Divyan Bavan
Introduction
To receive inputs from various sources, human cells have developed several classes of receptors. These are proteins which bind ligands—small molecules, proteins, and other signaling factors. The product of binding is a cellular response. The ligand can have a small effect or a large one; it can be a short response or a long-lasting one. This depends on the type of receptor mediating the response. In human cells, receptors can be grouped into four main classes: ionotropic receptors (IRs), G protein-coupled receptors (GPCRs), enzymatic receptors (ERs), and nuclear receptors (NRs). Each of these receptors has different properties which enable efficient signaling. This essay, by using examples, will discuss these receptors’ structures, signaling pathways, and utility to show how crucial they are for cellular activity.
Type I Ionotropic Receptors
Among the different receptor classes, IRs act on the fastest timescale. Upon binding to its respective ligand, this protein alters the permeability of ions across the cell membrane. This is particularly useful at synapses; neurotransmitters can induce various effects through binding to ionotropic receptors in the post-synaptic cell.
A well-documented example of an IR is the nicotinic acetylcholine receptor (nAChR). This protein is found at many synapses: the neuromuscular junction, autonomic ganglia, and in the brain (Ritter et al., 2024). When the presynaptic neuron at these synapses fires an action potential, it induces a calcium influx at the axon terminal. This causes the fusion of vesicles to the membrane and the release of acetylcholine. This is the ligand for nAChRs. When acetylcholine binds to nAChR, it causes the channel to open. This results in the influx of cations into the postsynaptic cell, causing subsequent depolarization. This enables the muscle contraction and other functions throughout the body (Alberts et al., 2022).
The structure of nAChR is similar to receptors for many other neurotransmitters—serotonin, GABA, and glycine. The protein consists of two identical and three related subunits. Each subunit has four transmembrane helices which anchor the receptor to the membrane. The main difference between the subunits is the presence of acetylcholine binding sites; only the two identical α subunits contain binding sites for the molecule. For the channel to open, both sites must be occupied (Alberts et al., 2022).
The structure of IRs is very diverse. However, the core mechanism remains the same—ligand binding induces channel opening, causing the flow of ions (Ritter et al., 2024). These types of receptors are found heavily at synapses due to the need for rapid signaling. In IRs, the effect is immediate; ligand binding directly causes the intended effect. There are several consequences of this. While signaling is rapid, there is not any amplification of the signal. This means a high quantity of ligand is needed for activation of the signaling pathway. IRs work in synaptic transmission as acetylcholine concentrations in the cleft are sufficient for enough nAChR activation (Alberts et al., 2022). However, this is not the case for sparse ligands. For the effective transduction of these signals, different receptors are needed.
Type II G Protein-Coupled Receptors
GPCRs are a family of receptors which amplify signals through the recruitment of proteins for intercellular signaling cascades. This amplification means that ligands can have long-lasting effects and cause larger consequences within the cell. The trade-off of this is their timescale; IRs act in milliseconds while GPCRs act in seconds. Nonetheless, this specialization is still advantageous for many signaling pathways (PT’s Lecture Slides).
GPCRs have a conserved structure—7 transmembrane helices, an extracellular N terminus, and an intracellular C terminus (Ritter et al., 2024). GPCRs can be split into three classes: A, B, and C. These receptors can be classified based on the length of the N terminus tail. For example, class A GPCRs—which can bind neuroamines—have a short tail. In this case, neuroamines will bind to the transmembrane helices to induce a signaling pathway. This is the case for GPCRs such as the β2 adrenergic receptor, which binds to noradrenaline (Ritter et al., 2024).
Unlike IRs, GPCRs are monomers which do not have channels for molecules to pass through. Instead, upon binding to its respective ligand, a GPCR will recruit a G protein. This protein, which is normally bound to GDP, has three subunits: α, β, and γ. When a GPCR binds to its ligand, it will induce a conformational change in the intracellular portion of the receptor, opening a binding site for the G protein. Upon binding of the protein, the GDP is replaced with GTP. This induces the separation of the α subunit from the βγ subunit. The α subunit contains the GTP, and can therefore influence the activity enzymes and ion channels. The specific targets are determined by the type of α subunit associated with the GPCR; different types can produce opposite effects in cells. Once the target has been activated or deactivated by the α-GTP complex, the subunit will hydrolyze GTP into GDP through its GTPase activity. This turns off the response, and the α subunit will reform the complex with βγ. It should be noted that although there is no GTP bound to βγ, it can still influence cellular responses through binding (Ritter et al., 2024).
The enzymes which G proteins act on are responsible for producing secondary messengers: intercellular ligands which influence the activity of their targets. The benefit of this strategy is amplification; one active G protein can activate an enzyme to make many secondary messengers. An example of this is adenylyl cyclase. When an active α subunit activates this enzyme, it converts ATP to cyclic AMP (cAMP). This secondary messenger can activate protein kinase A, which phosphorylates many targets. For the β2 adrenergic receptor, binding of noradrenaline to the receptor induces the increased breakdown of glycogen. This is induced through the cAMP pathway. Similar systems exist for other secondary messengers, such as phospholipase C for inositol triphosphate production. Furthermore, the G protein subunits can also directly activate calcium and potassium ion channels (Ritter et al., 2024).
Type III Kinase-Linked Receptors
his class of receptor acts on the timescale of hours. However, the downstream effects of binding to these receptors are much larger than IRs or GPCRs, often influencing gene expression in the cell. Kinase-linked receptors often bind to larger proteins such as insulin. Their structure often consists of a single transmembrane helix, followed by larger extracellular and intracellular domains. This is shown in Figure 2, with the structure of the insulin receptor. When insulin binds to this receptor, it induces a conformational change in the intracellular domains. In this circumstance, tyrosine residues can phosphorylate each other. This enables the phosphorylation of other proteins, leading to the activation or deactivation of transcription factors through phosphorylation (Goodsell, 2015). Receptors which use this tyrosine-mediated mechanism are often referred to as receptor tyrosine kinases (RTKs). This group includes many other receptors such as epidermal growth factor receptor (EGFR) and nerve growth factor receptor (NGFR). These ligands are important for regulating cell growth, showing how important this receptor class is for wider cell identity (Ritter et al., 2024).
Type IV Nuclear Receptors
The final class of receptors are the nuclear receptors. This class differs from the others as it is not a cell surface receptor. Instead, it stays within the cytosol. To bind to the receptor, the ligand must cross the cell membrane and activate it. For this reason, many nuclear receptors are specialized for steroid hormones such as estrogen and testosterone. The receptors for these hormones are the oestrogen and androgen receptors, respectively. Once the hormones have crossed the membrane, they bind to the receptors and induce conformational changes (Ritter et al., 2024).
The structure of the nuclear receptor superfamily is conserved. Each nuclear receptor contains two activation factors, two zinc finger motifs for DNA binding, and a ligand binding domain. When a ligand binds to its respective nuclear receptor, the activation factors interact and activate the receptor. Nuclear receptors act as transcription factors; upon binding to their respective ligand, they localize to the nucleus where they influence gene expression over long timescales. The structure of these receptors reveals this mechanism, and it has been shown through experiments with radiolabelled ligands (Ritter et al., 2024).
Conclusion
The study of cellular receptors and their mechanisms is central to the modern process of drug development. By understanding what specific receptors do, we can design drugs which bind to them and influence their activity. This can be for short durations, such as with ionotropic receptors, or for long term manipulation, as seen with nuclear receptors. While many drugs have been discovered for the receptors we know of, there are many potential targets waiting to be studied within our genome. It is only through a mechanistic understanding of these receptors and their functions that we can design new drugs for disease.
References
Alberts, B. et al. (2022) Molecular biology of the cell. New York, NY: W. W. Norton & Company.
Goodsell, D. (2015) PDB101: Molecule of the month: Insulin receptor, RCSB. Available at: https://pdb101.rcsb.org/motm/182 (Accessed: 25 November 2025).
Professor Paolo Tammaro’s Lecture Slides
Ritter, J. et al. (2024) Rang and Dale’s Pharmacology. London: Elsevier.