Function introduction to G proteins (more details in another lecture) Small proteins, bind GDP & GTP GDP bound at rest GTP becomes bound when G protein is activated by receptor G proteins consist of 3 subunits, , ,   -GTP is the active subunit different families of G proteins exist

At rest at rest, G protein has GDP bound inactive receptor membrane receptor Effector eg adenylate cyclase G protein

Agonist binding agonist binds to receptor causing a conformational change G protein becomes activated (ie binds GTP) R AC G Agonist GTP

Activation of 2nd messenger G protein diffuses in membrane and binds to enzymes present in membrane (eg adenylate cyclase, phospholipase C) and/or ion channels this activates or inactivates these enzymes or channels R AC GTP G active

Summary G protein then breaks down GTP to GDP and is inactivated R AC Agonist GTP R AC GTP G active R AC GDP G

Function II G proteins can interact with different receptors and effectors overall effect depends on receptor and cell eg adrenaline on beta-adrenoreceptor in smooth muscle activates adenylate cyclase increases cAMP activates a protein kinase muscle relaxation G proteins produce amplification a single agonist/receptor complex can activate several G proteins in turn and each can activate enzymes etc