the enzyme. The reaction is apparently first order in the first 5-10 min, but is more complex in the longer term and does not go to completion. Kinetic analysis of the initial phase suggests that there are two parallel reactions, one saturable (dye binds reversibly before reaction) and one non-saturable (biomolecular). The apparent rate constant kapp (i.e. the sum of the rate constants for the parallel reactions) varies only slightly over the pH range 7-10. In the presence of a number of active centre ligands, as well as the allosteric ligand UMP, there is a clear increase in kapp. This finding is contrary to the reduction in rate of inactivation (protection) normally provided by ligands against active-site directed reagents, suggesting that in the saturable reaction, there is a conformational change upon dye-binding that increases the exposure of the essential residue(s) with which the dye reacts. These results show that, although it probably inactivates by reaction with specific amino-acid residues, the dye is not bound at the substrate-binding or allosteric sites, i.e. it is not an affinity-labeling reagent in the usual sense.

Self-assembled organic monolayers: model systems for studying adsorption of proteins at surfaces

Self-assembled monolayers (SAMs) of omega-functionalized long-chain alkanethiolates on gold films are excellent model systems with which to study the interactions of proteins with organic surfaces. Monolayers containing mixtures of hydrophobic (methyl-terminated) and hydrophilic [hydroxyl-, maltose-, and hexa(ethylene glycol)-terminated] alkanethiols can be tailored to select specific degrees of adsorption: the amount of protein adsorbed varies monotonically with the composition of the monolayer. The hexa(ethylene glycol)-terminated SAMs are the most effective in resisting protein adsorption. The ability to create interfaces with similar structures and well-defined compositions should make it possible to test hypotheses concerning protein adsorption.

Involvement of tryptophan 209 in the allosteric interactions of Escherichia coli aspartate transcarbamylase using single amino acid substitution mutants

Five mutant versions of aspartate transcarbamylase have been isolated, all with single amino acid substitutions in the catalytic chain of the enzyme. A previously isolated pyrB nonsense mutant was suppressed with supB, supC, supD and supG to create enzymes with glutamine, tyrosine, serine or lysine, respectively, inserted at the position of the nonsense codon. Each of these enzymes was purified to homogeneity and kinetically characterized. The approximate location of the substitution was determined by using tryptic fingerprints of the wild-type enzyme and the enzyme obtained with a tyrosine residue inserted at the position of the nonsense codon. By first cloning the pyrBI operon, from the original pyrB nonsense strain, followed by sequencing of the appropriate portion of the gene, the exact location of the mutation was determined to be at position 209 of the catalytic chain. Site-directed mutagenesis was used to generate versions of aspartate transcarbamylase with tyrosine and glutamic acid at this position. The Tyr209 enzyme is identical with that obtained by suppression of the original nonsense mutation with supC. The two enzymes produced by site-directed mutagenesis were purified using a newly created overproducing strain. Kinetic analysis revealed that each mutant has an altered affinity for aspartate, as judged by variations in the substrate concentration at one-half maximal activity. In addition, the mutants exhibit altered Hill coefficients and maximal activities. In the wild-type enzyme, position 209 is a tryptophan residue that is involved in the stabilization of a bend in the molecule near the subunit interface region. The alteration in homotropic cooperativity seems to be due to changes induced in this bend in the molecule, which stabilizes alternate conformational states of the enzyme.