Overview of Cell Surface Receptors: Videos & Practice Problems

The three main types of cell surface receptors are ion channel-coupled receptors, G protein-coupled receptors (GPCRs), and enzyme-coupled receptors. Ion channel-coupled receptors respond to electrical gradients and can convert chemical signals into electrical signals. GPCRs activate intracellular G proteins, which then initiate a signaling cascade affecting various cellular functions. Enzyme-coupled receptors, often referred to as protein kinase receptors, function as enzymes and typically form complexes. These receptors are activated by ligand binding, leading to downstream signaling that can influence gene expression via transcription factors.

G protein-coupled receptors (GPCRs) work by binding to extracellular ligands, which activates the receptor. This activation causes a conformational change in the GPCR, allowing it to interact with and activate G proteins located in the cytosol. The activated G proteins then go on to influence various intracellular targets, such as enzymes, ion channels, and other proteins, initiating a signaling cascade. This cascade can result in a wide range of cellular responses, ultimately affecting processes like gene expression, cell growth, and metabolism.

Second messengers play a crucial role in cell signaling by acting as intermediaries that relay signals from activated cell surface receptors to target molecules within the cell. When a ligand binds to a receptor, it triggers the production or release of second messengers, such as cyclic AMP (cAMP), calcium ions (Ca²⁺), or inositol triphosphate (IP₃). These second messengers then propagate the signal downstream, leading to the activation of various signaling pathways and ultimately influencing cellular responses such as gene expression, metabolism, and cell proliferation.

When a ligand binds to an enzyme-coupled receptor, it induces a conformational change in the receptor, activating its enzymatic function. Typically, enzyme-coupled receptors have a domain in the cytosol that acts as an enzyme, often a kinase. Upon activation, these receptors can form complexes, such as dimers, and initiate a series of phosphorylation events. These phosphorylation events activate downstream signaling molecules and pathways, ultimately leading to changes in gene expression, cell growth, differentiation, or other cellular responses.

Ion channel-coupled receptors function in nerve signaling by responding to electrical gradients and allowing the passage of specific ions across the cell membrane. When a neurotransmitter binds to an ion channel-coupled receptor, it causes the channel to open, permitting ions such as Na⁺, K⁺, Ca²⁺, or Cl^- to flow into or out of the neuron. This ion movement generates an electrical signal, known as an action potential, which propagates along the nerve cell and transmits the signal to other neurons or target cells, facilitating communication within the nervous system.