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Reception
For cells to respond to signals, they need to have receptors that bind the signaling molecule (ligand). Cell receptors are protein molecules that bind to signaling molecules like hormones, drugs, and antigens. Like in enzymes, the binding sides on receptors are highly specific and recognize only one chemical messenger. Receptors allow for control over which cells receive the signal and prevent cells from being inundated by signal molecules. Some small or hydrophobic signaling molecules are able to cross the membrane and bind to intracellular receptors. Larger, hydrophilic signaling molecules bind to membrane-bound receptors. Ligand-to-ligand-gated channels, found in the membrane, open or close when a ligand binds. This can allow for (or stop) the flow of molecules across the membrane. Cell-surface receptors are embedded in the plasma membrane of target cells. They have an extracellular domain that binds to signaling molecules, a transmembrane domain, and an intracellular domain that initiates the transduction pathway.
G protein-coupled receptors are an example of a receptor protein in eukaryotes. This group of receptors are very large and diverse in their form and function. However, all of them have an extracellular region (that binds the signaling molecule), a region that is within the membrane, and an intracellular region. They are called G protein-coupled receptors because this intracellular region interacts with G proteins within the cell. As will be discussed, these interactions lead to cellular responses. G protein-coupled receptors play a role in a variety of cellular functions including neurotransmission, gene expression, and sensory perception.
Transduction
When a ligand binds to a receptor, it causes a conformational change which activates the receptor. The receptor then interacts with other molecules inside the cell to pass on the signal. Usually, there is a sequence of molecules that, in turn, activate or deactivate each other. This is called a signaling cascade. This relay uses second messengers, like cyclic AMP, to strengthen and spread the signal within the cell. The signal is passed from protein to protein using processes such as a phosphorylation cascade, dephosphorylation, and protein modification. Signaling cascades amplify the incoming signal, resulting in the appropriate responses by the cell, which could include cell growth, secretion of molecules, or gene expression. By amplifying the signal, a small amount of ligand can have a large impact inside the cell. This means that there can be an impactful response even if the signal is weak or the ligand is present in low concentrations.
Cellular Response
Signal transduction ultimately influences how a cell responds to its environment. It may result in changes in gene expression, enzyme activity, or cell function, which could alter phenotype or result in apoptosis. Once the cellular response has been achieved, the signal can be terminated to prevent overstimulation and prepare the cell to respond to new signals. This can be done through removing the receptor from the membrane through endocytosis, breaking down the ligand with enzymes, or deactivating other signaling molecules.
Changes in Signal Transduction Pathways
Just as with enzymes, protein structure is critical to the effectiveness of these pathways. Changes in the structure of any signaling molecule will affect the activity of the signaling pathway. If the pathway changes, this can significantly alter or prevent the appropriate cellular response. For example, mutated proteins may not be activated in the same way, halting transduction of the signal and the activation of downstream components. Other chemicals, if present, can also interfere with the pathway, influencing the effectiveness of the signal. If these chemicals interact with any component of the signaling pathway, the pathway may be activated or inhibited. Many drugs target these pathways to enhance or prevent cellular responses in a way that is beneficial to treating the patient. For instance, some mimic ligands and bind to receptors and others block the activation of proteins.
