Stable pentaammineruthenium(III) complexes of reductic acids: synthesis, linkage isomers, and autoxidation kinetics

Visualizing the protons in a metalloenzyme electron proton transfer pathway

In redox metalloenzymes, the process of electron transfer often involves the concerted movement of a proton. These processes are referred to as proton-coupled electron transfer, and they underpin a wide variety of biological processes, including respiration, energy conversion, photosynthesis, and metalloenzyme catalysis. The mechanisms of proton delivery are incompletely understood, in part due to an absence of information on exact proton locations and hydrogen bonding structures in a bona fide metalloenzyme proton pathway. Here, we present a 2.1-Å neutron crystal structure of the complex formed between a redox metalloenzyme (ascorbate peroxidase) and its reducing substrate (ascorbate). In the neutron structure of the complex, the protonation states of the electron/proton donor (ascorbate) and all of the residues involved in the electron/proton transfer pathway are directly observed. This information sheds light on possible proton movements during heme-catalyzed oxygen activation, as well as on ascorbate oxidation.

Proton delivery to ferryl heme in a heme peroxidase: enzymatic use of the Grotthuss mechanism

We test the hypothesized pathway by which protons are passed from the substrate, ascorbate, to the ferryl oxygen in the heme enzyme ascorbate peroxidase (APX). The role of amino acid side chains and bound solvent is demonstrated. We investigated solvent kinetic isotope effects (SKIE) for the wild-type enzyme and several site-directed replacements of the key residues which form the proposed proton path. Kinetic constants for H(2)O(2)-dependent enzyme oxidation to Compound I, k(1), and subsequent reduction of Compound II, k(3), were determined in steady-state assays by variation of both H(2)O(2) and ascorbate concentrations. A high value of the SKIE for wild type APX ((D)k(3) = 4.9) as well as a clear nonlinear dependence on the deuterium composition of the solvent in proton inventory experiments suggest the simultaneous participation of several protons in the transition state for proton transfer. The full SKIE and the proton inventory data were modeled by applying Gross-Butler-Swain-Kresge theory to a proton path inferred from the known structure of APX. The model has been tested by constructing and determining the X-ray structures of the R38K and R38A variants and accounts for their observed SKIEs. This work confirms APX uses two arginine residues in the proton path. Thus, Arg38 and Arg172 have dual roles, both in the formation of the ferryl species and binding of ascorbate respectively and to facilitate proton transfer between the two.

Crystal structure of the ascorbate peroxidase-salicylhydroxamic acid complex

Ascorbate peroxidase is a bifunctional peroxidase that catalyzes the H(2)O(2)-dependent oxidation of both ascorbate and various aromatic substrates. The ascorbate binding site was recently identified as being close to the gamma-heme edge [Sharp, K. H., Mewies, M., Moody, P. C. E., and Raven, E. L. (2003)Nat. Struct. Biol. 10, 303-307]. In this work, the X-ray crystal structure of recombinant soybean cytosolic ascorbate peroxidase (rsAPX) in complex with salicylhydroxamic acid (SHA) has been determined to 1.46 A. The SHA molecule is bound close to the delta-heme edge in a cavity that connects the distal side of the heme to the surface of the protein. There are hydrogen bonds between the phenolic hydroxide of the SHA and the main chain carbonyl of Pro132, between the carbonyl oxygen of SHA and the side chain guanadinium group of Arg38, and between the hydroxamic acid group and the indole nitrogen of Trp41. The structure provides the first information about the location of the aromatic binding site in ascorbate peroxidase and, together with our previous data [Sharp, K. H., et al. (2003) Nat. Struct. Biol. 10, 303-307], completes the structural description of the binding properties of ascorbate peroxidase. The mechanistic implications of the results are discussed in terms of our current understanding of how APX catalyzes oxidation of different types of substrates bound at different locations.

Photoaccumulation of two ascorbyl free radicals per photosystem I at 200 K

Illumination of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 at 200 K in the presence of ascorbate leads to the formation of two ascorbyl radicals per PSI, which are formed by P700(+) reduction by ascorbate. During photoaccumulation, one half of the ascorbyl radicals is formed with a halftime of 1 min and the other half with a halftime of 7 min. Pulsed electron paramagnetic resonance (EPR) experiments with protonated/deuterated PSI show that a PSI proton/deuteron is strongly coupled to the ascorbyl radical. Our data indicate that reactive ascorbate molecules bind to PSI at two specific locations, which might be symmetrically located with respect to the pseudo-C(2) axis of symmetry of the heterodimeric core of PSI. Reduction of P700(+) by ascorbate leads to multiple turnover of PSI photochemistry, resulting in partial photoaccumulation of the doubly reduced species (F(A)(-), F(B)(-)). A modified form of F(B)(-)-in accordance with Chamorovsky and Cammack [Biochim. Biophys. Acta 679 (1982) 146-155], but not of F(A)(-), is observed by EPR after illumination at 200 K, which indicates that reduction of F(B) at 200 K is followed by some relaxation process, in line with this cluster being the most exposed to the solvent.

Crystal structure of the ascorbate peroxidase-ascorbate complex

Heme peroxidases catalyze the H2O2-dependent oxidation of a variety of substrates, most of which are organic. Mechanistically, these enzymes are well characterized: they share a common catalytic cycle that involves formation of a two-electron, oxidized Compound I intermediate followed by two single-electron reduction steps by substrate. The substrate specificity is more diverse--most peroxidases oxidize small organic substrates, but there are prominent exceptions--and there is a notable absence of structural information for a representative peroxidase-substrate complex. Thus, the features that control substrate specificity remain undefined. We present the structure of the complex of ascorbate peroxidase-ascorbate. The structure defines the ascorbate-binding interaction for the first time and provides new rationalization of the unusual functional features of the related cytochrome c peroxidase enzyme, which has been a benchmark for peroxidase catalysis for more than 20 years. A new mechanism for electron transfer is proposed that challenges existing views of substrate oxidation in other peroxidases.