Topic 5 – Proteins

Introduction to Proteins

Proteins are the most abundant macromolecules in living cells and play a central role in virtually all biological processes. They are polymers composed of amino acid (a.a.) units linked by peptide bonds. The sequence of amino acids in a protein is determined by the genetic code within DNA, and each protein's unique sequence dictates its structure and function.

• Definition: Proteins are large, complex molecules with molecular weights typically greater than 10,000 Da.

• Abundance: Escherichia coli contains approximately 3,000 different proteins, while a human cell contains over 50,000 distinct proteins.

• Genetic Control: The sequence of amino acids in a protein is specified by a gene.

Biological Functions of Proteins

Proteins perform a wide variety of functions in biological systems, which can be classified into several major categories:

• Enzymes: Biological catalysts that accelerate chemical reactions (e.g., chymotrypsin, amylase).

• Transport Proteins: Facilitate the movement of molecules across membranes (e.g., hemoglobin, carrier proteins).

• Structural Proteins: Provide support and shape to cells and tissues (e.g., collagen, elastin).

• Defence Proteins: Protect the organism from pathogens (e.g., antibodies).

• Regulatory Proteins: Involved in the regulation of biological processes (e.g., hormones like insulin).

Example: Antibodies are Y-shaped proteins produced by the immune system. They consist of two identical heavy chains and two identical light chains, with the tips of the arms forming the antigen-binding region.

Industrial and Medical Applications of Proteins

Proteins have numerous applications in industry and medicine due to their diverse functions:

• Enzymes in Industry:

  • Proteases: Used in detergents, meat tenderization, cheese manufacture (rennin), leather processing, contact lens cleaning, and as thrombolytic agents (e.g., tissue plasminogen activator).

  • Starch-degrading enzymes (α-amylase, β-amylase, glucoamylase): Used in the production of syrups and other food products.

• Regulatory Proteins in Medicine:

  • Hormones: Insulin (diabetes treatment), follicle-stimulating hormone (FSH, infertility treatment), growth hormone (dwarfism treatment).

  • Interferons: Used to combat cancer and viral infections.

• Antibodies: Used in diagnostic test kits and as therapeutic agents.

Protein Purification

To study a protein, it must first be isolated from the cell. The purification process depends on whether the protein is extracellular or intracellular and is based on its unique properties such as size, shape, and charge.

1. Initial Steps: Cell lysis and precipitation with salts or organic solvents to decrease protein solubility and cause precipitation.

2. Chromatography: A key method for protein purification, separating proteins based on specific properties:

• Buffer Solutions: Used to maintain constant pH and preserve protein biological activity during purification.

Example: Gel filtration chromatography separates proteins based on their size, with larger proteins eluting first.

Multi-Subunit and Conjugated Proteins

Many proteins are composed of two or more polypeptide chains (subunits), which may be identical or different. Some proteins also contain non-polypeptide components called prosthetic groups, forming conjugated proteins.

• Multi-Subunit Proteins: For example, hemoglobin is a tetramer with two α and two β chains.

• Estimating Amino Acid Content: The approximate number of amino acid residues in a protein can be calculated by dividing its molecular weight by 110.

• Conjugated Proteins: Contain additional chemical groups (prosthetic groups) such as metals or vitamins.

• Vitamins as Prosthetic Groups: Many vitamins or their derivatives serve as prosthetic groups for specific enzymes.

Stability: Most proteins are only biologically active within a narrow range of pH and temperature.

Summary of Key Points

• Proteins are essential macromolecules with diverse structures and functions.

• They are involved in catalysis, transport, structure, defense, regulation, and more.

• Industrial and medical applications of proteins are vast, including enzymes and therapeutic proteins.

• Protein purification is a multi-step process involving techniques such as chromatography.

• Many proteins are multi-subunit or conjugated with prosthetic groups, contributing to their functional diversity.