L11 Carriers pumps 2011 - COPYRIGHT Mammalian Physiology...

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1/11 COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 2011 MEMBRANE TRANSPORT: PUMPS AND CARRIERS 1) Carrier-mediated transport: pumps, primary active transport. Like channels, pumps are membrane proteins. However, unlike channels, membrane pumps are also enzymes (e.g. ATPases) that hydrolyze ATP to ADP. The energy liberated in this hydrolysis is used to generate transmembrane concentration differences as, for example, for Na + and K + (Fig. 1). The Na/K ATPase is the example par excellence for pumps located in cell membranes. Intracellular structures such as mitochondria or vacuoles may have other types of pumps (Table 1). The Na/K pump aka Na/K ATPase is responsible for maintaining intracellular K + concentrations high and Na + concentrations low. Both K + and Na + are transported uphill, against their concentration differences. Since the transport of Na + and K + is energetically uphill, it is called active transport, in particular primary active transport because it is directly coupled to the hydrolysis of ATP. Fig. 1. The ubiquitous Na/K ATPase of eukaryotic cell membranes. Note the pump’s association with K + - and Na + channels, with symporters such as the Na + -glucose and the Na + /K + /2Cl - cotransporter and with antiporters such as the Na + /H + exchanger and 3Na + /Ca 2+ exchanger. These associations accomplish specific functional tasks: electrical communication, nutrient transport and the regulation of intracellular pH and Ca 2+ and Mg 2+ concentrations. Other ion pumps, their occurrence, and energy sources are shown in Table 1. Some pumps are electrogenic, others are electrically silent. For example, H + -pumps are highly electrogenic, generating current via the transport of H + . The Na/K pump is mildly electrogenic transporting 3 Na + ions out of the cell for every 2 K + ions into the cell (Fig. 1). The H/K pump secreting acid in the stomach is electroneutral, exchanging H + and K + with a stoichiometry of 2:2.
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2/11 With the exception of light-driven rhodopsin pumps, all pumps are reversible. Imposing pump-specific ion gradients can generate ATP from ADP. However, lightning bugs (fireflies) are capable of converting the energy of ATP hydrolysis back to light with nearly 100 % efficiency, using luciferin and luciferase. Table 1. Primary active transport pumps in biological membranes. PUMP ION ENERGY SOURCE PRESENCE bacteriorhodopsin H + light halobacteria halorhodopsin Cl - light halobacteria ion-translocating Na + decarboxylation bacteria NADH oxidase Na + redox energy alkalophilic bacteria cytochrome oxidase H + redox energy mitochondria, bacteria H-PPase H + hydrolysis of pyrophosphates plant vacuoles transport ATPases P-type H + , Na + , K + , Ca 2+ ATP hydrolysis ubiquitous transport ATPases V-type H + , Na + ATP hydrolysis ubiquitous a) Structure of the Na/K ATPase. The Na/K ATPase is a heterodimer, consisting of an α- and β-subunit (Fig. 2). The α-subunit has a molecular weight of about 112 kDa, and the β-subunit has a MW between 50 and 60 kDa. A γ-subunit of 7 kDa has been detected
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This note was uploaded on 04/09/2011 for the course BIOAP 4580 taught by Professor Beyenbach,k. during the Spring '11 term at Cornell University (Engineering School).

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L11 Carriers pumps 2011 - COPYRIGHT Mammalian Physiology...

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