Pharmacodynamics: Drug-receptor interactions

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Pharmacodynamics: Drug-receptor interactions

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Pharmacodynamics: Drug-receptor interactions

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The is achieved when the dosage is high enough that all available receptors for a medication are occupied.   

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Content Reviewers:

Yifan Xiao, MD

Pharmacodynamics refers to the mechanisms and effects of medications within the body. Or more simply, it’s what medications do to the body and how they do it.

Alright, so, in order to have an effect, most medications have to reach their target cells and bind to a receptor.

Receptors are specialized proteins both on the cell membrane and inside the cell, that can bind to a ligand and get triggered to alter their shape or activity.

This gives rise to a signal cascade of intracellular molecules - the second messengers, which, ultimately, results in some change in the cell’s function.

Intracellular receptors are typically located in the cytoplasm or nucleus of the cell and recognize small, hydrophobic, meaning water- hating, ligands. These include molecules like steroids, which are happy to diffuse across the phospholipid membrane.

Once bound to their ligand, the receptor- ligand complex attaches to specific DNA sequences that activate or inhibit specific genes.

On the cell membrane are cell-surface receptors, which are embedded into the plasma membrane and bind to ligands too large or hydrophilic, meaning water- loving, to pass through.

Based on their structure and properties, cell- surface receptors fall into three main types.

First are ligand- gated ion channels, which form channels or pores that are generally closed, but then open up once they bind a specific ligand.

They allow ions like chloride, calcium, sodium, and potassium to passively flow into the cell, down their gradient, and trigger the signaling pathway.

Next are the G- protein coupled receptors, also known as seven- pass transmembrane receptors, which means they are really long proteins that have one end that sits outside the cell, and then the snake- like protein dips in and out of the cell membrane seven times, and finally ends on the inside of the cell.

The ligand binds to the end sitting outside the cell, and the end of the protein that’s within the cell activates guanine nucleotide-binding proteins or G proteins, which contain an alpha, beta, and gamma subunit.

Normally, the alpha subunit binds to a guanosine diphosphate or GDP molecule and the G protein is inactive.

When a ligand binds to the receptor, the G protein changes shape, causing the alpha subunit to release the GDP and allowing a guanosine triphosphate or GTP, to bind. This causes the alpha subunit to detach and trigger other proteins in the signalling pathway.

Broadly speaking, there are three types of G proteins: Gq, Gi, and Gs. Each type has its own kind of alpha subunit.

The Gq protein activates the enzyme phospholipase C, which cleaves a phospholipid called phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate and diacylglycerol.

On the other hand, protein Gs and Gi stimulate or inhibit, respectively, the enzyme adenylate cyclase, which takes adenosine triphosphate or ATP, and removes two phosphate molecules and transforms it into cyclic adenosine monophosphate or cAMP.

Inositol trisphosphate, diacylglycerol and cAMP then go on stimulate and inhibit different sets of enzymes and molecular pathways.

Finally, there are the enzyme-coupled receptors. These are usually single- pass transmembrane proteins, meaning that they have only one transmembrane segment.

The extracellular end of these receptors binds to ligands, and their intracellular end has enzyme activity.

The enzymatic domain is usually a protein kinase which phosphorylates other molecules.

When a ligand binds, it triggers a conformational change in the enzymatic domain to form high-affinity binding sites for the second messengers.

These second messengers get phosphorylated by the tyrosine kinases before heading off to activate other proteins in the signal pathway.

Sources
  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Recent Insights from Molecular Dynamics Simulations for G Protein-Coupled Receptor Drug Discovery" International Journal of Molecular Sciences (2019)
  4. "Catalytic Receptors" British Journal of Pharmacology (2007)
  5. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)