Proteins: Structure, Composition, and Biological Significance

Introduction Proteins are essential components of food, required by every cell for structure and function. They are complex polymers composed of amino acids and exhibit amphoteric properties, behaving as both acids and bases. In their polymeric forms, proteins possess a specific three-dimensional folded structure. About 20 genetically coded amino acids make up food proteins, linked covalently by peptide bonds between the α-carboxyl group of one amino acid and the α-amino group of another. Proteins contribute significantly to food texture, color, flavor, and functional properties, serving as buffering agents, emulsifiers, and fat mimetics.

Amino Acids Amino acids are organic compounds containing amine (-NH2) and carboxyl (-COOH) functional groups, along with side chains (R groups) specific to each amino acid.

Composition of Proteins Proteins are large organic compounds containing carbon, hydrogen, oxygen, and approximately 16% nitrogen—a unique feature distinguishing them from carbohydrates and fats. Some proteins also contain sulfur, phosphorus, iron, and cobalt. Plants synthesize proteins using nitrates and ammonia from the soil, while herbivores obtain proteins from plants. Humans consume both plant and animal foods (milk and meat) to meet protein needs.

Structure of Proteins Proteins are composed of amino acids linked by peptide bonds. Each peptide has a free carboxyl group at one end and a free amino group at the other, allowing the addition of other amino acids. Proteins exhibit amphoteric properties due to the presence of both free acid and amino groups.

1. Primary Structure: Linear chains of polypeptides.
2. Secondary Structure: Formed by linking multiple polypeptide chains into helical, pleated, or random coil structures.
3. Tertiary Structure: Complex folding into globular forms.

Proteins consist of long chains (100–500 amino acids) forming three-dimensional structures known as the native state. Denaturation, which alters this structure, occurs due to factors such as heating, acid, beating, or freezing.

Essential and Non-Essential Amino Acids

• Essential Amino Acids: Nine amino acids must be obtained through diet, as the body cannot synthesize them in sufficient quantities. Arginine is semi-essential, particularly for infants during rapid growth.
• Non-Essential Amino Acids: The body synthesizes 13 amino acids, essential for tissue building, repair, and metabolism.

Food Sources of Protein

1. Plant Sources: Pulses, nuts, oilseeds, cereals, and their products.
2. Animal Sources: Milk, eggs, fish, poultry, and meat.
3. Other Sources: Dry, salted, or smoked fish and meat, commonly consumed in coastal regions.

Biological Significance and Functions of Proteins

1. Enzymatic Function: Most biochemical enzymes are proteins.
2. Hormonal Function: Many hormones, like insulin, are protein-based.
3. Genetic Regulation: Proteins control gene transcription and translation.
4. Transport: Hemoglobin transports oxygen, while lipoproteins transport lipids.
5. Protection: Immunoglobulins and interferons protect against infections.
6. Structural Support: Collagen and elastin provide strength and elasticity.
7. Nutritional Role: Ovalbumin (egg white) and casein (milk) serve as nutrients.
8. Osmotic Balance: Proteins maintain plasma osmotic pressure.
9. Muscle Contraction: Actin and myosin facilitate skeletal muscle contraction.

Classification of Proteins Proteins can be classified based on solubility, physical properties, shape, size, and function.

Based on Solubility and Physical Properties

1. Simple Proteins: Yield only amino acids upon hydrolysis.
   • Albumins: Soluble in water, coagulated by heat. Eg: Serum albumin, lactalbumin.
   • Globulins: Insoluble in pure water, soluble in dilute salt solutions. Eg: Serum globulin, ovoglobulin.
   • Glutelins: Soluble in dilute acids and alkalis. Eg: Glutenin (wheat).
   • Prolamines: Soluble in 70–80% alcohol. Eg: Zein (corn), gliadin (wheat).
   • Histones: Soluble in water, not coagulated by heat. Eg: Nucleic acid-associated histones.
   • Protamines: Soluble in water or ammonium hydroxide. Eg: Sperm cell protamines.
   • Albuminoids (Scleroproteins): Insoluble in neutral solvents, acids, and alkalis. Eg: Keratins, collagen.

2. Conjugated Proteins: Simple proteins combined with non-protein (prosthetic) groups.
   • Nucleoproteins: Associated with nucleic acids. Eg: Chromatin.
   • Glycoproteins: Contain carbohydrates (<4%). Eg: Mucin (saliva).
   • Lipoproteins: Associated with cholesterol and fatty acids.
   • Phosphoproteins: Contain phosphoric acid. Eg: Casein (milk), vitellin (egg yolk).
   • Metalloproteins: Bound to metal ions. Eg: Ceruloplasmin (copper), carbonic anhydrase (zinc).
   • Chromoproteins: Bound to colored pigments. Eg: Hemoglobin, cytochromes.

3. Derived Proteins: Formed from simple and conjugated proteins due to heat, enzymes, or chemicals.
   • Primary Derived Proteins: Products of partial hydrolysis.
   • Secondary Derived Proteins: Further degradation products.

Based on Shape and Size

• Globular Proteins: Spherical, soluble in water. Eg: Enzymes, hemoglobin.
• Fibrous Proteins: Elongated, insoluble, structural proteins. Eg: Collagen, keratin.

Based on Function

• Catalytic Proteins:
• Regulatory Proteins:
• Protective Proteins:
• Storage Proteins: Casein, ovalbumin.