Membrane protein functions Transport carriers in facilitated diffusion processes; Ion channels: Pumps involved in active transport Receptors for hormones and neurotransmitters, e.g. G-proteins, which act as mole- cular switches: signal transduction Cell to cell recognition an Junctional proteins in intercellular junctions: cell adhesion molecules; Second messenger enzymes Lipid-anchored proteins, which lie outside the lipid bilayer but are covalently linked to lipid molecules within the bilayer. Properties of integral membrane proteins Integral membrane proteins demonstrate asymmetrical orientation in the mem brane. They are amphipathic, with both hydrophobic and hydrophilic regions If they span the membrane, they are known as transmembrane proteins Removal from the membrane can be achieved by denaturation of the membrane, using either a detergent, e. g. ionic detergent sodium tetradecyl sulphonate, or he non-ionic detergent Triton X-100, or an organic solvent. Examples of integral membrane proteins include hormone receptors, ion channels, gap junction proteins, Na/K-ATPase and histocompatibility antigens. classification of cell membrane receptors Cell membrane receptors are classified according to the signal transduction nechanism involved into: Ion channel-linked(ionotropic)receptors, which are coupled directly to ligand-gated ion channels. Examples include nicotinic acetylcholine recep tors, ionotropic glutamate receptors, and gamma-aminobutyric acid(GABA A receptors. Catalytic receptors, which possess a cytoplasmic catalytic region that usually behaves as a tyrosine kinase. G-protein-linked receptors, which are further discussed in the chapter on endocrine physiology(see p 258) ll membrane receptors structurally comprise the following groups, depend on the number of times they span the membrane Single trans-luminal domain receptors, which are directly or indirectly coupled to intracellular kinase enzymes.Lipid-anchored proteins, which lie outside the lipid bilayer but are covalently linked to lipid molecules within the bilayer. Properties of integral membrane proteins Integral membrane proteins demonstrate asymmetrical orientation in the mem￾brane. They are amphipathic, with both hydrophobic and hydrophilic regions. If they span the membrane, they are known as transmembrane proteins. Removal from the membrane can be achieved by denaturation of the membrane, using either a detergent, e.g. ionic detergent sodium tetradecyl sulphonate, or the non-ionic detergent Triton X-100, or an organic solvent. Examples of integral membrane proteins include hormone receptors, ion channels, gap junction proteins, Naþ/Kþ-ATPase and histocompatibility antigens. Classification of cell membrane receptors Cell membrane receptors are classified according to the signal transduction mechanism involved into: * Ion channel-linked (ionotropic) receptors, which are coupled directly to ligand-gated ion channels. Examples include nicotinic acetylcholine recep￾tors, ionotropic glutamate receptors, and gamma-aminobutyric acid (GABA) A receptors. * Catalytic receptors, which possess a cytoplasmic catalytic region that usually behaves as a tyrosine kinase. * G-protein-linked receptors, which are further discussed in the chapter on endocrine physiology (see p. 258). Cell membrane receptors structurally comprise the following groups, depend￾ing on the number of times they span the membrane: * Single trans-luminal domain receptors, which are directly or indirectly coupled to intracellular kinase enzymes. Membrane protein functions Transport carriers in facilitated diffusion processes; Ion channels; Pumps involved in active transport; Receptors for hormones and neurotransmitters, e.g. G-proteins, which act as mole￾cular switches: signal transduction; Cell to cell recognition and interaction; Junctional proteins in intercellular junctions: cell adhesion molecules; Second messenger enzymes. Cell membrane 3