Selenocysteine Substitution in a Class I Ribonucleotide Reductase

Non-standard amino acid incorporation into thiol dioxygenases

Thiol dioxygenases (TDOs) are non‑heme Fe(II)‑dependent enzymes that catalyze the O2-dependent oxidation of thiol substrates to their corresponding sulfinic acids. Six classes of TDOs have thus far been identified and two, cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), are found in eukaryotes. All TDOs belong to the cupin superfamily of enzymes, which share a common β‑barrel fold and two cupin motifs: G(X)5HXH(X)3-6E(X)6G and G(X)5-7PXG(X)2H(X)3N. Crystal structures of TDOs revealed that these enzymes contain a relatively rare, neutral 3‑His iron‑binding facial triad. Despite this shared metal-binding site, TDOs vary greatly in their secondary coordination spheres. Site‑directed mutagenesis has been used extensively to explore the impact of changes in secondary sphere residues on substrate specificity and enzymatic efficiency. This chapter summarizes site-directed mutagenesis studies of eukaryotic TDOs, focusing on the tools and practicality of non‑standard amino acid incorporation.

Role of Water in Proton-Coupled Electron Transfer between Tyrosine and Cysteine in Ribonucleotide Reductase

Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides and is critical for DNA synthesis and repair in all organisms. Its mechanism requires radical transfer along a ∼32 Å pathway through a series of proton-coupled electron transfer (PCET) steps. Previous simulations suggested that a glutamate residue (E623) mediates the PCET reaction between two stacked tyrosine residues (Y730 and Y731) through a proton relay mechanism. This work focuses on the adjacent PCET reaction between Y730 and a cysteine residue (C439). Quantum mechanical/molecular mechanical free energy simulations illustrate that when Y730 and Y731 are stacked, E623 stabilizes the radical on C439 through hydrogen bonding with the Y730 hydroxyl group. When Y731 is flipped away from Y730, a water molecule stabilizes the radical on C439 through hydrogen bonding with Y730 and lowers the free energy barrier for radical transfer from Y730 to C439 through electrostatic interactions with the transferring hydrogen but does not directly accept the proton. These simulations indicate that the conformational motions and electrostatic interactions of the tyrosines, cysteine, glutamate, and water strongly impact the thermodynamics and kinetics of these two coupled PCET reactions. Such insights are important for protein engineering efforts aimed at altering radical transfer in RNR.

Using selenocysteine-specific reporters to screen for efficient tRNASec variants

The unique properties of selenocysteine (Sec) have generated an interest in the scientific community to site-specifically incorporate Sec into a protein of choice. Current technologies have rewired the natural Sec-specific translation factor-dependent selenoprotein biosynthesis pathway by harnessing the canonical elongation factor (EF-Tu) to simplify the requirements for Sec incorporation in Escherichia coli. This strategy is versatile and can be applied to Sec incorporation at any position in a protein of interest. However, selenoprotein production is still limited by yield and serine misincorporation. This protocol outlines a method in E. coli to design and optimize tRNA libraries which can be selected and screened for by the use of Sec-specific intein-based reporters. This provides a fast and simple way to engineer tRNAs with enhanced Sec-incorporation ability.

Radicals in Biology: Your Life Is in Their Hands

Radicals in biology, once thought to all be bad actors, are now known to play a central role in many enzymatic reactions. Of the known radical-based enzymes, ribonucleotide reductases (RNRs) are pre-eminent as they are essential in the biology of all organisms by providing the building blocks and controlling the fidelity of DNA replication and repair. Intense examination of RNRs has led to the development of new tools and a guiding framework for the study of radicals in biology, pointing the way to future frontiers in radical enzymology.

Diversity of structures and functions of oxo-bridged non-heme diiron proteins

Oxo-bridged diiron proteins are a distinct class of non-heme iron proteins. Their active sites are composed of two irons that are coordinated by amino acid side chains, and a bridging oxygen that interacts with each iron. These proteins are members of the ferritin superfamily and share the structural feature of a four α-helix bundle that provides the residues that coordinate the irons. The different proteins also display a wide range of structures and functions. A prototype of this family is hemerythrin, which functions as an oxygen transporter. Several other hemerythrin-like proteins have been described with a diversity of functions including oxygen and iron sensing, and catalytic activities. Rubrerythrins react with hydrogen peroxide and rubrerythrin-like proteins possess a rubredoxin domain, in addition to the oxo-bridged diiron center. Other redox enzymes with oxo-bridged irons include flavodiiron proteins that act as O2 or NO reductases, ribonucleotide reductase and methane monooxygenase. Ferritins have an oxo-bridged diiron in the ferroxidase center of the protein, which plays a role in the iron storage function of these proteins. There are also bacterial ferritins that exhibit catalytic activities. The structures and functions of this broad class of oxo-bridged diiron proteins are described and compared in this review.

Introducing Selenocysteine into Recombinant Proteins in Escherichia coli

Selenoproteins contain the 21st amino acid, selenocysteine. Selenocysteine is the only amino acid that is synthesized on its cognate tRNA, and it is inserted at specific recoded UGA stop codons via a complex translation system. Although highly similar to cysteine, selenocysteine has unique properties, including a stronger nucleophilic ability and lower reduction potential. Efforts to site-specifically incorporate selenocysteine to create recombinant selenoproteins involve a recoded UAG stop codon and expression of the necessary selenocysteine translation machinery. This article presents a protocol for expressing and purifying selenoproteins in Escherichia coli. © 2021 Wiley Periodicals LLC. Basic Protocol: Recombinant selenoprotein production in E. coli using a rewired translation system.

Site-Specific Selenocysteine Incorporation into Proteins by Genetic Engineering

Selenocysteine (Sec), a rare naturally proteinogenic amino acid, is the major form of essential trace element selenium in living organisms. Selenoproteins, with one or several Sec residues, are found in all three domains of life. Many selenoproteins play a role in critical cellular functions such as maintaining cell redox homeostasis. Sec is usually encoded by an in-frame stop codon UGA in the selenoprotein mRNA, and its incorporation in vivo is highly species-dependent and requires the reprogramming of translation. This mechanistic complexity of selenoprotein synthesis poses a big challenge to produce synthetic selenoproteins. To understand the functions of natural as well as engineered selenoproteins, many strategies have recently been developed to overcome the inherent barrier for recombinant selenoprotein production. In this review, we will describe the progress in selenoprotein production methodology.

Selective cysteine-to-selenocysteine changes in a [NiFe]-hydrogenase confirm a special position for catalysis and oxygen tolerance

In [NiFe]-hydrogenases, the active-site Ni is coordinated by four cysteine-S ligands (Cys; C), two of which are bridging to the Fe(CO)(CN)2 fragment. Substitution of a single Cys residue by selenocysteine (Sec; U) occurs occasionally in nature. Using a recent method for site-specific Sec incorporation into proteins, each of the four Ni-coordinating cysteine residues in the oxygen-tolerant Escherichia coli [NiFe]-hydrogenase-1 (Hyd-1) has been replaced by U to identify its importance for enzyme function. Steady-state solution activity of each Sec-substituted enzyme (on a per-milligram basis) is lowered, although this may reflect the unquantified presence of recalcitrant inactive/immature/misfolded forms. Protein film electrochemistry, however, reveals detailed kinetic data that are independent of absolute activities. Like native Hyd-1, the variants have low apparent KMH2 values, do not produce H2 at pH 6, and display the same onset overpotential for H2 oxidation. Mechanistically important differences were identified for the C576U variant bearing the equivalent replacement found in native [NiFeSe]-hydrogenases, its extreme O2 tolerance (apparent KMH2 and Vmax [solution] values relative to native Hyd-1 of 0.13 and 0.04, respectively) implying the importance of a selenium atom in the position cis to the site where exogenous ligands (H-, H2, O2) bind. Observation of the same unusual electrocatalytic signature seen earlier for the proton transfer-defective E28Q variant highlights the direct role of the chalcogen atom (S/Se) at position 576 close to E28, with the caveat that Se is less effective than S in facilitating proton transfer away from the Ni during H2 oxidation by this enzyme.

Gated Proton Release during Radical Transfer at the Subunit Interface of Ribonucleotide Reductase

The class Ia ribonucleotide reductase of Escherichia coli requires strict regulation of long-range radical transfer between two subunits, α and β, through a series of redox-active amino acids (Y₁₂₂•[β] ↔ W₄₈?[β] ↔ Y₃₅₆[β] ↔ Y₇₃₁[α] ↔ Y₇₃₀[α] ↔ C₄₃₉[α]). Nowhere is this more precarious than at the subunit interface. Here, we show that the oxidation of Y₃₅₆ is regulated by proton release involving a specific residue, E₅₂[β], which is part of a water channel at the subunit interface for rapid proton transfer to the bulk solvent. An E₅₂Q variant is incapable of Y₃₅₆ oxidation via the native radical transfer pathway or non-native photochemical oxidation, following photosensitization by covalent attachment of a photo-oxidant at position 355[β]. Substitution of Y₃₅₆ for various F_nY analogues in an E₅₂Q-photoβ₂, where the side chain remains deprotonated, recovered photochemical enzymatic turnover. Transient absorption and emission data support the conclusion that Y₃₅₆ oxidation requires E₅₂ for proton management, suggesting its essential role in gating radical transport across the protein-protein interface.