66 research outputs found
Formal Reduction Potential of 3,5-Difluorotyrosine in a Structured Protein: Insight into Multistep Radical Transfer
The reversible YâOâą/YâOH redox properties of the α[subscript 3]Y model protein allow access to the electrochemical and thermodynamic properties of 3,5-difluorotyrosine. The unnatural amino acid has been incorporated at position 32, the dedicated radical site in α[subscript 3]Y, by in vivo nonsense codon suppression. Incorporation of 3,5-difluorotyrosine gives rise to very minor structural changes in the protein scaffold at pH values below the apparent pK (8.0 ± 0.1) of the unnatural residue. Square-wave voltammetry on α[subscript 3](3,5)F[subscript 2]Y provides an E°âČ(YâOâą/YâOH) of 1026 ± 4 mV versus the normal hydrogen electrode (pH 5.70 ± 0.02) and shows that the fluoro substitutions lower the E°âČ by â30 ± 3 mV. These results illustrate the utility of combining the optimized α[subscript 3]Y tyrosine radical system with in vivo nonsense codon suppression to obtain the formal reduction potential of an unnatural aromatic residue residing within a well-structured protein. It is further observed that the protein E°âČ values differ significantly from peak potentials derived from irreversible voltammograms of the corresponding aqueous species. This is notable because solution potentials have been the main thermodynamic data available for amino acid radicals. The findings in this paper are discussed relative to recent mechanistic studies of the multistep radical-transfer process in Escherichia coli ribonucleotide reductase site-specifically labeled with unnatural tyrosine residues.National Institutes of Health (U.S.) (Grant GM29595
Function of the Diiron Cluster of Escherichia coli Class Ia Ribonucleotide Reductase in Proton-Coupled Electron Transfer
The class Ia ribonucleotide reductase (RNR) from Escherichia coli employs a free-radical mechanism, which involves bidirectional translocation of a radical equivalent or âholeâ over a distance of ~35 Ă
from the stable diferric/tyrosyl-radical (Y[subscript 122]âą) cofactor in the ÎČ subunit to cysteine 439 (C[subscript 439]) in the active site of the α subunit. This long-range, intersubunit electron transfer occurs by a multistep âhoppingâ mechanism via formation of transient amino acid radicals along a specific pathway and is thought to be conformationally gated and coupled to local proton transfers. Whereas constituent amino acids of the hopping pathway have been identified, details of the proton-transfer steps and conformational gating within the ÎČ sununit have remained obscure; specific proton couples have been proposed, but no direct evidence has been provided. In the key first step, the reduction of Y[subscript 122]âą by the first residue in the hopping pathway, a water ligand to Fe[subscript 1] of the diferric cluster was suggested to donate a proton to yield the neutral Y[subscript 122]. Here we show that forward radical translocation is associated with perturbation of the Mössbauer spectrum of the diferric cluster, especially the quadrupole doublet associated with Fe[subscript 1]. Density functional theory (DFT) calculations verify the consistency of the experimentally observed perturbation with that expected for deprotonation of the Fe[subscript 1]-coordinated water ligand. The results thus provide the first evidence that the diiron cluster of this prototypical class Ia RNR functions not only in its well-known role as generator of the enzymeâs essential Y[subscript 122]âą, but also directly in catalysis.National Institutes of Health (U.S.) (GM-29595
Angular Momentum of Early- and Late-type Galaxies: Nature or Nurture?
We investigate the origin, the shape, the scatter, and the cosmic evolution
in the observed relationship between specific angular momentum and
the stellar mass in early-type (ETGs) and late-type galaxies (LTGs).
Specifically, we exploit the observed star-formation efficiency and chemical
abundance to infer the fraction f_\rm inf of baryons that infall toward the
central regions of galaxies where star formation can occur. We find f_\rm
inf\approx 1 for LTGs and for ETGs with an uncertainty of about
dex, consistent with a biased collapse. By comparing with the locally
observed vs. relations for LTGs and ETGs we estimate the
fraction of the initial specific angular momentum associated to the
infalling gas that is retained in the stellar component: for LTGs we find
, in line with the classic disc formation
picture; for ETGs we infer , that can be
traced back to a evolution via dry mergers. We also show that the
observed scatter in the vs. relation for both galaxy
types is mainly contributed by the intrinsic dispersion in the spin parameters
of the host dark matter halo. The biased collapse plus mergers scenario implies
that the specific angular momentum in the stellar components of ETG progenitors
at is already close to the local values, in pleasing agreement with
observations. All in all, we argue such a behavior to be imprinted by nature
and not nurtured substantially by the environment
Magnetic Inflation and Stellar Mass. I. Revised Parameters for the Component Stars of the Kepler
Non-native Intermediate Conformational States of Human Growth Hormone in the Presence of Organic Solvents
Voltammetric and Amperometric Investigations of Azide Oxidation at the Basal Plane of Highly Oriented Pyrolytic Graphite
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