Biochemistry 313: Membrane Transport

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Porins
-simplest membrane transporters
-located in outer membranes of bacteria, mitochondria and chloroplasts
-are trimers in which each subunit forms a 16 or 18 strand B barrel that reaches across the membrane
-water filled core lined with hydrophilic side chaines
-passage for ions and molecules up to 1000 Daltons
-side groups on loops give selectivity

Ion channels
-found in neurons and in other cells
-more complicated proteins than porins
-multimers of identical or similar subunits with alpha-helices that spane the membrane
-well studied: K+ channel from streptomyces lividans, 10,000 times more permeable to K+ than Na+ because its carbonyl groups coordinate the ion on the way through
-ion passageway lies along central axis where subunits meet and is large enough for K+ ion to be surrounded by water
-most of the pore is narrow and hydrophobic except for hydrophilic cavity midway that lowers E needed
-voltage gated K+ channel in neurons is similar to S. lividans channel.
-All mammalian K channels are tetramers

Aquaporins
-water-specific pores
-plants have as many as 50 different kinds
-mammals have 10
-found where water transport is important, esp kidney, salivary glands, lacrimal glands
-glycerol and urea cannot pass thru
-best defined one is aquaporin 1 or AQP1, a homotetramer with carbohydrate chains on its extracellular surface. Six membrane spanning alpha helices plus 2 shorter helices that are inside the bilayer
-water doesn't go through the central hole between subunits, rather it goes through a path inside each subunit
-blocks proton transport by H-bonding each water to asparagine residues so no H-jumping


Gated channels
-undergo conformational changes to open & close
-not well understood
-can be voltage gated or ligand gated (aka: LGICs or ionotropic receptors)
-the ligands are often neurotransmitters in synapses
-voltage gated: membrane potential changes opens gated channels in nervous system (Calcium at synapses and Sodium for impulse transmission along axon, and potassium for returning nerve axon to resting potential)

What is the mechanism of the active transporter known as the sodium-potassium ATPase?
Aka the Sodium Potassium Pump, is an enzyme located in the membrane. It is an antiporter because it takes something in and something out. Uniport moves one thing one way, and symport moves two things same way.
Reaction cycle:
1. 3 intracellular Na+ bind
2. ATP binds and is hydrolyzed
3. a phsphoryl group is transferred from ATP to an Asp side chain of the pump, ADP is released
4. protein conformation changes, exposing bound Na+ to cell exterior, Na+ ions dissociate
5. 2 extracellular K+ ions bind
6. Asp phsophate group is hydrolyzed, inorganic phosphate is released
7. protein conformation changes, exposing K+ to cell interior, where it dissociates
Summary: uses one ATP and one H2O to transport 3 Na+ out and 2 K+ in.