An examination of the utility of photogenerated reagents by using alpha-chymotrypsin

Plant polyketide synthases leading to stilbenoids have a domain catalyzing malonyl-CoA:CO2 exchange, malonyl-CoA decarboxylation, and covalent enzyme modification and a site for chain lengthening

Stilbene synthases and the related bibenzyl synthases are plant polyketide synthases whose biological functions lie in the formation of antimicrobial phytoalexins. The formation of hydroxystilbenes from one molecule of acyl-CoA and three molecules of malonyl-CoA is catalyzed by a homodimeric 90 kDa protein and includes Claisen condensations and cleavage of a thioester followed by decarboxylation. Combining inhibitor studies, protein modifications, and site-directed mutagenesis, we were able to differentiate between the binding sites for malonyl-CoA and the regions responsible for the selection of the primer, p-coumaroyl-CoA or m-hydroxyphenylpropionyl-CoA, respectively. Mutations in the C-terminal part of the molecule or modification by photolabeling with p-azidocinnamoyl-CoA influence the overall reaction, the formation of hydroxystilbenes, but leave partial reactions, such as the malonyl-CoA:CO2 exchange and the malonyl-CoA-dependent modification of the enzyme, unaffected. Data obtained with several kinds of stilbene synthase and mutant forms suggest that the malonyl-CoA-dependent covalent modification takes place at a cysteine residue in the N-terminal part of the enzyme. Mutations in the C-terminal half of the enzyme molecule do not interfere with the malonyl-CoA-dependent reactions.

Photo-induced labelling of Escherichia coli ribosomes by a tobramycin analog

An [3H]azidobenzyl derivative of tobramycin, a 4,6-disubstituted 2-deoxystreptamine aminoglycoside, has been synthesized, and its ability to label Escherichia coli 70-S ribosomes under photoactivation has been studied. Two concentrations of the photolabel, corresponding to the saturation of the two classes of tobramycin sites on the ribosomes, were used. The results show that, at high antibiotic concentrations which induce maximal misreading during protein synthesis, most of the ribosomal proteins are labelled. At low antibiotic concentration, which results in the saturation of the first-class sites, a few proteins of both subunits are labelled, including L6, S4, S5, and, to a lesser extent, L2, L13 and S18. The 30-S subunit is, on the whole, labelled more efficiently than the 50-S subunit.

Identification by photoaffinity labelling of 16S RNA as a component of IF2-binding site of E. coli MRE 600 ribosome

A photoactivated radioactive derivative of IF2 was obtained by treatment with p-azidobenzaldehyde in the presence of NaBH4, followed by reductive methylation with [14C] formaldehyde. The modified IF2 preserved the functional activity of the native factor in specific binding to 30S ribosomal subparticles to which it specifically cross-linked during mild irradiation. The main affinity-labelled component of the 30S subparticle was found to be 16S RNA.

Photolytic inhibition and labeling of proteins with aryl diazonium compounds

In the course of preparing aryl azide derivatives for use as photoprobes, we have observed significant light sensitivity in the precursor aryl diazonium compounds. The photosensitive properties of this class of compounds are of interest since they will seek out cationic binding sites in biological targets, and can be employed to inhibit complementary targets at acid pH. The relationship between photolytic change in the structure of diazonium compounds and the corresponding change in function of a biological target are presented. Experiments are described in which the dark and light sensitive properties of a model diazonium compound, diazobenzene sulfonate (DBS), were determined. The ultraviolet spectra were used to evaluate the dark stability and light sensitivity of DBS. Chymotrypsin and trypsin served as functioning targets for further evaluation of the photochemical properties. Both enzymes are stable to the probe in the dark at acid pH. A rapid loss of enzyme activity was observed following flash photolysis of DBS-enzyme solutions. Photolytic incorporation of radioactive DBS into chymotrypsin was observed. Aryl diazonium salts can be employed to probe the availability of complementary sites in biological targets at different acid pH values.

Inactivation of alpha-chymotrypsin by new bifunctional reagents: halomethylated derivatives of dihydrocoumarins

alpha-Chymotrypsin is rapidly and completely inactivated by a series of halomethylated derivatives of dihydrocoumarins at pH 7 and 25 degrees. The inactivation is pH-dependent and optimal at neutral pH; it is also more or less complete depending on the excess of inhibitor with respect to the enzyme. These compounds are substrates for alpha-chymotrypsin and, during the catalytic process, a latent alkylating function of the reagent is activated at the active site of the enzyme and reacts with some vicinal nucleophilic amino acid residues. The stoichiometry of the reaction of the corresponding radioactive reagents with the enzyme is slightly superior to one, but one histidine residue of alpha-chymotropsin is mainly modified. This histidine is identified as histidine-57 by the diagonal peptide mapping method. In comparison with other reagents, the efficiency of these suicide substrates" and particularly that one of a derivative (compound 2) carrying a substrate-like side chain is found to be very high.