Proteins- Definition, Properties, Structure, Classification, Functions


Proteins are the most abundant and versatile biological macromolecules, occurring in all living cells and playing a crucial role in many vital processes. Proteins are composed of smaller units called amino acids, which are linked together by peptide bonds to form long chains. The sequence of amino acids in a protein determines its unique shape and function.

There are 20 different kinds of amino acids that can be found in proteins, each with a specific chemical structure and properties. Some amino acids can be synthesized by the body, while others must be obtained from the diet. These are called essential amino acids and they are important for growth and repair.

Proteins have different levels of structural organization, from the primary structure (the linear sequence of amino acids) to the quaternary structure (the interaction of multiple polypeptide chains). The secondary and tertiary structures are formed by various types of interactions between the amino acid residues, such as hydrogen bonds, hydrophobic interactions, electrostatic interactions, and disulfide bonds. These interactions result in different folding patterns, such as alpha-helices, beta-sheets, and loops.

The structure of a protein determines its function, which can be very diverse and specific. Some examples of protein functions are:

  • Enzymes: Proteins that catalyze chemical reactions in the body, such as digestion, metabolism, and synthesis of biomolecules.
  • Structural proteins: Proteins that provide support and shape to cells and tissues, such as collagen, keratin, and elastin.
  • Hormones: Proteins that act as chemical messengers and regulate various physiological processes, such as insulin, growth hormone, and adrenaline.
  • Antibodies: Proteins that recognize and bind to foreign substances (antigens) and help the immune system to fight infections.
  • Transport proteins: Proteins that carry substances across membranes or in the blood, such as hemoglobin, albumin, and glucose transporter.
  • Receptors: Proteins that bind to specific molecules (ligands) and trigger cellular responses, such as nerve impulses, gene expression, and cell growth.
  • Transcription factors: Proteins that bind to DNA and regulate gene expression, such as p53, NF-kB, and STATs.

Proteins are species-specific and organ-specific, meaning that they differ from one species to another and from one organ to another within the same organism. Proteins are also dynamic molecules that can undergo various modifications after their synthesis on the ribosome. These modifications can affect their activity, stability, localization, and interactions with other molecules.

Proteins are essential for life and health, but they can also be involved in diseases when they are defective or dysfunctional. For example, mutations in protein-coding genes can cause genetic disorders such as cystic fibrosis, sickle cell anemia, and Huntington`s disease. Misfolding or aggregation of proteins can cause neurodegenerative diseases such as Alzheimer`s disease, Parkinson`s disease, and prion diseases. Overexpression or underexpression of proteins can cause cancer or autoimmune diseases.

In this article, we will explore the properties, structure, classification, and functions of proteins in more detail. We will also discuss how proteins can be studied using various techniques and tools.