The next abundant constituent of cell is  Proteins. They perform numerous functions vital to all organisms.

These are made from amino acids

They function as structural components of cells and subcellular entities.

They act as sources of nutrients, as atom- and energy-storage reservoirs, and

They are functional species such as hormones, enzymes, receptors, and transport molecules.

Aminoacids

These are organic molecules .

They contain a hydrogen atom, a carboxyl group (–COOH), and an amino group (–NH₂) and all of these are bonded to the same carbon atom, called α carbon.

The fourth group bonded to the α carbon varies among the different amino acids and is called a residue or a side chain, represented in structural formulas by the letter R.

When two or more amino acids combine, water molecule is removed and the two amino acids are connected by a covalent bond called the Peptide bond, which is formed by the reaction of the carboxylic acid group of one amino acid molecule with the amine group of another amino acid molecule. The resulting molecule is called a Peptide.

 Prefixes are often used to specify these numbers of amino acids that join for example dipeptides (two amino acids), tripeptides (three amino acids) etc.,

In general oligopeptides are formed by joining up to approximately 20 amino acids, whereas polypeptides are synthesized from up to approximately 50 amino acids.

“Proteins” by OpenStax is licensed under CC BY 4.0

When the number of amino acids linked together becomes very large, or when multiple polypeptides are used as building subunits, the macromolecules that result are called proteins.

Protein Structure

The size (length) and specific amino acid sequence of a protein are major determinants of its shape, and the shape of a protein is critical to its function.

Protein structure is categorized in terms of four levels: primary, secondary, tertiary, and quaternary.

The primary structure : is simply the sequence of amino acids that make up the polypeptide chain. 

The interactions of the functional groups and R groups of the amino acids form hydrogen, ionic, and disulfide bonds, along with polar/nonpolar interactions. These interactions lead to the formation of secondary, tertiary, and quaternary protein structures.

These groups are composed primarily of carbon, hydrogen, oxygen, nitrogen, and sulfur, in the form of hydrocarbons, acids, amides, alcohols, and amines.

The secondary structure : When the chain is sufficiently long, hydrogen bonding may occur between amine and carbonyl functional groups within the peptide backbone (excluding the R side group),

This results in localized folding of the polypeptide chain into helices and sheets.

These shapes constitute a protein’s secondary structure. The most common secondary structures are the α-helix and β-pleated sheet.

In the α-helix structure, the helix is held by hydrogen bonds between the oxygen atom in a carbonyl group of one amino acid and the hydrogen atom of the amino group that is just four amino acid units farther along the chain.

In the β-pleated sheet, the pleats are formed by similar hydrogen bonds between continuous sequences of carbonyl and amino groups that are further separated on the backbone of the polypeptide chain

The tertiary structure :  This is the  three-dimensional shape of a single polypeptide chain.

Tertiary structure is determined by interactions between amino acid residues that are far apart in the chain.

A variety of interactions give rise to protein tertiary structure, such as disulfide bridges, which are bonds between the sulfhydryl (–SH) functional groups on amino acid side groups; hydrogen bonds; ionic bonds; and hydrophobic interactions between nonpolar side chains.

All of these interactions, weak and strong, combine to determine the final three-dimensional shape of the protein and its function

As the result polypeptide chain assumes a large-scale, three-dimensional shape is called protein folding.

Folded proteins that are fully functional in their normal biological role are said to possess a native structure.

When a protein loses its three-dimensional shape, it may no longer be functional. These unfolded proteins are denatured.

Denaturation implies the loss of the secondary structure and tertiary structure and, the quaternary structure) without the loss of the primary structure.

The Quarternary structure  : Some proteins are assemblies of several separate polypeptides, also known as protein subunits. The interactions that hold these subunits together constitute the quaternary structure of the protein.

The overall quaternary structure is stabilized by relatively weak interactions. Hemoglobin, for example, has a quaternary structure of four globular protein subunits: two α and two β polypeptides, each one containing an iron-based heme .

Another important class of proteins is the conjugated proteins that have a nonprotein portion. If the conjugated protein has a carbohydrate attached, it is called a glycoprotein. If it has a lipid attached, it is called a lipoprotein.

“Primary structure of protein” by Openstax is licensed under CC BY 4.0

“Secondary structure of protein” by Openstax is licensed under CC BY 4.0

“Tertiary structure of protein” by Openstax is licensed under CC BY 4.0

“Quaternary structure of protein” by Openstax is licensed under CC BY 4.0

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