Membrane Proteins

Define Membrane Protein

• protein molecule that is attached to, or associated with, the membrane of a cell or an organelle

general ratio of proteins to phospholipid molecules

1 : 25

Two categories of membrane proteins

• Integral or Intrinsic Proteins

• Peripheral or Extrinsic Proteins

Integral/Intrinsic proteins

• permanently embedded in the bilayer

• transmembrane

• amphiphilic

  • non polar hydrophilic regions anchor them in lipid layer

  • polar hydrophilic ends

• Some can drift slowly

• Held in place by cytoskeleton - cannot move

Peripheral membrane proteins

• loosely bound with membrane or integral proteins

• hydrophilic

• do not enter lipid layer

• surface of membrane

some facts about glycoproteins and glycolipids

• extracellular side of membrane

• cell to cell recognition

• in cancerous cells the identity of these change which allows them to go undetected by the immune system

• cell - to - cell junctions

what determine membrane properties

• proteins

• comprise 50% of membrane mass

Overview of Functions of membrane proteins

• transport across membranes

• enzymatic activity

• cell - to - cell junctions

• cytochromes

• structural support

• cell recognition

• signal reception

Transport Proteins

• enable transport of polar molecules

  • Channel or Carrier Proteins

Channel Proteins

• are always open and passive

  • Gated channels are the same but not always open (opened when triggered by stimulus)

Carrier Membrane Proteins

• pumps for active transport (primary or secondary A.T)

• passage ways for passive transport (facilitated diffusion)

Enzymes

• proteins catalyse reactions on inner/outer membrane surface

• maltase in the intestine

  • maltose (a disaccharide) into two glucose molecules

Cell-surface receptor proteins

• proteins have a specific shape, specific to the signaling molecule such as hormones

• the binding triggers events in cell membrane or in cell

Cell-surface identity markers/antigens

• glycoproteins - distinguish cell types

• cells carry their own identity surface markers which identify them to other cells

• antigen proteins on the cell identify the cell as ‘self’ or ‘non-self’

  • may elicit an immune response

Cell-to-cell adhesion proteins

• use glycoproteins to adhere to one another

  • tight/anchoring junctions

Structural Proteins

• some membrane proteins are anchored to cytoskeleton

• provides shape and stability

• anchors ECM

Cytochromes

• peripheral membrane proteins

• chloroplasts and mitochondria

• energy transfer systems such as electron carriers

• cytochromes facilitate respiration and photosynthesis