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Laura Parker

ION AND SOLUTE PUMPS Active transport to develop gradients Complex mechanisms due to energy coupling Slow transport rates, highly specific Variety of energy sources Light – rhodopsin Redox potential Decarboxylation Pyrophosphate hydrolysis **ATP** - ubiquitous Ion gradients drive many processes Mechanical Chemical (ATP synth) Signal trans. Chemiosmotic (glucose) Osmotic volume regulation BACTERIORHODOPSIN: light-driven proton pump Protons return to generate ATP Structure: 7TM helices enclose RETINAL (chromophore – light absorbing) Retinal divides proton conducting pathway in half Cytoplasmic – hydrophobic/narrow  barrier to proton movement Extracellular – hydrophilic/wide  allows proton to leave once halfway through D96 – pKa of 10  protonated under neutral circumstances D85 – pKa of 2  deprotonated usually Proton flow: Cytoplasm  D96  retinal  light driven isomerization  D85  another retinal conf. change  D85 pKa drops  extracellular **small conf. changes in retinal make it act as transient switch** similarities between pumping and signal transduction Electroneutral pumps: all energy of ATP hydrolysis (7.3 kcal/mol) goes into generating chem.. gradient electrogenic pumps squander much energy as membrane potential (elec. grad.) **can generate huge gradients using electroneutral pumps (H+/K+)** Transport ATPases: F – H+ transport to make ATP V (vacuolar) – ATP hydrolysis to transport H+ P – monovalent/divalent cation transport ABC – variety of substrate transport F TYPE ATP SYNTHASES freely reversible in vitro and in bacteria; counterproductive in eukaryotes (generate ion gradient for other processes) (only function is to make ATP  inhibitory protein binds under anoxic condition to prevent ATP hydrolysis) STRUCTURE: F1 – water soluble; catalyzes ATP hydrolysis (B-subunit) F0 – membrane embedded; conducts protons (via proton binding) FUNCTION: BINDING CHANGE MECHANISM - strongly cooperative and cyclical sequence of 3 catalytic sites o cycle only occurs if both tight and loose sites are bound - energy is required for cyclical activity tight – high affinity (binds A
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