Nonspecific target engagement by test compounds and purported chemical probes is a significant source of assay interference and promiscuous bioactivity in high-throughput screening (HTS) and chemical biology. Most counter-screens for thiol-reactive compounds utilize mass spectrometry or fluorescence detection, and non-proteinaceous reporters like glutathione that may not always approximate the reactivity of protein side-chains. By contrast, a La assay to detect reactive molecules by nuclear magnetic resonance (ALARM NMR) is an industry-developed protein-based [H-C]-heteronuclear multiple quantum coherence (HMQC) NMR counter-screen to identify nonspecific protein interactions by test compounds by reporting their tendencies to modulate the human La antigen conformation. This is a users-guide to the production of theC-labeled La antigen reporter protein, the reaction of test compounds with this reporter protein, as well as the collection and analysis of characteristic NMR spectra. Combined with other assay interference counter-screens, this assay will enhance chemical biology by helping researchers better prioritize chemical matter and which will increase the number of tractable HTS screening actives and aid in the development of better chemical probes.
Peptides containing C-terminal esters are an important class of bioactive molecules that includes a-factor, a farnesylated dodecapeptide, involved in the mating of S. cerevisiae. Here, results that expand the scope of solid phase peptide synthetic methodology that uses trityl side-chain anchoring for the preparation of peptides with C-terminal cysteine alkyl esters are described. In this method, Fmoc-protected C-terminal cysteine esters are anchored to trityl chloride resin and extended by standard solid phase procedures followed by acidolytic cleavage and HPLC purification. Analysis using a Gly-Phe-Cys-OMe model tripeptide, revealed minimal epimerization of the C-terminal cysteine residue under basic conditions used for Fmoc deprotection. 1H-NMR analysis of the unfarnesylated a-factor precursor peptide confirmed the absence of epimerization. The side-chain anchoring method was used to produce wild type a-factor that contains a C-terminal methyl ester along with ethyl-, isopropyl- and benzyl-ester analogs in good yield. Activity assays using a yeast-mating assay demonstrate that while the ethyl and isopropyl esters manifest near-wild type activity, the benzyl ester-containing analog is ca. 100-fold less active. This simple method opens the door to the synthesis of a variety of C-terminal ester modified peptides that should be useful in studies of protein prenylation and other structurally related biological processes.
The hydroxylase component (S5HH) of salicylate-5-hydroxylase catalyzes C5 ring hydroxylation of salicylate but switches to methyl hydroxylation when a C5 methyl substituent is present. The use of 18 O 2 reveals that both aromatic and aryl-methyl hydroxylations result from monooxygenase chemistry. The functional unit of S5HH comprises a nonheme Fe(II) site located 12 Å across a subunit boundary from a one-electron reduced Riesketype iron−sulfur cluster. Past studies determined that substrates bind near the Fe(II), followed by O 2 binding to the iron to initiate catalysis. Stopped-flow-single-turnover reactions (STOs) demonstrated that the Rieske cluster transfers an electron to the iron site during catalysis. It is shown here that fluorine ring substituents decrease the rate constant for Rieske electron transfer, implying a prior reaction of an Fe(III)-superoxo intermediate with a substrate. We propose that the iron becomes fully oxidized in the resulting Fe(III)-peroxo-substrate-radical intermediate, allowing Rieske electron transfer to occur. STO using 5-CD 3 -salicylate-d 8 occurs with an inverse kinetic isotope effect (KIE). In contrast, STO of a 1:1 mixture of unlabeled and 5-CD 3 -salicylate-d 8 yields a normal product isotope effect. It is proposed that aromatic and aryl-methyl hydroxylation reactions both begin with the Fe(III)-superoxo reaction with a ring carbon, yielding the inverse KIE due to sp 2 → sp 3 carbon hybridization. After Rieske electron transfer, the resulting Fe(III)-peroxo-salicylate intermediate can continue to aromatic hydroxylation, whereas the equivalent aryl-methyl intermediate formation must be reversible to allow the substrate exchange necessary to yield a normal product isotope effect. The resulting Fe(III)-(hydro)peroxo intermediate may be reactive or evolve through a high-valent iron intermediate to complete the arylmethyl hydroxylation.
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