Describe the relationships between membrane channel proteins, membrane enzyme proteins, direct activation and indirect activation, including the role of G proteins and the creation and role of second messengers.
Both membrane channel proteins and membrane enzyme proteins are proteins found on the phospholipid bilayer of cell membranes and form class of cell surface receptors. Thus they play a major role in signal transduction.
Membrane channel proteins form channels for molecules to pass through the cell membrane. Most common membrane channel proteins are ion channels like ligand gated ion channels or voltage gated ion channels.
Membrane enzyme proteins are usually single-pass transmembrane receptors with their enzymatic domain being intracellular. They are also known as enzyme linked receptors and are most commonly protein kinases or associated with protein kinases. eg. RTK (Receptor Tyrosine Kinase).
G-Protein Coupled Receptors (GPCRs) are a class of membrane proteins that have G-proteins attached to their intracellular domains. The G-proteins are directly activated by by the GPCR when a ligand binds to its extracellular domain. Then the G protein can act via one of the many pathways like adenylate cyclase-cAMP pathway or IP3-DAG pathway. The cAMP or IP3 are the second messengers formed inside the cell due to the activation of the receptor and transmit the information carried by the ligand to the nucleus of the cell and elicit response. Thus a ligand binding to the cell-surface receptor directly activates the receptor and associated proteins while indirectly activating the second messengers and protein kinases withing the cell. Thus the signal gets transduced.