Various Ligand Types

There are many distinct types of ligands, which are produced by signaling cells and interact with receptors in or on target cells. Others are hydrophobic molecules like steroids, while still others are gases like nitric oxide. Some are proteins.

Ligands that can enter the Cell

• The female sex hormone estradiol, which is a form of estrogen, and the male sex hormone testosterone are both common steroid hormones. 
• Another example of a steroid hormone is the substance known as vitamin D, which is created in the skin using light energy. 
• These hormones must connect to carrier proteins in order to pass through the (watery) bloodstream because they are hydrophobic and can easily permeate the plasma membrane.
• A ligand is a gas called nitric oxide (NO). Its small size allows it to permeate straight across the plasma membrane, similar to steroid hormones. 
• One of its primary functions is to trigger a signaling pathway that causes the smooth muscle around blood vessels to relax, allowing the blood vessels to enlarge (dilate). 
• In truth, nitroglycerin is a medication that treats heart illness by causing the production of NO, which dilates blood vessels and improves heart blood flow.
• NO has gained more recognition recently since Viagra and other erectile dysfunction prescription drugs target the pathway that NO impacts.

Ligands that bind on the outside of the Cell

• Because they are polar or charged, water-soluble ligands find it difficult to pass the plasma membrane.
• As a result, the majority of water-soluble ligands attach to the extracellular domains of cell-surface receptors and remain on the cell’s surface.
• The broadest class of water-soluble ligands is composed of peptide (protein) ligands. For instance, this group includes hormones like insulin, some neurotransmitters, and growth factors. 
• Peptide ligands can be as short as a few amino acids, as the pain-relieving enkephalins, or as long as a hundred amino acids or more.
• Neurotransmitters can also be proteins. However, many additional neurotransmitters are tiny chemical molecules called hydrophilic (water-loving) molecules. 
• Standard amino acids like glutamate and glycine are found in some neurotransmitters, whilst other modified or non-standard amino acids are found in others. 

In biochemistry and pharmacology, receptors are protein-based chemical structures that receive and transmit signals that can be integrated into biological systems. A ligand is an ion or molecule that binds to a central metal atom to form a coordination complex in coordination chemistry. Bonding with metals typically entails the formal donation of one or more of the ligand’s electron pairs, often via Lewis Bases Although there are many kinds of receptors, they can be categorized into two groups: 

• Cell surface receptors, which are found in the plasma membrane, and 
• Intracellular receptors are found inside the cell.