Bioelectrochemistry: Fundamentals, Experimental Techniques and Applications - Hardcover

Bartlett, Philip N.

Hardcover

ISBN 10: 0470843640 ISBN 13: 9780470843642

Verlag: John Wiley & Sons Inc, 2008

Inhaltsangabe

Bioelectrochemistry: Fundamentals, Experimental Techniques and Application, covers the fundamental aspects of the chemistry, physics and biology which underlie this subject area. It describes some of the different experimental techniques that can be used to study bioelectrochemical problems and it describes various applications of biolelectrochemisty including amperometric biosensors, immunoassays, electrochemistry of DNA, biofuel cells, whole cell biosensors, in vivo applications and bioelectrosynthesis.

 By bringing together these different aspects, this work provides a unique source of information in this area, approaching the subject from a cross-disciplinary viewpoint.

Über diesen Titel

Über die Autorin bzw. den Autor

Philip N. Bartlett is Head of the Electrochemistry Section, Deputy Head of Chemistry for Strategy, and Associate Dean for Enterprise in the Faculty of Natural and Environmental Sciences at the University of Southampton. He received his PhD from Imperial College London and was a Lecturer at the University of Warwick and a Professor for Physical Chemistry at the University of Bath, before moving to his current position. His research interests include bioelectrochemistry, nanostructured materials, and chemical sensors.

Von der hinteren Coverseite

Bioelectrochemistry is the study and application of biological electron transfer processes. Over the last 25 years we have learnt some of the important factors which control the interaction between biological redox partners, including how to apply this knowledge and to start to design electrode surfaces, through deliberate chemical modification, so that the biological molecules will interact in a productive way with the electrode surface and facilitate efficient electron transfer. Over the same period significant parallel developments in physical electrochemistry have meant that the tools and techniques, such as in situ infrared spectroscopy, SERS, EQCM, STM and AFM, now exist to study the electrode solution interface at the molecular level. These techniques are now being used to characterise chemically modified electrode surfaces and to study their interaction with biological molecules.

Bioelectrochemistry: Fundamentals, Experimental Techniques and Applications, covers the fundamental aspects of the chemistry, physics and biology which underlie this subject area. It describes some of the different experimental techniques that can be used to study bioelectrochemical problems and it describes various applications of bioelectrochemistry including amperometric biosensors, in vivo applications and bioelectrosynthesis.

This volume provides a modern view of the field and is appropriate for graduate students and final year undergraduate students in chemistry and biochemistry as well as researchers in related disciplines including biology,physics, physiology and pharmacology.

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Bioelectrochemistry

Fundamentals, Experimental Techniques and Applications

John Wiley & Sons

Copyright © 2008 John Wiley & Sons Ltd
All right reserved.
ISBN: 978-0-470-84364-2

Chapter One

Bioenergetics and Biological Electron Transport

Philip N. Bartlett School of Chemistry, University of Southampton, Southampton, SO17 1BJ, UK

1.1 INTRODUCTION

Electron transfer reactions play a central role in all biological systems because they are essential to the processes by which biological cells capture and use energy. These electron transfer reactions occur in highly organised ways, in electron transport chains in which electron transfer occurs in an ordered way between specific components, and these electron transfer reactions occur at interfaces. In this chapter we will explore the principles behind the organisation and operation of these electron transfer chains from an electrochemical perspective. We will examine the guiding physical principles which govern the efficient operation of biological electron transfer. As we will see, several guiding principles emerge: the tuning of redox potentials for different components in the electron transfer chain to optimise energy efficiency, the control of distance between redox centres to control the kinetics of electron transfer and to achieve specificity, the role of an insulating lipid bilayer to separate charge and store electrochemical energy. Such a study is informative, not only because it tells us about the structure, organisation and function of biological systems, but also because we can learn useful lessons from the study of biological electron transfer systems which have evolved over millions of years which we can use to guide our design of electrochemical systems. For example, electrocatalysis of the four-electron reduction of oxygen to water at neutral pH remains a key barrier to the development of efficient polymer electrolyte membrane (PEM) fuel cells. This same reaction is an important component in the mitochondrial electron transport chain where it is achieved using non-noble metal catalytic sites. A detailed understanding of this biological reaction may give clues to the design of new electrocatalysts for fuel cells. Similarly an understanding of the organisation, light harvesting and electron transfer reactions in the photosynthetic systems in plants and bacteria can inform our design of artificial photosynthetic systems for solar energy conversion. Closer to home, an understanding of the principles which govern efficient biological electron transfer is essential if we wish to exploit biological electron transfer components, such as oxidoreductase enzymes, NADH-dependent dehydrogenases or redox proteins, in biosensors, biofuel cells or bioelectrosynthesis.

This chapter is conceived as a general introduction to biological electron transfer processes for those with little or no prior knowledge of the subject, but with a background in chemistry or electrochemistry. As such it should serve as an introduction to the more specific material to be found in the chapters which follow. At the same time I have tried to emphasise the underlying principles, as seen from an electrochemical perspective, and to bring out similarities rather than to emphasise the differences in detail between the different electron transport chains. Such an interest in the organisation and principles which guide biological electron transfer is directly relevant to current interest in integrated chemical systems.

Broadly this chapter is organised as follows. We begin with a very simple description of the different types of biological cell, bacterial, plant and animal, their internal organisation and the different structures within them. We then consider the structure of biological cells from an electrochemical perspective focusing on the processes of energy transduction and utilisation. This is followed by a more detailed description of the electron transfer chains in the mitochondria and in the photosynthetic membrane and the different redox centres that make up the electron transport chains in these systems. We then describe the governing principles which emerge as the important features in all of these electron transfer processes, before concluding with a discussion of the way these processes are used to drive the thermodynamically uphill synthesis of adenosine triphosphate (ATP).

1.2 BIOLOGICAL CELLS

All living matter is made up of cells and these cells share many common features in terms of their structure and the chemical components which make up the cell. The different types of living cell, from the simplest bacteria to complex plant and mammalian cells, carry out many of their fundamental processes in the same way. Thus the production of chemical energy by conversion of glucose to carbon dioxide is carried out in a similar way across all biological cells. This similarity reflects the common ancestry of all living cells and the process of evolution.

In this section we focus on the internal structure of the different types of biological cell. For a more detailed discussion of biological cells the reader is directed to modern biochemistry or cell biology texts such as that by Lodish and colleagues which provides a beautifully illustrated account of the subject. Biological cells can be divided into two classes: eukaryotic and prokaryotic (Figure 1.1). The eukaryotes include all plants and animals as well as many single cell organisms. The prokaryotes have a simpler cell structure and include all bacteria; they are further divided into the eubacteria and archaebacteria. In the prokaryotic cell there is a single plasma membrane, a phospholipid bilayer, which separates the inside of the cell from the outer world, although in some cases there can also be simple internal photosynthetic membranes.

In contrast, in eukaryotes the inner space within the cell is further divided into a number of additional structures called organelles. These are specialised structures surrounded by their own plasma membranes. Thus, within the eukaryotic cell we find specialised structures, such as the nucleus, which contains the cells DNA and which directs the synthesis on RNA within the cell, peroxisomes, which metabolise hydrogen peroxide, mitochondria, where ATP is generated by oxidation of small molecules, and, in plants, chloroplasts, where light is captured. It is the last two of these, the mitochondria and the chloroplasts which are of most interest to us here since both are central to biological energy transduction and both function electrochemically. These same functions of energy transduction occur in prokaryotes, but in this case they are associated with the outer cell membrane.

Below we give a more detailed account of the electron transfer processes which occur in the mitochondrion and chloroplast, but for now we concentrate on the essential common features. Both processes, the oxidation of small molecules to generate energy in the mitochondrion and the capture of light and its transduction into energy that the cell can use in the chloroplast, occur across energy transducing membranes. In both cases the final product is ATP (see below), a high energy species that is used elsewhere in the cell to drive catabolism (the synthesis of molecules within the cell) and other living processes. An essential feature of the phospholipid bilayers which make up the plasma membranes of the cell and the different organelles within the eukaryotic cell is that although they are permeable to gases such as oxygen and carbon dioxide, they are impermeable to larger molecules such as amino acids or sugars and they are impermeable to ions such as [H.sup.+], [K.sup.+], [Cl.sup.-]. This allows the cell to control the composition of the solution on the two sides of the membrane separately, a process which is achieved by the presence of specific transmembrane proteins, or permeases, within the cell membrane, which control transport of molecules and ions across the membrane.

The energy transducing membranes of eukaryotes and prokaryotes, that is the plasma membrane of simple prokaryotic cells such as bacteria and blue-green algae, the inner membrane of mitochondria and the thylakoid membrane of chloroplasts in eukaryotes, share many common features. All of these membranes have two distinct protein assemblies: the ATPase at which ADP is converted to adenosine triphosphate (ATP) and the energy source electron transport chain which provides the thermodynamic driving force to the synthesis of ATP. These two processes are linked by the directed flow of electrons and protons across the membrane in order to establish an electrochemical potential which is used to drive ATP synthesis. This chemiosmotic model of biological energy transduction, which is essentially an electrochemical model, was first described by Mitchell in 1961 and was recognised by the award of the Nobel prize for chemistry in 1978.

1.3 CHEMIOSMOSIS

The key concept in the chemiosmotic theory is that the synthesis of ATP is linked to the energy source electron transfer chain through the transmembrane proton motive force that is set up. This proton motive force is made up of a contribution from the proton concentration gradient across the membrane, as well as the potential difference across the membrane. Figure 1.2 shows a simplified picture for the mitochondrial membrane and the thylakoid membrane of the chloroplast. In the mitochondria and aerobic bacteria, energy from the oxidation of carbon compounds, such as glucose, is used to pump protons across the membrane. In photosynthesis, energy absorbed from light is used to pump protons across the membrane. In both cases the protons are pumped from the inside, cytoplasmic face, to the outside, exoplasmic face, of the membrane. In addition to the production of ATP, this proton motive force can also be used by the cell to drive other processes such as the rotation of flagella to allow bacteria to swim around in solution or to drive the transport of species across the cell membrane against the existing concentration gradient. Clearly, an essential feature of this energy coupling between the transduction and its use in ATP synthesis or other processes is the presence of an insulating, closed membrane which is impermeable to the transport of protons, since without this it would not be possible to build up a transmembrane proton motive force.

1.3.1 The Proton Motive Force

In this section we consider the thermodynamics of the proton motive force. The proton motive force is a combination of the potential difference across the membrane and the difference in proton concentration across the membrane. Both contribute to the available free energy.

Consider an impermeable membrane separating two solutions, [alpha] and , as shown in Figure 1.3. The electrochemical potential of the proton in solution [alpha], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] [alpha], is given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.1)

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is the chemical potential of the proton in solution [alpha] under standard conditions, R is the gas constant, T the temperature in Kelvin, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] [alpha] is the activity of protons in solution [alpha], F the Faraday constant and [phi]([alpha]) the potential. Notice that there are three contributions to the electrochemical potential of the proton in solution: a chemical term given by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] [alpha], an activity (or concentration) dependent term, and a term which depends on the potential. Similarly, for the protons in solution , we can write

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.2)

At equilibrium, by definition the electrochemical potentials in the two solutions will be equal. That is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.3)

However, a living cell is not at equilibrium. The difference in the electrochemical potential of the proton across the membrane, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], is a measure of the distance of the system from equilibrium and is given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.4)

so that

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.5)

We can simplify Equation (1.5) since the standard chemical potential of the proton is the same in both solutions, thus

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.6)

After collecting together terms Equation (1.5) becomes

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.7)

or

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.8)

The proton motive force itself is then

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.9)

At room temperature

pmf = mV -59 x [DELTA]pH + [DELTA][phi] (1.10)

Thus the proton motive force is made up of two components: the contribution from the difference in proton concentration across the membrane, and the contribution from the potential difference across the membrane. Thus if the membrane is permeable to [Cl.sup.-] or if [H.sup.+] exchanges with another cation (such as [K.sup.+]) the contribution from the potential difference [DELTA][phi] will be small but [DELTA]pH can still be large. This is the situation for the thylakoid membrane in photosynthesis. In contrast, if the membrane is impermeable to anions, [DELTA][phi] can make a more significant contribution. This is the case in the respiring mitochondrion where the total proton motive force of around 220 mV is made up of a transmembrane potential of 160 mV (with the inside of the mitochondrion at a negative potential with respect to the outside - the protons are pumped from inside to out) together with a 60 mV contribution from the one unit pH difference across the membrane.

1.3.2 The Synthesis of ATP

The second part of the story is the synthesis of ATP from ADP and inorganic phosphate and we now turn our attention to the thermodynamics of this process. Adenosine triphosphate, ATP (Figure 1.4), is found in all types of living organism and is the universal mode of transferring energy around the cell in order to drive all the endergonic ([[DELTA].sub.rxn] > 0) reactions necessary for life. These include the synthesis of cellular macromolecules, such as DNA, RNA, proteins and polysaccharides, the synthesis of cellular constituents, such as phospholipids and metabolites, and cellular motion including muscle contraction. In humans it is estimated that on average 40 kg of ATP are used every day corresponding to 1000 turnovers between ADP, ATP and back to ADP for every molecule of ADP in the body each day. In ATP the energy is stored in high-energy phosphoanhydride bonds and hydrolysis of these bonds to produce ADP or AMP (adenosine monophosphate) releases this energy

[ATP.sup.4-] + [H.sub.2]O [right arrow] [ADP.sup.3-] + [H.sup.+] + [HP[O.sub.4].sup.2-] (1.11)

or

[ATP.sup.4-] + [H.sub.2]O [right arrow] [AMP.sup.2-] + [H.sup.+] + [H[P.sub.2][O.sub.7].sup.3-] (1.12)

where [HP[O.sub.4].sup.2-] is inorganic phosphate and [H[P.sub.2][O.sub.7].sup.3-] is inorganic pyrophosphate. In the case of Reaction (1.12), the inorganic pyrophosphate produced is hydrolysed to inorganic phosphate by the enzyme pyrophosphatase. Both reactions, (1.11) and (1.12), have a free energy change of -30.5 kJ [mol.sup.-1] in the standard state at pH 7. When we take account of the actual concentrations of the different species (2.5 mM for ATP, 0.25 mM for ADP and 2.0 mM for [HP[O.sup.4].sup.2-]) this gives a value of about -52 kJ [mol.sup.1] in the living cell. If we assume a transmembrane proton motive force of, say, 200 mV this corresponds to a free energy change for each proton translocated across the membrane of -19.3 kJ [mol.sup.-1]. Thus it is necessary to transfer at least two protons across the membrane for each ATP molecule synthesised.

1.4 ELECTRON TRANSPORT CHAINS

We now turn our attention in more detail onto the electron transport chains in mitochondria, in the chloroplast and in bacteria and focus on the processes occurring in these electron transport chains. As we have seen, electron transport in these systems is central to the process of energy generation in living systems. We can therefore expect these systems to have evolved to operate efficiently and it is of interest to study the way that they operate and the underlying principles involved. We begin by considering the mitochondrial electron transport chain.

(Continues...)

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Details zur Ausgabe

• Verlag: John Wiley & Sons Inc
• Erscheinungsdatum: 2008
• ISBN 10: 0470843640
• ISBN 13: 9780470843642
• Einband: Tapa dura
• Sprache: Englisch
• Auflage: 1
• Anzahl der Seiten: 496
• Kontakt zum Hersteller: Nicht verfügbar