membrane proteins Flashcards
liquid-ordered (Lo) state
gel-like state in which all types of motion of individual molecules are strongly constrained
liquid-disordered (Ld) state
state in which individual hydrocarbon chains are in constant motion (lateral & rotational)
membrane dynamics
at mammalian physiologic temperatures (20-40 C) biological membranes are fluid and are maintained in a transition state between Lo & Ld
membrane proteins facilitate…
translocation of individual lipid molecules
flippase
(P-type ATPase) moves PE and PS from outer to inner leaflet
floppase
(ABC transporter) moves phospholipids from inner to outer leaflet
scramblase
moves lipids in either direction, toward equilibrium
membrane proteins are limited in movement by:
associating to form large aggregates (patches)
anchoring to internal structures
microdomains (rafts)
clusters of cholesterol and sphingolipids that make the bilayer slightly thicker and more ordered than neighboring, phospholipid-rich regions- can be up to 50% of the cell surface
_______________ are central to the ability of biological membranes to undergo fusion with other membranes w/o losing continuity
curvature changes
integrins
surface adhesion proteins that mediate a cell’s interaction with the extracellular matrix and with other cells
-> carry signals in both directions across membrane
-> heterodimeric proteins composed of two unlike subunits, a & B
cadherins
involved in surface adhesion
-> undergo homophilic interactions with identical cadherins in an adjacent cell
selectins
have extracellular domains that bind specific polysaccharides on the surface of an adjacent cell
-> requires Ca2+
-> essential part of the blood-clotting process
passive transporters
facilitate movement down a concentration gradient, increasing the transport rate
-> process is called passive transport or facilitated diffusion
active transporters
move substrates across a membranes against a concentration gradient or an electrical potential; process called active transport