Understanding Proteins: Structure, Properties, and Classification

Proteins: Composition and Structure

Proteins consist of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), and may also contain sulfur (S), phosphorus (P), iron (Fe), magnesium (Mg), and copper (Cu). Proteins are made up of smaller molecules called amino acids, which are linked by peptide bonds.

Amino Acids and Their Properties

Amino acids with uncharged polar side chains are hydrophilic and are usually found on the outside of the cell. Nonpolar side chains tend to aggregate in the interior.

Peptide Bonds and Polypeptides

Amino acids are commonly attached through amide links, called peptide bonds. The union of multiple amino acids results in a peptide. A peptide with fewer than 10 amino acids is called an oligopeptide, and if it contains more than 10 amino acids, it is called a polypeptide. If the number of amino acids is greater than 50, we refer to it as a protein.

Protein Structure

Primary Structure

Primary structure is simply the order of amino acids. If you change the order of amino acids, you change the type of protein.

Secondary Structure

Secondary structure occurs when the amino acid sequences interact through hydrogen bonding.

Alpha Helix

The alpha helix is formed when a polypeptide chain is wound on itself, with the C=O group of a peptide bond located 4 residues below forming a linear polypeptide chain.

Beta Sheet

The beta sheet is a structure where linear parallel and antiparallel strands associate in adjacent planes.

Tertiary Structure

Tertiary structure occurs when certain attractions are present between alpha and beta helices, such as covalent bonds between cysteine residues, hydrogen bonds between side chains, van der Waals interactions between side chains, and hydrophobic effects.

Quaternary Structure

Quaternary structure involves the interaction between several polypeptide chains in proteins composed of more than one polypeptide.

Properties of Proteins

Specificity

Specificity refers to a protein's function. Each protein performs a specific function and has a particular structure due to its elementary composition and spatial conformation. A change in the structure of the protein can lead to a loss of function. Examples include structural, enzymatic, hormonal, defense, and transportation proteins, as well as those involved in reservation.

Denaturation

Denaturation is the loss of tertiary structure, caused by breaking the bonds that maintain the structure. This may be caused by changes in pH or temperature.

Classification of Proteins

Holoproteins

Holoproteins are simple proteins and can be globular or fibrous.

Heteroproteins

Heteroproteins are formed by a protein fraction and a non-protein group called a prosthetic group (e.g., glycoproteins, lipoproteins, nucleoproteins, chromoproteins).