Flow of Electrons and Protons

In the P450cam system, putidaredoxin is an effector protein because it supplies electrons into the active site. Although the first electron to form the ferrous center (B) can come from various sources, it is critical for prolonging catalysis that putidaredoxin deliver the second electron that leads to cleavage of the 0-0 bond (Fig. 6.1). The electron flow into the active site is coupled with proton shuttling that is mediated by the H-bond network surrounding the Fe-02 unit failure in either of these processes leads to dysfunction [4]. It thus appears that metallo-proteins have evolved to synchronize electron and proton transfer to ensure that oxidation of substrates occurs. 

There are three classes of proteins that reversibly bind dioxygen. Hemoglobins occur in a wide variety of organisms, having active sites with a single iron pro-toprophyrin IX cofactor. Hemerythrins are nonheme diiron proteins present in certain species of seaworms. The hemocyanins are dinuclear copper proteins found in the hemolymph of mollusks and arthropods. There have been extensive studies on the physical and structural properties of these proteins, and only a brief mention of their active site structures will be discussed here [9]. 

Dioxygen binding in hemoglobins occurs at the monomeric Fe(II) center in an rj manner - a similar binding mode as described for P450s. There is overwhelming biochemical and physical data to support that in most hemoglobins 

---

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

FIGURE 19-15 Summary of the flow of electrons and protons through the four complexes of the respiratory chain. Electrons reach Q through Complexes I and II. QH2 serves as a mobile carrier of electrons and protons. It passes electrons to Complex III, which passes them to another mobile connecting link, cytochrome c. Complex IV... 

Figure 5 The modus operandi of xanthine oxidase (one subunit) showing the sites of substrate oxidation and oxygen reduction and the flow of electrons and protons (H+).

Cause of auroras An aurora is attributed to solar wind, which is a continuous flow of electrons and protons from the sun. These high-energy, electrically charged particles become trapped by Earth s magnetic field, and they penetrate to the ionosphere. There, the particles collide with oxygen and nitrogen molecules and transfer energy to them. 

The biosynthesis of ATP involves the flow of both electrons (e ) and protons (H ) in the respiratory chain to form ATP by the process known as oxidative phosphorylation. The respiratory chain comprises four structures known as complex I, complex II, complex III and complex IV and a mushroom-shaped structure (ATP synthetase alias Fq/Fi or complex V) that synthesises ATP from ADP and inorganic phosphate (Pi). We will consider the flow of electrons and protons (i) first from complex I, and (ii) from complex II. 

The flow of electrons and protons from NADH+H and FADH2 via complexes I and II, respectively, to complex III of the respiratory chain are shown in Fig. 13.1. Electrons are then transported to complex IV where they combine with oxygen. Meanwhile, protons are pumped into the intermembrane space and are returned to the matrix via the... 

II, III, and IV parts. These four parts form an internal circuit as indicated by the arrows for the flows of electrons and protons in the figure. The half reaction at Part I is the common hydrogen oxidation reaction with an electrode/ PEM interfacial potential around 0 V the half reaction at Parts II and III is the... 

I 6 Dioxygen Binding and Activation Reactive Intermediates 6.1.2.3 Flow of Electrons and Protons... 

Taking a final overview of proteins we have to observe how remarkably suitable they are as semi-soft materials. The different variety of sequences and the different ways their folds enable them to act in a variety of ways within the temperature range of water may well be unique. Remember that their value rests not just in structure but in structure associated with thermodynamically controlled features, i.e. concentration, mobility, and temperature. These structures are dynamic and are an essential feature of physical flow, e.g. of electrons and protons and metabolic activity and as such their connectivity is of the essence of energy uptake and degradation. 

NADH, flavins, and quinones all bind protons in addition to electrons when they undergo reduction. Suppose that an electron-transfer reaction in which protons are taken up from the solution occurs on the matrix side of the inner membrane, and a reaction that releases protons to the solution occurs facing the intermembrane space. If protons are transferred along with electrons between the two sites, the flow of electrons ferries protons across the membrane (fig. 14.20). The key point here is that, unlike reactions in free solution, enzymatic reactions in organized structures such as membranes can have a directional, or vectorial, character. 

Fig. 9. The coupling of electron and proton flow in succinate iquinone oxidoreduc-tases in aerobic (a,c) and anaerobic respiration (b,d), respectively. Positive and negative sides of the membrane are as described for Fig. 1. (a) and (b) Electroneutral reactions as catalyzed by C-type SQR enzymes (a) and D-type E. coli QFR (b). (c) Utilization of a transmembrane electrochemical potential Ap as possibly catalyzed by A-type and B-type enzymes, (d) Electroneutral fumarate reduction by B-type QFR enzymes with a proposed compensatory E-pathway. ...

In the previous chapter we indicated that the components involved with electron flow are situated in the lamellar membranes of chloroplasts such that they lead to a vectorial or unidirectional movement of electrons and protons (see Fig. 5-19). We now return to this theme and focus on the gradients in H+ (protons) thus created. In the light, the difference in the chemical potential of H+ from the inside to the outside of a thylakoid acts as the energy source to drive photophosphorylation. This was first clearly recognized in the 1960s by Peter Mitchell, who received the 1978 Nobel Prize in chemistry for his enunciation of what has become known as the chemiosmotic hypothesis for interpreting the relationship among electron flow, proton movements, and ATP formation. 

PROBLEM 9.4 Complete each of the following equations to show the conjugate acid and the conjugate base formed by proton transfer between the indicated species. Use curved arrows to show the flow of electrons, and specify whether the position of equilibrium lies to the side of reactants or products. 

In contrast, when cells are full of ATP, the reentry of protons into the matrix is minimal, and the high mitochondrial membrane potential slows down the flow of electrons and mitochondrial respiration. 

A Br0nsted-Lowry acid is a proton donor, whereas a Lewis acid is an electron pair acceptor. If all bases donate two electrons, the electron flow is from the base to the acid, rather than from the acid to the base. Therefore, an acid does not donate the proton, but rather the proton is attacked by the base to form a new bond to the proton, as shown previously for water and H+ giving the hydronium ion. As noted, a blue curved arrow is used to indicate the flow of electrons, and the electron flow is always from a source of high electron density to a point of low electron density. In this case, the direction of the arrow is always from the base to the proton of the acid, as illustrated. Although the color blue is used for the electron-rich base as well as the electron flow represented by the arrow, the color red is used for the electron-deficient acid—here, a proton. This convention is used throughout this book. 

In Figure 5.45, the curved arrows are now written to show loss of the protons attached to a hydroxyl group to some generalized base B . Read from the lower left to the upper left, the curved arrows are again used to indicate the flow of electrons and the eventual disposition of the proton and to consummate the movement of nuclei in the reversal of the process of Figure 5.44. 

As the atom (A) to which H is bonded becomes more electronegative the polarization H—A becomes more pronounced and H is more easily lost as H An alternative approach to the same conclusion is based on the equation for proton transfer especially with regard to the flow of electrons as shown by curved arrows... 

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

The oxidation of one NADH and the reduction of one UQ by NADH-UQ reductase results in the net transport of protons from the matrix side to the cytosolic side of the inner membrane. The cytosolic side, where H accumulates, is referred to as the P (for positive) face similarly, the matrix side is the N (for negative) face. Some of the energy liberated by the flow of electrons... 

At the cathode, the electrons and protons combine with oxygen to form water, which flows out of the cell... 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

The enzymic processes appear exactly equivalent, except that protons are removed and supplied through the involvement of peptide side-chains. It is unlikely that a distinct enolate anion is formed instead, we should consider the process as concerted with a smooth flow of electrons. Thus, as a basic group removes a proton from one part of the molecule, an acidic group supplies a proton at another. 

By the use of an anode and cathode system to isolate the electron and proton of hydrogen, fuel cell circuits capture the electron flow and generate electricity. The power plant of the fuel cell vehicle utilizes this electricity to operate motor drive mechanisms. 

---