2.1: GPCRs in Vision

G Protein-Coupled Receptors (GPCRs) play a crucial role in the visual process, with rhodopsin being the most well-studied GPCR involved in this process. Rhodopsin is a light-sensitive GPCR found in the rod cells of the retina, which are responsible for low-light vision.

Structure and Function of Rhodopsin

Rhodopsin is a complex protein made up of two main components: opsin, a transmembrane protein, and retinal, a light-sensitive molecule derived from vitamin A. The opsin protein spans the membrane seven times, forming a barrel-like structure that surrounds the retinal molecule. When light hits the retinal molecule, it undergoes a conformational change, which activates the rhodopsin protein.

Activation by Light

When light hits the retinal molecule, it changes from a bent, or cis, configuration to a straight, or trans, configuration. This change in configuration causes the retinal to separate from the opsin protein, activating it. The activated opsin protein then activates a G protein called transducin, which initiates a signaling cascade that ultimately leads to the perception of light.

Signaling Pathways

Once activated, transducin activates a phosphodiesterase enzyme, which reduces the levels of cyclic guanosine monophosphate (cGMP) in the rod cell. This reduction in cGMP causes the closure of cGMP-gated ion channels, which leads to a hyperpolarization of the rod cell membrane. This hyperpolarization is then transmitted to the brain, where it is interpreted as light.

Summary

In summary, GPCRs play a crucial role in the visual process, with rhodopsin being the most well-studied GPCR involved in this process. Rhodopsin is a light-sensitive GPCR found in the rod cells of the retina, which are responsible for low-light vision. When light hits the retinal molecule, it undergoes a conformational change, which activates the rhodopsin protein. The activated opsin protein then activates a G protein called transducin, which initiates a signaling cascade that ultimately leads to the perception of light.

2.2: GPCRs in Olfaction

GPCRs also play a crucial role in the sense of smell, with olfactory receptors being the GPCRs responsible for detecting different odorant molecules.

Structure and Function of Olfactory Receptors

Olfactory receptors are transmembrane proteins found in the olfactory neurons of the nose. Each olfactory neuron expresses only one type of olfactory receptor, and each type of olfactory receptor can detect a limited range of odorant molecules. When an odorant molecule binds to an olfactory receptor, it activates a signaling pathway that ultimately leads to the perception of the odor.

Interaction with Odorant Molecules

Odorant molecules bind to olfactory receptors through a process called ligand-receptor binding. The shape and chemical properties of the odorant molecule determine which olfactory receptors it can bind to. Once an odorant molecule binds to an olfactory receptor, it changes the conformation of the receptor, activating it.

Signaling Pathways

Once activated, the olfactory receptor activates a G protein called Gαolf, which activates a signaling cascade that ultimately leads to the perception of the odor. The signaling cascade involves the activation of adenylyl cyclase, which increases the levels of cyclic adenosine monophosphate (cAMP) in the olfactory neuron. This increase in cAMP leads to the opening of cAMP-gated ion channels, which depolarizes the olfactory neuron membrane. This depolarization is then transmitted to the brain, where it is interpreted as the perception of the odor.

Summary

In summary, GPCRs play a crucial role in the sense of smell, with olfactory receptors being the GPCRs responsible for detecting different odorant molecules. Olfactory receptors are transmembrane proteins found in the olfactory neurons of the nose, and each olfactory neuron expresses only one type of olfactory receptor. When an odorant molecule binds to an olfactory receptor, it changes the conformation of the receptor, activating it. The activated olfactory receptor activates a G protein called Gαolf, which activates a signaling cascade that ultimately leads to the perception of the odor.

2.3: GPCRs in Taste

GPCRs also play a crucial role in the sense of taste, with taste receptors being the GPCRs responsible for detecting different taste molecules.

Structure and Function of Taste Receptors

Taste receptors are transmembrane proteins found in the taste cells of the tongue. There are five basic tastes: sweet, salty, sour, bitter, and umami. Each taste has its own specific taste receptor, and each taste receptor can detect a limited range of taste molecules. When a taste molecule binds to a taste receptor, it activates a signaling pathway that ultimately leads to the perception of the taste.

Interaction with Taste Molecules

Taste molecules bind to taste receptors through a process called ligand-receptor binding. The shape and chemical properties of the taste molecule determine which taste receptors it can bind to. Once a taste molecule binds to a taste receptor, it changes the conformation of the receptor, activating it.

Signaling Pathways

Once activated, the taste receptor activates a G protein called gustducin, which activates a signaling cascade that ultimately leads to the perception of the taste. The signaling cascade involves the activation of phospholipase C, which increases the levels of inositol trisphosphate (IP3) in the taste cell. This increase in IP3 leads to the release of calcium ions from intracellular stores, which depolarizes the taste cell membrane. This depolarization is then transmitted to the brain, where it is interpreted as the perception of the taste.

Summary

In summary, GPCRs play a crucial role in the sense of taste, with taste receptors being the GPCRs responsible for detecting different taste molecules. Taste receptors are transmembrane proteins found in the taste cells of the tongue, and there are five basic tastes: sweet, salty, sour, bitter, and umami. Each taste has its own specific taste receptor, and when a taste molecule binds to a taste receptor, it changes the conformation of the receptor, activating it. The activated taste receptor activates a G protein called gustducin, which activates a signaling cascade that ultimately leads to the perception of the taste.

2.4: GPCRs in Cardiovascular Function

GPCRs also play a crucial role in regulating cardiovascular functions, including the control of heart rate, blood pressure, and vascular tone.

Specific GPCRs Involved

The GPCRs involved in cardiovascular function include β-adrenergic receptors, muscarinic receptors, and angiotensin receptors. β-adrenergic receptors are responsible for increasing heart rate and contractility, while muscarinic receptors are responsible for decreasing heart rate and contractility. Angiotensin receptors are responsible for increasing blood pressure by constricting blood vessels.

Activation Mechanisms

The activation mechanisms of these GPCRs involve the binding of specific ligands, such as hormones or neurotransmitters, to the receptor. This binding causes a conformational change in the receptor, which activates a G protein.

Signaling Pathways

The activated G protein then activates a signaling pathway that ultimately leads to the regulation of cardiovascular functions. For example, the activation of β-adrenergic receptors leads to an increase in cyclic adenosine monophosphate (cAMP) levels, which increases heart rate and contractility. The activation of muscarinic receptors leads to a decrease in cAMP levels, which decreases heart rate and contractility. The activation of angiotensin receptors leads to an increase in intracellular calcium levels, which constricts blood vessels and increases blood pressure.

Summary

In summary, GPCRs play a crucial role in regulating cardiovascular functions, including the control of heart rate, blood pressure, and vascular tone. The GPCRs involved in cardiovascular function include β-adrenergic receptors, muscarinic receptors, and angiotensin receptors. The activation mechanisms of these GPCRs involve the binding of specific ligands, such as hormones or neurotransmitters, to the receptor. The activated G protein then activates a signaling pathway that ultimately leads to the regulation of cardiovascular functions.

2.5: GPCRs in Respiratory Function

GPCRs also play a crucial role in regulating respiratory functions, including the control of airway smooth muscle tone and mucus production.

Specific GPCRs Involved

The GPCRs involved in respiratory function include β2-adrenergic receptors, muscarinic receptors, and prostaglandin receptors. β2-adrenergic receptors are responsible for relaxing airway smooth muscle, while muscarinic receptors are responsible for contracting airway smooth muscle. Prostaglandin receptors are responsible for regulating mucus production.

Activation Mechanisms

The activation mechanisms of these GPCRs involve the binding of specific ligands, such as hormones or neurotransmitters, to the receptor. This binding causes a conformational change in the receptor, which activates a G protein.

Signaling Pathways

The activated G protein then activates a signaling pathway that ultimately leads to the regulation of respiratory functions. For example, the activation of β2-adrenergic receptors leads to an increase in cAMP levels, which relaxes airway smooth muscle. The activation of muscarinic receptors leads to a decrease in cAMP levels, which contracts airway smooth muscle. The activation of prostaglandin receptors leads to the regulation of mucus production.

Summary

In summary, GPCRs play a crucial role in regulating respiratory functions, including the control of airway smooth muscle tone and mucus production. The GPCRs involved in respiratory function include β2-adrenergic receptors, muscarinic receptors, and prostaglandin receptors. The activation mechanisms of these GPCRs involve the binding of specific ligands, such as hormones or neurotransmitters, to the receptor. The activated G protein then activates a signaling pathway that ultimately leads to the regulation of respiratory functions.

2.6: GPCRs in Immune Function

GPCRs also play a crucial role in regulating immune functions, including the control of inflammation and immune cell activation.

Specific GPCRs Involved

The GPCRs involved in immune function include chemokine receptors, formyl peptide receptors, and histamine receptors. Chemokine receptors are responsible for regulating the migration of immune cells to sites of inflammation, while formyl peptide receptors are responsible for regulating the activation of immune cells. Histamine receptors are responsible for regulating the inflammatory response.

Activation Mechanisms

The activation mechanisms of these GPCRs involve the binding of specific ligands, such as chemokines or histamine, to the receptor. This binding causes a conformational change in the receptor, which activates a G protein.

Signaling Pathways

The activated G protein then activates a signaling pathway that ultimately leads to the regulation of immune functions. For example, the activation of chemokine receptors leads to the migration of immune cells to sites of inflammation, while the activation of histamine receptors leads to the inflammatory response.

Summary

In summary, GPCRs play a crucial role in regulating immune functions, including the control of inflammation and immune cell activation. The GPCRs involved in immune function include chemokine receptors, formyl peptide receptors, and histamine receptors. The activation mechanisms of these GPCRs involve the binding of specific ligands, such as chemokines or histamine, to the receptor. The activated G protein then activates a signaling pathway that ultimately leads to the regulation of immune functions.