Definition of Proteins

Proteins are large, complex molecules composed of one or more long chains of amino acids. They play crucial roles in the structure, function, and regulation of the body’s tissues and organs. Proteins are essential for various biological processes, including catalysis of metabolic reactions, DNA replication, responding to stimuli, and transporting molecules.

Classification of Proteins Based on Composition

Simple Proteins
Composed only of amino acids.
Examples: Albumin (egg white), globulins (serum proteins).

2. Conjugated Proteins

Composed of amino acids and a non-protein component (prosthetic group).
Examples: Hemoglobin (with heme), glycoproteins (with carbohydrates).

3. Derived Proteins

Formed by partial hydrolysis of simple or conjugated proteins.
Examples: Peptones, peptides.

Based on Solubility

Albumins
Soluble in water and dilute salt solutions.
Examples: Serum albumin.

2. Globulins

Soluble in dilute salt solutions but insoluble in pure water.
Examples: Immunoglobulins.

3. Prolamins

Soluble in 70-80% ethanol.
Examples: Gliadin (wheat), zein (corn).

4. Glutelins

Soluble in dilute acids or bases.
Examples: Glutenin (wheat).

5. Scleroproteins

Insoluble in water and all neutral solvents.
Examples: Collagen, keratin.

Classification of Amino Acids Based on Chemical Nature

Nonpolar (Hydrophobic)
Glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan.

2. Polar (Hydrophilic)

Uncharged: Serine, threonine, cysteine, tyrosine, asparagine, glutamine.
Charged: Aspartic acid, glutamic acid (acidic); lysine, arginine, histidine (basic).

Based on Nutritional Requirements

Essential Amino Acids
Cannot be synthesized by the human body and must be obtained from the diet.
Examples: Lysine, tryptophan, methionine, valine, leucine, isoleucine, threonine, phenylalanine, histidine.

2. Non-Essential Amino Acids

Can be synthesized by the human body.
Examples: Alanine, aspartic acid, asparagine, glutamic acid, serine.

Structure of Proteins

Proteins have four levels of structural organization:

1. Primary Structure

The linear sequence of amino acids in a polypeptide chain.

2. Secondary Structure

Local folding of the polypeptide chain into helices or sheets.
- Examples: Alpha-helix, beta-pleated sheet.

3. Tertiary Structure

The three-dimensional folding pattern of a protein due to side chain interactions.

4. Quaternary Structure

The structure formed by the assembly of multiple polypeptide chains.
- Examples: Hemoglobin (four subunits).

Qualitative Tests for Proteins

Biuret Test
Add a few drops of biuret reagent to the test solution. A violet color indicates the presence of proteins.

2. Ninhydrin Test

Heat the test solution with ninhydrin. A blue or purple color indicates the presence of free amino acids and proteins.

3. Xanthoproteic Test

Add concentrated nitric acid to the test solution and heat. A yellow color indicates the presence of aromatic amino acids.

4. Millon’s Test

Add Millon’s reagent to the test solution and heat. A red color indicates the presence of tyrosine.

Biological Role of Proteins and Amino Acids

Structural Role: Proteins like collagen provide structure to cells and tissues.
Enzymatic Role: Enzymes are proteins that catalyze biochemical reactions.
Transport Role: Hemoglobin transports oxygen in the blood.
Defense Role: Antibodies are proteins that protect the body from pathogens.
Regulatory Role: Hormones like insulin regulate physiological processes.
Storage Role: Ferritin stores iron in the liver.

Diseases Related to Malnutrition of Proteins

Kwashiorkor
Cause: severe protein deficiency.
Symptoms: Edema, fatty liver, skin lesions, hair changes.
Treatment: increase protein in diet

2. Marasmus
Cause: severe deficiency of protein and calories.
Symptoms: severe wasting, stunted growth, weakness, anemia.
Treatment: increasing the proteins, carbohydrates, fats and other essential nutrients in diet .

3. Phenylketonuria (PKU)

Cause: A genetic disorder resulting in the inability to metabolize phenylalanine.
Symptoms: Intellectual disability, seizures, behavioral problems.
Treatment:
1. Dietary Management
a. Supplementation
b. Gene Therapy

4. Sickle Cell Anemia

Cause: a mutation in the hemoglobin gene, leading to the production of abnormal hemoglobin.
Symptoms: Anemia, pain, swelling in hands and feet, frequent infections.
Treatment:
Vaccinations: Regular immunizations (e.g., pneumococcal, meningococcal, and influenza vaccines) to prevent infections, which can be more severe in individuals with sickle cell anemia.
1. Prophylactic Antibiotics: Daily antibiotics (e.g., penicillin) for young children to prevent bacterial infections.
2. Folic Acid Supplements: To support red blood cell production and counteract anemia.