Catálogo de publicaciones - libros

This chapter reviews some of the evidence and the postulated proposals for how oxygenic photosynthesis first emerged as a distinct form of photoautotrophic metabolism using water as an electron donor. This form of photosynthesis is the most successful photoautotrophic metabolism in the contemporary biosphere and is found in all higher plants, green and red algae and both cyano- and oxyphoto-bacteria. We summarize the timetable for emergence and the biogeochemical consequences of oxygenic photosynthesis. Particular attention is paid to evolution of the inorganic core of the enzyme that catalyzes water oxidation, chemical speciation of the inorganic cofactors and possible alterative substrates. We discuss possible mineral remnants of early oxygenic photosynthesis and the emerging role of bicarbonate in assembly of the inorganic core and as an hypothesized evolutionary cofactor.

Photosystem II (PS II) uses light to split water and form O, protons and electrons. Cytochrome oxidase (CcO) is the respiratory enzyme found in mitochondria that catalyzes the reverse reaction: the four-electron reduction of molecular oxygen to water. Because these two enzymes catalyze the opposite directions of the same chemical reaction, they share mechanistic similarities. For example, in both systems, the four-electron oxygen/water reaction occurs in essentially a single reaction step, without requirement of electron or proton transfer from a source outside the active site. In this chapter, the chemistry of water oxidation/oxygen reduction by PS II/CcO is discussed with a consideration of the thermodynamics of the reactions, the roles of protons, and the analogies in the reaction mechanisms of the two systems.

The core structural motif of the photosynthetic pigment-protein complexes that generate a charge-separated state by the absorption of light is remarkably conserved between reaction centers from purple bacteria and Photosystem II (PS II). These systems differ in the functional ability of PS II to make use of much higher energies to oxidize water through electron transfer events involving a tyrosine residue and manganese complex. In this chapter we present models of the evolutionary developments of photosynthesis from the standpoint of the alterations of the pigment-protein complexes needed to perform increasingly complex photochemical reactions. We also describe experimental efforts that are designed to mimic these evolutionary developments by altering the bacterial reaction center such that it gains specific functional features of the oxygen-evolving complex.

An emerging biomimetic approach to exploring protein structure/function relationships has grown out of the creation of proteins from first principles, i.e., design. Over the past two decades, protein design has progressed from demonstrating the feasibility of designing simple secondary structures such as monomeric α-helices to synthesizing holoproteins containing a variety of different metals, light-absorbing pigments and redox cofactors. A range of prosthetic groups are involved in energy transfer, electron transfer and proton-coupled electron transfer in biological energy transduction and, by using rational design, computational and combinatorial approaches, many of these have been introduced into designed proteins. In this chapter we review synthetic work aimed at revealing the engineering specifications and tolerances for the protein/cofactor interactions as well as the influence by the protein matrix on the chemical and redox properties of the six most prominent groups of cofactors involved in the respiratory and photosynthetic processes. Redesign strategies and design of heme proteins, (bacterio)chlorophyll proteins, flavoproteins, amino-acid radical proteins, iron-sulfur proteins, and copper-containing proteins are described. In addition, the structural and functional properties of more complex designed proteins containing multiple cofactors are summarized. The overall engineering principles can then be applied to the understanding of Photosystem II structure and function.

Inspired by the Photosystem II reaction center and the water oxidation chemistry that it performs, we aim to develop artificial photosynthesis for fuel production. Besides the original work we do in this direction, we also acquire knowledge feedback from our novel compounds. Our man-made systems create new perspectives on electron and proton transfer, bioinorganic chemistry, excitation energy transfer and other issues that are central to photosynthesis research. In this chapter we describe some of the highlights in our research and the conclusions they have generated.