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A4. Transport of Protons - Biology


Driven by oxidation - The proton gradient formed during aerobic oxidation and photosynthesis in mitochondria and chloroplast, respectively, is paid for by free energy decreases associated with oxidation of organic molecules.

Driven by ATP cleavage - As mentioned above, protons are transported into the the lumen of the stomach by a K+-H+ ATPase.

Driven by light - Photosynthetic bacteria have a membrane protein called bacteriorhodopsin which contains retinal, a conjugated polyene derived from beta-carotene. The retinal is covalently attached to the protein through a Schiff base linkage to an epsilon amino group of Lys (much as pyridoxal phosphate is in PLP-dependent enzymes). Bacteriorhodopsin is analogous to the visual pigment protein rhodopsin in retinal cells. Absorption of light by the retinal induces a conformational changes in the all trans-retinal, which causes an associated conformational change in bacteriorhodopsin. The initial state (BR) changes through a series of intermediates (K, L, M, N, and O). Various side chains and the protonated N of the Schiff base of retinal change their relative positions with respect to each other, which leads to changes in protonation states of the side chains and ultimately vectorial discharge of protons through the membrane. As the M state forms, H+ is moved to the extracellular side of the membrane (as shown below). Later a H+ is taken up on the cytoplasmic side (at the Schiff base of the retinal link) leading to reformation of the BR state. Experiments have been done to trap the protein in some of these intermediate states. In one (Leuke et al, 1999), a mutant (Asp 96 to Asparagine or D96N) trappped the protein in a state, MN, that occurs after a H+ has been moved to the extracellular side but before a compensatory H+ has been taken up on the cytoplasmic face. The mutation hinders the reuptake of the proton.

Animation of bacteriorhodopsin

Figure: BACTERIORHODOPSIN AND PROTON TRANSPORT


Figure: A NEW VERSION SHOWING PROTON TRANSFER IN BACTERIORHODOPSIN

Jmol: Updated Bacteriorhodopsin Crystallized From Bicelles - JSMol (HTML5)

Contributors

  • Prof. Henry Jakubowski (College of St. Benedict/St. John's University)

Proton Transport

Proton-transfer reactions

Proton transfer, either from a protonated molecule to a neutral molecule or from a neutral molecule to a negative ion, often proceeds at or near the collision rate when exothermic (see Figure 4 ). When the exothermicity is large, internal energy appearing in the protonated product may be sufficient to cause dissociation with the loss of one or more neutral fragments. Thus, as was the case with dissociative electron-transfer reactions, the product ions again provide a signature of the reactant molecule so that these reactions also are useful in the analysis of gases by chemical ionization mass spectrometry. Virtually any molecule can be chemically ionized with an appropriate proton donor and often this is possible with high efficiency and selectivity. Solvation can influence the rate of proton transfer as is illustrated in Figure 5 , but proton transfer generally remains fast until solvation renders the reaction endothermic. There is a growing interest in the ion chemistry of multiply protonated biological ions such as multiply protonated peptides and proteins. Proton transfer from multiply protonated cations may occur according to the following reaction:

Figure 4 . A correlation between the efficiency of proton transfer, kexp/kc, and its overall change in enthalpy, ΔH°, at room temperature. The measurements were taken using the flowing-afterglow technique. Reproduced with permission from Bohme DK (1981) Transactions of the Royal Society of Canada 19: 265–284.

Figure 5 . Observed variations with the number of water molecules in the rate coefficient for proton-transfer reactions between hydrated hydronium ions and various molecules B at room temperature. The measurements were taken using the flowing-afterglow technique. Adapted with permission from Bohme DK, Mackay GI and Tanner SD (1979) Journal of the American Chemical Society 101: 3724–3730.

Again, a ‘reverse activation energy’ is involved in such reactions as a consequence of the Coulombic repulsion between the product ions.


Show/hide words to know

ATP: adenosine triphosphate. ATP is the energy-carrying molecule of all cells. more

Gradient: a slow or gradual change from one thing to another a slope or hill.

Ion: an atom or molecule that does not have the same number of electrons as it has protons. This gives the atom or molecule a negative or positive charge. more

Potential energy: energy that an object or molecule has due to its position or structure.

Proton: the part of a molecule that has a positive electric charge a hydrogen ion that has lost its electron (written as H+).

Transporter: a protein found in the cell membrane that helps control what can enter or exit a cell.

A cell membrane can be thought of as a dam. Click for more detail.


Translocation into the Mitochondrial Matrix Depends on a Signal Sequence and Protein Translocators

Proteins imported into the matrix of mitochondria are usually taken up from the cytosol within seconds or minutes of their release from ribosomes. Thus, in contrast to the protein translocation into the ER described later, mitochondrial proteins are first fully synthesized as precursor proteins in the cytosol and then translocated into mitochondria by a posttranslational mechanism. Most of the mitochondrial precursor proteins have a signal sequence at their N terminus that is rapidly removed after import by a protease (the signal peptidase) in the mitochondrial matrix. The signal sequences are both necessary and sufficient for import of the proteins that contain them: through the use of genetic engineering techniques, these signals can be linked to any cytosolic protein to direct the protein into the mitochondrial matrix. Sequence comparisons and physical studies of different matrix signal sequences suggest that their common feature is the propensity to fold into an amphipathic α helix, in which positively charged residues are clustered on one side of the helix, while uncharged hydrophobic residues are clustered on the opposite side (Figure 12-23). This configuration—rather than a precise amino acid sequence—is recognized by specific receptor proteins that initiate protein translocation.

Figure 12-23

A signal sequence for mitochondrial protein import. Cytochrome oxidase is a large multiprotein complex located in the inner mitochondrial membrane, where it functions as the terminal enzyme in the electron-transport chain (discussed in Chapter 14). (A) (more. )

Protein translocation across mitochondrial membranes is mediated by multi-subunit protein complexes that function as protein translocators: the TOM complex functions across the outer membrane, and two TIM complexes, the TIM23 and TIM22 complexes, function across the inner membrane (Figure 12-24). TOM and TIM stand for translocase of the outer and inner mitochondrial membranes, respectively. These complexes contain some components that act as receptors for mitochondrial precursor proteins and other components that form the translocation channel. The TOM complex is required for the import of all nucleus-encoded mitochondrial proteins. It initially transports their signal sequences into the intermembrane space and helps to insert transmembrane proteins into the outer membrane. The TIM23 complex then transports some of these proteins into the matrix space, while helping to insert transmembrane proteins into the inner membrane. The TIM22 complex mediates the insertion of a subclass of inner membrane proteins, including the carrier protein that transports ADP, ATP, and phosphate. A third protein translocator in the inner mitochondrial membrane, the OXA complex, mediates the insertion of inner membrane proteins that are synthesized within the mitochondria. It also helps to insert some proteins that are initially transported into the matrix by the TOM and TIM complexes.

Figure 12-24

Three protein translocators in the mitochondrial membranes. The TOM and TIM complexes and the OXA complex are multimeric membrane protein assemblies that catalyze protein transport across mitochondrial membranes. The protein components of the TIM22 and (more. )


Integration of a 'proton antenna' facilitates transport activity of the monocarboxylate transporter MCT4

Monocarboxylate transporters (MCTs) mediate the proton-coupled transport of high-energy metabolites like lactate and pyruvate and are expressed in nearly every mammalian tissue. We have shown previously that transport activity of MCT4 is enhanced by carbonic anhydrase II (CAII), which has been suggested to function as a 'proton antenna' for the transporter. In the present study, we tested whether creation of an endogenous proton antenna by introduction of a cluster of histidine residues into the C-terminal tail of MCT4 (MCT4-6xHis) could facilitate MCT4 transport activity when heterologously expressed in Xenopus oocytes. Our results show that integration of six histidines into the C-terminal tail does indeed increase transport activity of MCT4 to the same extent as did coexpression of MCT4-WT with CAII. Transport activity of MCT4-6xHis could be further enhanced by coexpression with extracellular CAIV, but not with intracellular CAII. Injection of an antibody against the histidine cluster into MCT4-expressing oocytes decreased transport activity of MCT4-6xHis, while leaving activity of MCT4-WT unaltered. Taken together, these findings suggest that transport activity of the proton-coupled monocarboxylate transporter MCT4 can be facilitated by integration of an endogenous proton antenna into the transporter's C-terminal tail.

Keywords: Xenopus oocytes ion-selective microelectrodes proton-collecting antenna transport metabolon.


Contents

A carrier is not open simultaneously to both the extracellular and intracellular environments. Either its inner gate is open, or outer gate is open. In contrast, a channel can be open to both environments at the same time, allowing the molecules to diffuse without interruption. Carriers have binding sites, but pores and channels do not. [5] [6] [7] When a channel is opened, millions of ions can pass through the membrane per second, but only 100 to 1000 molecules typically pass through a carrier molecule in the same time. [8] Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases. [9]

Active transport is the movement of a substance across a membrane against its concentration gradient. This is usually to accumulate high concentrations of molecules that a cell needs, such as glucose or amino acids. If the process uses chemical energy, such as adenosine triphosphate (ATP), it is called primary active transport. Secondary active transport involves the use of an electrochemical gradient, and does not use energy produced in the cell. [10] Unlike channel proteins which only transport substances through membranes passively, carrier proteins can transport ions and molecules either passively through facilitated diffusion, or via secondary active transport. [11] A carrier protein is required to move particles from areas of low concentration to areas of high concentration. These carrier proteins have receptors that bind to a specific molecule (substrate) needing transport. The molecule or ion to be transported (the substrate) must first bind at a binding site at the carrier molecule, with a certain binding affinity. Following binding, and while the binding site is facing the same way, the carrier will capture or occlude (take in and retain) the substrate within its molecular structure and cause an internal translocation so that the opening in the protein now faces the other side of the plasma membrane. [12] The carrier protein substrate is released at that site, according to its binding affinity there.

Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific transport proteins and requires no energy input. Facilitated diffusion is used especially in the case of large polar molecules and charged ions once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the bilayers. The type of carrier proteins used in facilitated diffusion is slightly different from those used in active transport. They are still transmembrane carrier proteins, but these are gated transmembrane channels, meaning they do not internally translocate, nor require ATP to function. The substrate is taken in one side of the gated carrier, and without using ATP the substrate is released into the cell. They may be used as potential biomarkers.

Reverse transport, or transporter reversal, is a phenomenon in which the substrates of a membrane transport protein are moved in the opposite direction to that of their typical movement by the transporter. [13] [14] [15] [16] [17] Transporter reversal typically occurs when a membrane transport protein is phosphorylated by a particular protein kinase, which is an enzyme that adds a phosphate group to proteins. [13] [14]

1: Channels/pores Edit

  • α-helical protein channels such as voltage-gated ion channel (VIC), ligand-gated ion channels(LGICs)
  • β-barrel porins such as aquaporin
  • channel-forming toxins, including colicins, diphtheria toxin, and others
  • Nonribosomally synthesized channels such as gramicidin which function in export of enzymes that digest bacterial cell walls in an early step of cell lysis.

Facilitated diffusion occurs in and out of the cell membrane via channels/pores and carriers/porters.

Channels are either in open state or closed state. When a channel is opened with a slight conformational switch, it is open to both environment simultaneously (extracellular and intracellular)

Pores are continuously open to these both environment, because they do not undergo conformational changes. They are always open and active.

2: Electrochemical potential-driven transporters Edit

Also named carrier proteins or secondary carriers.

3: Primary active transporters Edit

    3.A: P-P-bond-hydrolysis-driven transporters :
      (ABC transporter), such as MDR, CFTR ( "V" related to vacuolar ). ( "P" related to phosphorylation), such as :

    4: Group translocators Edit

    The group translocators provide a special mechanism for the phosphorylation of sugars as they are transported into bacteria (PEP group translocation)

    5: Electron carriers Edit

    The transmembrane electron transfer carriers in the membrane include two-electron carriers, such as the disulfide bond oxidoreductases (DsbB and DsbD in E. coli) as well as one-electron carriers such as NADPH oxidase. Often these redox proteins are not considered transport proteins.

    Every carrier protein, especially within the same cell membrane, is specific to one type or family of molecules. For example, GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer. Other specific carrier proteins also help the body function in important ways. Cytochromes operate in the electron transport chain as carrier proteins for electrons. [10]

    A number of inherited diseases involve defects in carrier proteins in a particular substance or group of cells. Cysteinuria (cysteine in the urine and the bladder) is such a disease involving defective cysteine carrier proteins in the kidney cell membranes. This transport system normally removes cysteine from the fluid destined to become urine and returns this essential amino acid to the blood. When this carrier malfunctions, large quantities of cysteine remain in the urine, where it is relatively insoluble and tends to precipitate. This is one cause of urinary stones. [18] Some vitamin carrier proteins have been shown to be overexpressed in patients with malignant disease. For example, levels of riboflavin carrier protein (RCP) have been shown to be significantly elevated in people with breast cancer. [19]

    1. ^Membrane+transport+proteins at the US National Library of Medicine Medical Subject Headings (MeSH)
    2. ^ Perland, Emelie Bagchi, Sonchita Klaesson, Axel Fredriksson, Robert (2017-09-01). "Characteristics of 29 novel atypical solute carriers of major facilitator superfamily type: evolutionary conservation, predicted structure and neuronal co-expression". Open Biology. 7 (9): 170142. doi:10.1098/rsob.170142. ISSN2046-2441. PMC5627054 . PMID28878041.
    3. ^
    4. Hediger, Matthias A. Romero, Michael F. Peng, Ji-Bin Rolfs, Andreas Takanaga, Hitomi Bruford, Elspeth A. (February 2004). "The ABCs of solute carriers: physiological, pathological and therapeutic implications of human membrane transport proteinsIntroduction". Pflügers Archiv: European Journal of Physiology. 447 (5): 465–468. doi:10.1007/s00424-003-1192-y. ISSN0031-6768. PMID14624363. S2CID1866661.
    5. ^ ab
    6. Perland, Emelie Fredriksson, Robert (March 2017). "Classification Systems of Secondary Active Transporters". Trends in Pharmacological Sciences. 38 (3): 305–315. doi:10.1016/j.tips.2016.11.008. ISSN1873-3735. PMID27939446.
    7. ^ Sadava, David, et al. Life, the Science of Biology, 9th Edition. Macmillan Publishers, 2009. 1-4292-1962-9. p. 119.
    8. ^
    9. Cooper, Geoffrey (2009). The Cell: A Molecular Approach. Washington, DC: ASM Press. p. 62. ISBN9780878933006 .
    10. ^ Thompson, Liz A. Passing the North Carolina End of Course Test for Biology. American Book Company, Inc. 2007. 1-59807-139-4. p. 97.
    11. ^
    12. Assmann, Sarah (2015). "Solute Transport". In Taiz, Lincoln Zeiger, Edward (eds.). Plant Physiology and Development. Sinauer. p. 151.
    13. ^ Sadava, David, Et al. Life, the Science of Biology, 9th Edition. Macmillan Publishers, 2009. 1-4292-1962-9. p. 119.
    14. ^ ab Ashley, Ruth. Hann, Gary. Han, Seong S. Cell Biology. New Age International Publishers. 8122413978. p. 113.
    15. ^ Taiz, Lincoln. Zeigler, Eduardo. Plant Physiology and Development. Sinauer Associates, 2015. 978-1-60535-255-8. pp. 151.
    16. ^ Kent, Michael. Advanced Biology. Oxford University Press US, 2000. 0-19-914195-9. pp. 157–158.
    17. ^ ab
    18. Bermingham DP, Blakely RD (October 2016). "Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters". Pharmacol. Rev. 68 (4): 888–953. doi:10.1124/pr.115.012260. PMC5050440 . PMID27591044.
    19. ^ ab
    20. Miller GM (January 2011). "The emerging role of trace amine-associated receptor 1 in the functional regulation of monoamine transporters and dopaminergic activity". Journal of Neurochemistry. 116 (2): 164–176. doi:10.1111/j.1471-4159.2010.07109.x. PMC3005101 . PMID21073468.
    21. ^
    22. Scholze P, Nørregaard L, Singer EA, Freissmuth M, Gether U, Sitte HH (2002). "The role of zinc ions in reverse transport mediated by monoamine transporters". The Journal of Biological Chemistry. 277 (24): 21505–13. doi: 10.1074/jbc.M112265200 . PMID11940571.
    23. ^
    24. Robertson SD, Matthies HJ, Galli A (2009). "A closer look at amphetamine-induced reverse transport and trafficking of the dopamine and norepinephrine transporters". Molecular Neurobiology. 39 (2): 73–80. doi:10.1007/s12035-009-8053-4. PMC2729543 . PMID19199083.
    25. ^
    26. Kasatkina LA, Borisova TA (November 2013). "Glutamate release from platelets: exocytosis versus glutamate transporter reversal". The International Journal of Biochemistry & Cell Biology. 45 (11): 2585–2595. doi:10.1016/j.biocel.2013.08.004. PMID23994539.
    27. ^ Sherwood, Lauralee. 7th Edition. Human Physiology. From Cells to Systems. Cengage Learning, 2008. p. 67
    28. ^ Rao, PN, Levine, E et al. Elevation of Serum Riboflavin Carrier Protein in Breast Cancer. Cancer Epidemiol Biomarkers Prev. Volume 8 No 11. pp. 985–990

    Anderle, P., Barbacioru,C., Bussey, K., Dai, Z., Huang, Y., Papp, A., Reinhold, W., Sadee, W., Shankavaram, U., & Weinstein, J. (2004). Membrane Transporters and Channels: Role of the Transportome in Cancer Chemosensitivity and Chemoresistance. Cancer Research, 54, 4294-4301.


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    The competition transport of sodium ions and protons in the cytoplasmic access channel of the Na+,K+-ATPase

    The changes in capacitance and conductance of lipid bilayer membranes have been studied with adsorbed membrane fragments containing Na+,K+-ATPase. These changes have been initiated by fast release of protons from a bound form (&ldquocaged H+&rdquo) induced by an UV flash. The changes of the capacitance in the presence of Na+,K+-ATPase were affected by the frequency of the applied voltage, pH and the concentration of sodium ions. Addition of sodium ions altered the changes of capacitance caused by a pH jump in the medium due to caged H+ photolysis, and the magnitude and sign of this effect depended on the initial pH. These results are explained by competitive binding of sodium ions and protons to the ion-binding sites of the Na+,K+-ATPase at its cytoplasmic side. The pH at which the sign of the sodium ion effect changed allows the evaluation of the pK of the proton binding site, which is about 7.6.

    Journal

    Biochemistry (Moscow) Supplement Series A: Membrane and Cell Biology &ndash Springer Journals


    Generating an Energy Carrier: ATP

    As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

    To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 2). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.

    In Summary: Light-Dependent Reactions

    The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of ions. The ions flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions.


    Proton Gradient Across Membranes

    A vital active transport process that occurs in the electron transport process in the membranes of both mitochondria and chloroplasts is the transport of protons to produce a proton gradient. This proton gradient or proton potential powers the phosphorylation of ATP associated with ATP synthase.

    The electron transport process in the thylakoid membranes of chloroplasts provides energetic electrons to the cytochrome complex which pumps protons across the membrane in the direction opposite the concentration gradient. The potential provided by this proton gradient then powers the conversion of ADP to ATP.

    In the case of the mitochondrial membrane, the goal is to produce ATP as an energy currency for cell processes by oxidizing a food material (oxidative phosphorylation). The Complexes I, III and IV of the electron transport process pump protons against their concentration gradient . That proton potential provides the energy for ATP synthase to accomplish the ADP to ATP process of phosphorylation.


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