Protein binding depends on the structure of the protein. A protein may have up to four levels of structure:
Primary structure - the chemical formula of the protein, which is usually simplified by using the component amino acids to describe it. It concerns the covalent bonds excluding cross-linking disulphide and hydrogen bonds
Secondary structure - the relative spatial positions of neighbouring covalently bonded molecules to each other. Free rotation occurs about single covalent bonds (but not double bonds). This rotation allows the molecule to settle into its most stable orientation resulting in a specific coiled protein layout held in place by disulphide bridges. This basic coil is often an alpha-helix
Tertiary structure - the shape in which the alpha-helix is subsequently arranged, which may be long and straight but usually it curls around itself
Quaternary structure - the inter-relationship between individual protein molecule subunits when more than one subunit constitutes the protein. Subunits are held together by weak bonds such as hydrogen bonds and van der Waals forces
Proteins are highly complex molecules and potentially have multiple areas or sites with individual properties. These sites vary in size, shape, electrostatic charge and their relationship to other sites. They have the potential to attract molecules of a suitable size, shape and charge in a similar manner to a three-dimensional jigsaw or key. Plasma proteins act in this way to transport poorly soluble molecules to other locations in the body. The bound molecules are generally in equilibrium with their unbound molecules, and are released as the free concentration falls.
Enzymes are specific proteins that bind molecules and then facilitate the formation or destruction of covalent bonds within molecules allowing synthesis of new molecules and destruction of other molecules. Receptors are usually proteins or glycoproteins that bind or receive specific molecules (agonists and antagonists) producing a conformational change in the receptor that is responsible for the effect. The effect of agonist-receptor complex may be to open an ion channel in a membrane, for example.
Drugs may interact with binding sites in the following ways:
• Agonist drugs have a similar structure to the intended molecule, bind to the receptor site and mimic the endogenous agonist
• Antagonist drugs have a similar structure to the intended molecule, bind to the receptor site without causing any change but prevent the endogenous agonist binding to the receptor
• Drugs may bind to another part of the protein causing a configuration change that prevents the receptor site from binding to the agonist
• Drugs may bind to another part of the protein preventing the configuration change necessary for the physiological effect
• Drugs may bind to the protein and prevent ions reaching the opened channel Plasma Protein Binding
The degree of drug binding to plasma proteins is relevant to the transport of poorly soluble drugs and also in determining dose requirements.
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