Proton bidirectional

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

Kottra, G. and Daniel, H. (2001) Bidirectional electrogenic transport of peptides by the proton-coupled carrier PEPT1 in Xenopus laevis oocytes its asymmetry and symmetry. The Journal of Physiology, 536 (Pt 2), 495-503. 

Irisawa, H. and Sato, R. (1986). Intra- and extracellular actions of proton on the calcium current of isolated guinea pig ventricular cells. Circulation Research 59 348-355. lessen, T.M. and Kandel, E.R. (1993). Synaptic transmissiuon A bidirectional and self-modifiable form of cell-cell communication. Neuron 10 (Suppl.) l-30. 

In the present case, where the experimentally determined barrier (AG ) to rotation is approximately 24.5 kcal mol-1 [44], then at 160 °C a single rotamer has a half life of about 0.17 s. Thus, if one selectively polarizes one of the three Ha, Hb and Hc (see 46 in Fig. 17) protons and - after appropriate time delays -assays for the location of that polarization, a clearcut distinction between predominantly unidirectional rotation and bidirectional rotation is available. In the former case, a disproportionate share of the polarization that has moved should be transferred to a resonance for only one of the two other protons. In the latter case, the polarization that moves should be transferred equally to the remaining two resonances. 

Many definitions and descriptions of HAT, prior to the emergence of PCET as a field of study, did not adequately take into account the complexity embodied by Fig. 17.1. HAT is traditionally defined as the transfer of an electron and a proton from one location to another along a spatially coincidental pathway. In this case, the electron and proton are donated from one atom and they are accepted by another atom. These transfers are well described mechanistically as the diagonal pathway of Fig. 17.1 and they have been treated formally by a number of investigators [3, 4]. However, many reactions treated within a formalism of HAT are more complex as ET and PT are site-differentiated either along uni- or bidirectional pathways. As will be discussed in this chapter, traditional descriptions of HAT do not address how the electron and proton transfer events are coordinated mechanistically in these more complicated reactions and more general treatments of PCET are warranted. 

Figure 17.20 Schematic depiction of metal-centered bidirectional PCET. Oxidation (or reduction) at the M-OH center is coupled to loss (or gain) of a proton and an increase (or decrease) in the M-O bond order.

Other oxidases also derive function from bidirectional PCET pathways at the enzyme active site. The recent crystal structures of PSII [206, 207] support suggestions that as the oxygen evolving complex (OEC) steps through its various S-states [208, 209], substrate derived protons are shuttled to the lumen via a proton exit channel, the headwater of which appears to be the Dgi residue hydrogen-bonded to Mn-bound water [210]. The protons are liberated with the proton-coupled oxidation of the Mn-OH2 site. As shown by the structure reproduced in Fig. 17.23, Dgj is diametrically opposite to Y, which has long been known [148, 151, 152] to be the electron relay between the PS II reaction center and OEC. Notwithstanding,... 

Bidirectional PCET is also featured on the reduction side of the photosynthetic apparatus. In the bacterial photosynthetic reaction center, two sequential photo-induced ET reactions from the P680 excited state to a quinone molecule (Qg) are coupled to the uptake of two protons to form the hydroquinone [213-215]. This diffuses into the inter-membrane quinone pool and is re-oxidized at the Qq binding site of the cytochrome bcj and coupled to translocation of the protons across the membrane, thereby driving ATP production. These PCET reactions are best described by a Type D mechanism because the PCET of Qg appears to involve specifically engineered PT coordinates among amino acid residues [215]. In this case PT ultimately takes place to and from the bulk solvent. Coupling remains tight in... 

Bidirectional PCET also manifests itself in reductases. Crystal structures of hydrogenases [216-218] indicate that the mechanism for hydrogen production occurs by transporting protons into the active site along pathways distinct from those traversed by the electron equivalents. Electrons are putatively injected into the active site via a chain of [FeS] clusters, while proton channels and acid-base residues at the active site manage the substrate inventory. 

OCTNl (SLC22A4) is expressed in the kidney, trachea, and bone marrow and operates as an organic cation-proton exchanger OCTNl likely functions as a bidirectional pH- and ATP-dependent transporter at the apical membrane in renal tubular epithelial cells. 

Scheme 5.1 (a) Concerted bidirectional proton and electron transfer in of tyrosine in... 

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