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Receptors as Proteins with Binding Sites for Specific Signalling Chemicals

Chemical signalling is a fundamental method of communication between cells. In this process, a signalling cell releases specific molecules known as ligands. These ligands serve as chemical messages that travel to a target cell to initiate a response. The interaction occurs when the ligand binds to a specific protein called a receptor located on or within the target cell. The primary mechanism involves the release of the ligand from the signalling cell and its subsequent binding to the receptor on the target cell, which triggers internal changes. There is no standard level (SL) content for the C2.1 module; it is exclusively Additional Higher Level (AHL) material.

Cell Signalling by Bacteria in Quorum Sensing

Quorum sensing (QS) is a sophisticated bacterial cell-to-cell communication process. It involves the production, detection, and collective response to extracellular signalling molecules called autoinducers. As the population density of a bacterial colony increases, these autoinducers accumulate in the surrounding environment. Bacteria use dedicated receptors to monitor the concentration of these molecules, allowing them to track changes in their cell numbers. When a certain threshold is reached, the bacteria collectively alter their gene expression to perform activities that are only beneficial when executed by a large group in synchrony, such as antibiotic production or bioluminescence.

An essential example of quorum sensing is found in the marine bacterium $Vibrio\,fischeri$. This aquatic organism secretes an autoinducer that binds to a cytoplasmic receptor known as LuxR. Once the LuxR-autoinducer complex forms, it binds to a specific region of DNA to prompt the expression of genes that code for the enzyme luciferase. Luciferase catalyzes an oxidation reaction where the majority of the energy is released as blue/green light. This bacterium maintains a mutualistic relationship with several animals, such as the bobtail squid. When bacterial population density is high, the resulting bioluminescence helps the squid camouflage against moonlight, protecting it from predation. In exchange, the squid provides the bacteria with a steady supply of sugar and amino acids.

Functional Categories of Signalling Chemicals in Animals

Animals utilize several functional categories of signalling chemicals, including hormones, neurotransmitters, cytokines, and calcium ions ($Ca^{2+}$). The classification of these ligands is determined by their function rather than their specific chemical structure. Hormones are produced in small quantities by endocrine glands and are primarily transported through the bloodstream. They target specific cells across the body to upregulate or inhibit various physiological processes. Because their target cells can be widely distributed, hormones typically have broad, systemic effects.

In contrast, neurotransmitters are secreted by presynaptic neurons and travel across a very short distance called the synaptic cleft. They bind to receptors on a postsynaptic neuron to either stimulate or inhibit a nerve impulse. Neurotransmitters are characterized by their very localized and specific effects and are rapidly neutralized, either through enzymatic breakdown or reabsorption by the presynaptic neuron. Cytokines are small proteins that generally act on the cell that produced them (autocrine) or nearby cells (paracrine). A single type of cytokine can be secreted by various cell types, and one cell can secrete numerous different cytokines. Because they are proteins and cannot cross the plasma membrane, cytokines bind to membrane receptors to trigger a signalling cascade that alters gene expression, often related to inflammation or immune responses.

Calcium ions ($Ca^{2+}$) serve as internal chemical signals within neurons and muscle fibers. In the context of muscle contraction, an influx of $Ca^{2+}$ from the sarcoplasmic reticulum binds to the protein troponin. This binding exposes actin filaments, which is the necessary step for contraction to occur. In neurons, the opening of calcium channels allows $Ca^{2+}$ to diffuse into the presynaptic membrane, which directly causes the secretion of neurotransmitters.

Chemical Diversity and Scale of Effects in Signalling Molecules

Signalling molecules exhibit significant chemical diversity, which accounts for the vast range of unique shapes and chemical properties required for specific receptor binding. Hormones are grouped into three main chemical classes: amines, proteins, and steroids (which are derived from cholesterol). Neurotransmitters also display diversity, including amino acids, peptides, amines, and even gases like nitrous oxide ($NO$).

There are distinct differences in the distance and scale of effects manageable by these molecules. Hormones travel large distances via the bloodstream or ducts, affecting many target cells and producing widespread effects. Neurotransmitters travel very small distances across synapses, affecting only one neuron (or a few) with high specificity. Cytokines travel small distances, typically affecting the same cell or nearby cells through the bloodstream or local diffusion. This variety in range and specificity allows for complex coordination throughout the organism.

Transmembrane and Intracellular Receptors

Receptors are specialized based on the properties of the ligands they receive, specifically whether the ligand is hydrophilic or hydrophobic. Signalling chemicals that are hydrophobic, such as steroid hormones, can passively diffuse through the lipid bilayer of the plasma membrane. These ligands bind to intracellular receptors located in the cytoplasm or the nucleus. Although they bind to hydrophobic ligands, intracellular receptors themselves are composed of hydrophilic amino acids because they must remain dissolved within the aqueous environment of the cytoplasm.

Hydrophilic ligands cannot pass through the hydrophobic core of the plasma membrane and must bind to extracellular receptors, which are transmembrane proteins. These receptors are structured with hydrophobic amino acids on their outer surfaces to anchor them within the plasma membrane's lipid core. However, their top and bottom sections (extracellular and intracellular domains) consist of hydrophilic amino acids so they can interact with the aqueous solutions outside and inside the cell. The binding of a ligand to these receptors sets off a sequence of responses known as a signal transduction pathway.

Signal Transduction and G-Protein-Coupled Receptors

Signal transduction refers to the entire process by which a cell receives an external signal, undergoes internal changes, and subsequently alters its behavior or activity. While intracellular receptors often move to the nucleus to directly alter gene expression, transmembrane receptors undergo conformational changes upon ligand binding to activate internal mechanisms involving effectors and second messengers. There are three primary categories of transmembrane receptors: G-protein-coupled receptors (GPCRs), enzyme-coupled receptors, and ion-channel-coupled receptors.

G-protein-coupled receptors are linked to G-proteins on the cytoplasmic side of the membrane. In humans, approximately $3\%$ of the genome codes for around $800$ different GPCRs. G-proteins are named for their binding to guanine nucleotides: guanosine triphosphate ($GTP$) and guanosine diphosphate ($GDP$). They are heterotrimeric, consisting of three subunits: $\text{G}\alpha$ (which binds the nucleotides), $\text{G}\beta$, and $\text{G}\gamma$.

The activation mechanism for a GPCR involves several steps: in its inactive state, the $\text{G}\alpha$ subunit is bound to $GDP$. When a ligand (the first messenger) binds to the GPCR, the receptor undergoes a conformational change that triggers the $\text{G}\alpha$ subunit to exchange its $GDP$ for a $GTP$. This exchange activates the $\text{G}\alpha$ subunit, causing it to dissociate from the $\text{G}\beta\gamma$ dimer. The activated $\text{G}\alpha$ subunit then activates an effector molecule, such as adenylyl cyclase, which stimulates the synthesis of second messengers. These second messengers then initiate intracellular processes like metabolism changes or gene expression. Finally, the $\text{G}\alpha$ subunit hydrolyzes the $GTP$ back into $GDP$, allowing the subunits to reassemble into an inactive heterotrimer.

Mechanism of Epinephrine Action in Liver Cells

The action of epinephrine (adrenaline) in liver cells provides a classic example of GPCR signal transduction. Epinephrine, a hydrophilic ligand, binds to its transmembrane receptor. This binding activates the associated G-protein, which in turn activates the effector enzyme adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of adenosine triphosphate ($ATP$) into cyclic adenosine monophosphate ($cAMP$), which acts as a secondary messenger. The $cAMP$ activates another enzyme called protein kinase. This initiates a phosphorylation cascade where various cellular proteins are phosphorylated. Ultimately, the enzyme glycogen phosphorylase is activated, which catalyzes the breakdown of glycogen into glucose phosphate. This glucose is then rapidly released into the bloodstream to provide energy.

Tyrosine Kinase Receptors and Insulin

Enzyme-coupled receptors, such as tyrosine kinase receptors, represent another major class of transmembrane receptors. A kinase is an enzyme that transfers phosphate groups from $ATP$ to a protein. For a tyrosine kinase, the phosphate is transferred specifically to the amino acid tyrosine. When a ligand like insulin binds to the extracellular domains of two adjacent tyrosine kinase receptors, they dimerize (bond together). This dimerization triggers auto-phosphorylation on the intracellular domain, where phosphate groups are added to tyrosine residues. These phosphorylated residues serve as docking sites for signalling proteins that relay the signal to target proteins. In the case of insulin, this cascade causes vesicles containing glucose transporters to move to and fuse with the plasma membrane, allowing glucose to enter the cell for respiration.

Ion-Channel-Coupled Receptors and Steroid Hormones

Ion-channel-coupled receptors change their conformation upon ligand binding to open a channel that allows specific ions to pass through the membrane. Acetylcholine is a prime example; when it is secreted by a presynaptic neuron and binds to receptors on a postsynaptic neuron, it causes ligand-gated sodium ($Na^+$) channels to open. The resulting influx of positive sodium ions depolarizes the membrane, allowing the neural signal to propagate.

Intracellular receptors behave differently; steroid hormones like oestradiol, progesterone, and testosterone diffuse directly through the plasma membrane. Once inside, they bind to a receptor in the cytoplasm to form a hormone-receptor complex. This complex migrates to the nucleus and binds to specific DNA segments, acting as a transcription factor to promote the transcription of target genes into $mRNA$. For instance, testosterone binds to the androgen receptor to upregulate the $FADS1$ gene, increasing fat production in prostate cells. Oestradiol in the hypothalamus enhances the transcription of $GnRH$ (gonadotropin-releasing hormone) $mRNA$, while progesterone in the endometrium promotes genes necessary for maintaining the uterine lining during the menstrual cycle.

Feedback Regulation and Naming Conventions

Cell signalling pathways are regulated by feedback loops. In positive feedback, the end product of a pathway amplifies the original stimulus, leading to an even greater production of the product. An example is oxytocin during childbirth: uterine contractions trigger oxytocin release, which causes more contractions and more oxytocin until the process of parturition is complete. In negative feedback, the rise of an end product inhibits the starting point of the pathway to maintain homeostasis. For example, high levels of testosterone inhibit the secretion of $GnRH$ and $LH$ (luteinizing hormone), which brings testosterone levels back down to a normal range.

Regarding nomenclature, "adrenaline" and "epinephrine" refer to the same hormone. The dual naming reflects international scientific history: "adrenaline" is derived from the Latin "ad" (at) and "ren" (kidney), while "epinephrine" comes from the Greek "epi" (above) and "nephros" (kidney). Both names describe the hormone’s origin in the adrenal glands. Although "epinephrine" sounds like the drug "ephedrine," both terms remain in common use globally, illustrating a unique case in international scientific cooperation.

Questions & Discussion

Q: Cholera is a water-borne disease which results in the modification of a G-protein that controls the opening of chloride channels in intestinal cells, causing them to remain continuously open. Outline the consequences of this illness.

A: The continuous opening of chloride channels leads to a massive outflux of chloride ions into the intestinal lumen. To maintain osmotic balance, water follows the ions, resulting in severe watery diarrhea and rapid dehydration.

Q: Outline the evolutionary advantages of having complex signaling networks in eukaryotes.

A: Complex networks allow for fine-tuned coordination of multicellular functions, the ability to process multiple environmental inputs simultaneously, and sophisticated regulation of growth, development, and homeostasis.

Q: Neurons rely heavily on the proper functioning of neurotransmitter signaling for communication. Outline the consequences on the human body if neurotransmitter reuptake is inhibited in neurons.

A: If reuptake is inhibited, neurotransmitters remain in the synaptic cleft longer, leading to prolonged stimulation or inhibition of the postsynaptic neuron. This can disrupt normal nervous system function, leading to issues like muscle spasms or altered mood.

Q: Outline why certain inherited diseases related to puberty are caused by mutations in G-protein receptors.

A: Puberty is driven by hormonal signals (like $GnRH$ and $LH$) that often rely on GPCRs to trigger developmental changes. Mutations can cause these receptors to be non-functional or constitutively active, disrupting the timing and progression of puberty.

Q: Outline the evolutionary advantages of quorum sensing in bacterial populations.

A: Quorum sensing allows bacteria to save energy by only expressing certain genes (like those for virulence or bioluminescence) when the population is large enough for the activity to be effective, enhancing their survival and ability to colonize hosts.

Q: Given the critical role of second messengers in cell signaling, outline the process by which cyclic AMP (cAMP) acts as a second messenger and its impact on cellular functions.

A: $cAMP$ is synthesized from $ATP$ by adenylyl cyclase following G-protein activation. It acts by activating protein kinases, which then phosphorylate other proteins, leading to changes in cell metabolism, gene expression, or protein activity.

Q: Explain how the same second messenger is used by many cells but induces different cellular responses.

A: The specific response depends on the type of receptors present on the cell and the specific suite of target proteins and enzymes available within that particular cell type to be activated by the second messenger.

Q: Outline the effects of estradiol, progesterone, and testosterone on target cells.

A: Estradiol stimulates $GnRH\,mRNA$ transcription in the hypothalamus. Progesterone maintains the endometrial lining by promoting gene transcription in uterine cells. Testosterone binds to androgen receptors to regulate genes like $FADS1$ for lipid production or development of male characteristics.

Q: Compare and contrast neurotransmitters and cytokines.

A: Both are signalling chemicals, but neurotransmitters act over very short distances across synapses for rapid nerve impulses, whereas cytokines are proteins involved in immune responses that act locally or on the cell that secreted them through signalling cascades.

Q: Explain the activation of G proteins during signal transduction.

A: Ligand binding to a GPCR causes $\text{G}\alpha$ to exchange $GDP$ for $GTP$. This triggers the dissociation of $\text{G}\alpha$ from the $\text{G}\beta\gamma$ subunits, allowing the active $\text{G}\alpha$ to trigger an effector molecule.

Q: Compare and contrast hormones and neurotransmitters.

A: Hormones travel long distances via blood for widespread effects, while neurotransmitters travel short distances via synapses for localized effects. Both bind to specific receptors to elicit a cellular response.

Q: Describe how cells respond to signals from lipid-soluble ligands.

A: Lipid-soluble ligands diffuse through the plasma membrane, bind to intracellular receptors to form a complex, and then enter the nucleus to bind to DNA and directly regulate gene transcription.

Q: Explain how receptors are adapted to the type of signalling chemical they receive.

A: Receptors for hydrophilic ligands are transmembrane proteins with hydrophobic anchors and hydrophilic domains. Receptors for hydrophobic ligands are located inside the cell and are composed of hydrophilic amino acids to remain soluble in the cytoplasm.

Q: Describe the mechanism of action of epinephrine (adrenaline) receptors.

A: Epinephrine binds to a GPCR, activating a G-protein which activates adenylyl cyclase. This produces $cAMP$, which activates protein kinases, leading to a phosphorylation cascade that results in the breakdown of glycogen into glucose.

Q: Describe how tyrosine kinase receptors work, giving an example.

A: Ligand binding (e.g., insulin) causes two receptors to dimerize and auto-phosphorylate. This creates binding sites for relay proteins. In the case of insulin, this leads to the translocation of glucose transporter vesicles to the plasma membrane.

Q: Discuss the role of quorum signalling in bacteria, giving an example.

A: Quorum sensing enables bacteria to sense population density via autoinducers. In $V.\,fischeri$, this leads to bioluminescence via the LuxR receptor and luciferase enzyme when density is high, aiding in mutualistic camouflage.

Q: Describe four functional categories of signalling chemicals in animals.

A: These include hormones (widespread blood-borne signals), neurotransmitters (fast synaptic signals), cytokines (immune and local signals), and calcium ions (intracellular signals for contraction and secretion).

Q: Describe the three categories of transmembrane receptors used in cell signalling.

A: These are GPCRs (activate G-proteins), enzyme-coupled receptors (like tyrosine kinases that phosphorylate proteins), and ion-channel-coupled receptors (open channels for ion flow upon ligand binding).

Q: Discuss the differences and similarities between how cells respond to hydrophilic and hydrophobic ligands.

A: Both involve ligand-receptor binding to change cell activity. Hydrophilic ligands require transmembrane receptors and second messengers, while hydrophobic ligands use intracellular receptors to directly alter gene transcription in the nucleus.