Membrane-Bound Receptors and the Second Messenger Concept
Membrane-Bound
Receptors and the Second
Messenger Concept
Many hormones, such as most amino acid derivatives, and the peptide hormones that are too large, or too polar, to pass through cell membranes, bind to receptor sites present on the surface of target cell membranes. The hormone and receptor form a complex that triggers a cascade of molecular events within a cell. The hormone thus behaves as a first messenger that causes the activation of a second messenger system in the cytoplasm. At least six different molecules have been identified as second messengers. Each works via a specific kinase, which causes activation or inactivation of rate-limiting enzymes which modify the direction and rate of cytoplasmic processes (Figure 36-2). Since many molecules of the second messenger are activated after a single hormone molecule has been bound, the message is amplified, perhaps many thousands of times.
Second messenger systems known to participate in hormone actions are cyclic AMP (cAMP), cyclic GMP (cGMP), Ca++/calmodulin, inositoltrisphosphate (IP3), and diacylglycerol (DAG). Cyclic AMP was the first to be investigated, and has been shown to mediate actions of many peptide hormones, including parathyroid hormone, glucagon, adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), melanophore-stimulating hormone (MSH), and vasopressin. It also mediates action of epinephrine (also called adrenaline), an amino acid derivative.
Many hormones, such as most amino acid derivatives, and the peptide hormones that are too large, or too polar, to pass through cell membranes, bind to receptor sites present on the surface of target cell membranes. The hormone and receptor form a complex that triggers a cascade of molecular events within a cell. The hormone thus behaves as a first messenger that causes the activation of a second messenger system in the cytoplasm. At least six different molecules have been identified as second messengers. Each works via a specific kinase, which causes activation or inactivation of rate-limiting enzymes which modify the direction and rate of cytoplasmic processes (Figure 36-2). Since many molecules of the second messenger are activated after a single hormone molecule has been bound, the message is amplified, perhaps many thousands of times.
Second messenger systems known to participate in hormone actions are cyclic AMP (cAMP), cyclic GMP (cGMP), Ca++/calmodulin, inositoltrisphosphate (IP3), and diacylglycerol (DAG). Cyclic AMP was the first to be investigated, and has been shown to mediate actions of many peptide hormones, including parathyroid hormone, glucagon, adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), melanophore-stimulating hormone (MSH), and vasopressin. It also mediates action of epinephrine (also called adrenaline), an amino acid derivative.
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