Linderstrom-Lang first defined the four basic structural levels of organisation, which are referred to as primary, secondary, tertiary, and quaternary. Three of these structural levels (primary, secondary, and tertiary) can exist in molecules composed of a single polypeptide chain, whereas the fourth (quaternary) involves interactions of polypeptide within a multi-chained protein molecule. The basic structure of…

The nutritive value of proteins is determined by the composition of essential amino acids. From the nutritional point of view, proteins are classified into three categories.

Based on the function they perform, proteins are classified into the following groups:

Based on the composition and solubility, proteins are broadly classified into three major groups.

Globular or corpuscular proteins: They have an axial ratio (length: width) of less than ten (usually 3–4) and hence possess a relatively spherical or orbit shape. These are usually soluble in water or in aqueous media, containing acids, bases, salts, or alcohol, and diffuse readily. Nearly all enzymes are globular proteins. Example: Protein hormones, blood transport…

The proteins are all remarkably similar in structure as they contain amino acids. Proteins are classified based on the following:

Antibiotics such as gramicidin, bacitracin, tyrocidine, and actinomycin are peptides in nature. Gastrointestinal hormones like gastrin, secretin, and so on are gastrointestinal peptides which serve as hormones. Substance P is a decapeptide which acts as neurotransmitter.

Angiotensin 1 is a decapeptide (ten amino acids), which is converted to angiotensin 2 (eight amino acids). The latter has more hypertensive effect and also stimulates the release of aldosterone from adrenal gland. Methionine enkephalin is a pentapeptide found in brain. Functions: It has opiate-like function and inhibits the sense of pain. Bradykinin and kallidin are…

ADH is also a nanopeptide produced by posterior pituitary gland. Functions: It stimulates the kidney to retain water and thus increases blood pressure.

It’s a hormone secreted by posterior pituitary gland and contains nine amino acids (nanopeptide). Functions: It stimulates the contraction of smooth muscles, especially during childbirth.