Proteins: Structure, Classification, and Functions
Proteins: An Overview
Proteins are essential biomolecules primarily composed of carbon, hydrogen, oxygen, and nitrogen. They may also contain sulfur, and some types contain phosphorus, iron, magnesium, copper, and other elements. Proteins are polymers of smaller molecules called amino acids, which are linked together by peptide bonds.
Amino Acids
Amino acids are characterized by a carboxyl group (-COOH) and an amino group (-NH2). The remaining two carbon valencies are saturated with a hydrogen atom and a variable radical group (R). Based on the R group, there are 20 different types of amino acids.
Amino Acid Behavior in Aqueous Solutions
In aqueous solutions, amino acids exhibit amphoteric behavior, meaning they can ionize depending on the pH. They can act as an acid by releasing protons (forming -COO’) or as a base by accepting protons (forming -NH3+). They can also exist as both an acid and a base simultaneously, forming a dipolar ion called a zwitterion.
Peptide Bonds
Peptides are formed by the joining of amino acids through peptide bonds. This covalent bond is established between the carboxyl group of one amino acid and the amino group of the next, releasing a water molecule. The peptide bond behaves like a double bond, exhibiting rigidity and immobilizing the atoms involved in a plane.
Protein Structure
Protein structure is defined by four levels: primary, secondary, tertiary, and quaternary. Each level builds upon the previous one.
Primary Structure
The primary structure is the linear sequence of amino acids in a protein. It determines the protein’s function and the shapes it can adopt.
Secondary Structure
The secondary structure refers to the spatial arrangement of the amino acid sequence. Two common types of secondary structure are alpha-helix and beta-sheet.
Tertiary Structure
The tertiary structure describes the three-dimensional folding of the polypeptide chain, resulting in a globular conformation. This conformation enhances solubility in water and enables proteins to perform various functions, such as transport, enzymatic activity, and hormonal regulation.
Quaternary Structure
The quaternary structure involves the association of multiple polypeptide chains (each called a protomer) through weak, non-covalent bonds to form a protein complex.
Protein Properties
Specificity
Protein specificity refers to the unique function performed by each protein. This specificity arises from the protein’s primary structure and its spatial conformation. Any alteration in protein structure can lead to a loss of function.
Denaturation
Denaturation is the loss of a protein’s tertiary structure due to the disruption of the bonds maintaining its shape. Denatured proteins typically have an open conformation and increased interaction with the solvent. This can lead to insolubility in water and precipitation. Denaturation can be caused by changes in temperature (e.g., boiling an egg) or pH. In some cases, denatured proteins can regain their original conformation through a process called renaturation.
Protein Classification
Proteins are classified into two main categories:
- Holoproteins: Composed only of amino acids.
- Heteroproteins: Composed of a protein fraction and a non-protein component called a prosthetic group.
Examples of Holoproteins
- Globular: Prolamins (e.g., zein in maize), glutenins (e.g., glutenin in wheat), albumins (e.g., serum albumin in blood), hormones (e.g., insulin), enzymes (e.g., hydrolases, oxidases).
- Fibrous: Collagen (in connective tissues), keratins (in hair, nails), elastin (in tendons), fibroin (in silk).
Examples of Heteroproteins
- Glycoproteins: Ribonuclease, mucoproteins, antibodies.
- Lipoproteins: HDL, LDL, VLDL (transport lipids in blood).
- Nucleoproteins: Chromatin, ribosomes.
- Chromoproteins: Hemoglobin, hemocyanin, myoglobin (carry oxygen), cytochromes (transport electrons).