Proteins: Occurrence and Importance

Proteins: Occurrence and Importance

• Proteins are found in every cell of humans, animals, plants, and microorganisms.
• They constitute about 50% of a cell's dry weight.
• The term "protein" comes from the Greek word "proteios," meaning "holding first place."

Medical and Biological Importance

• Proteins play essential roles in various bodily functions.
• Transport: Examples include hemoglobin, which carries oxygen.
• Enzymatic activity: Proteins catalyze chemical reactions in the body.
• Defense: Proteins like immunoglobulins fight bacterial and viral infections.
• Hormonal regulation: Hormones, such as insulin, control biochemical processes.
• Muscle contraction: Muscle proteins are involved in muscle movement.
• Gene expression: Histones control gene expression and translation.
• Nutrients and storage: Proteins like casein and ferritin serve as nutrients and store substances.
• Buffering: Proteins act as buffers, maintaining stable pH levels.
• Anti-vitamins: Some proteins act as inhibitors, like avidin in eggs.
• Infectious agents: Prions, proteins, cause diseases like mad cow disease.

Chemical Nature of Proteins

• Proteins are polymers of amino acids.
• Amino acids are linked together by peptide bonds.
• Sometimes proteins are also called polypeptides due to multiple peptide bonds.

Properties of Proteins

• Proteins have high molecular weights. Examples include lactalbumin (17,000) and pyruvate dehydrogenase (7 x 10⁶).
• Proteins are colloids.
• Proteins have large particle sizes.
• Solubility varies in different solvents.
• Proteins exhibit different shapes.
• Hydrolysis of some proteins yields only amino acids, and others yield amino acids plus additional molecules.
• Charge properties depend on the environment (e.g., pH). Isoelectric point is where net charge is zero.
• Proteins can act as buffers due to their amphoteric nature.
• Hemoglobin and plasma proteins are significant buffers in blood.

Protein Classification

• Proteins can be classified based on composition, solubility, structure, or function.
• Simple proteins: Composed only of amino acids. Examples include albumin, trypsin, etc.
• Conjugated proteins: Contain non-protein components (prosthetic groups). Subclasses include lipoproteins, glycoproteins, phosphoproteins, nucleoproteins, hemoproteins/chromoproteins, flavoproteins, and metalloproteins.
• Derived proteins: Formed from simple or conjugated proteins via processes like denaturation or hydrolysis (proteoses, peptones, gelatin, and peptides).

Protein Structure

• Primary structure: Linear sequence of amino acids.
• Secondary structure: Ordered folding patterns (e.g., alpha-helices, beta-sheets).
• Alpha-helix: Hydrogen bonds between groups along the chain. 3.6 amino acids per turn.
• Beta-sheets: Adjacent polypeptide chains held together by hydrogen bonds.
• Tertiary structure: Three-dimensional arrangement of the entire protein. Driven by various interactions (hydrophobic, electrostatic, hydrogen bonds, and van der Waals).
• Quaternary structure: Arrangement of multiple polypeptide chains in a complex. Formed through various interactions like hydrogen bond, electrostatic, van der waals, and disulfide bonds.

Denaturation of Proteins

• Loss of native conformation.
• Physical or chemical changes can lead to denaturation. Examples include high temperature, extreme pH, or the presence of urea, guanidine, or heavy metal salts.
• Denaturation often results in altered solubility, biological activity, and other properties.
• Sometimes, denaturation is irreversible. Renaturation is possible in some cases.
• Denaturation has significance in many biological and medical processes (e.g., enzyme function, food preservation).

Plasma Proteins

• Plasma contains various proteins that play crucial roles.
• Albumin is a significant plasma protein.
• The protein concentration and subtypes in serum are clinically significant.