Proteins and Amino Acids

 

Proteins and Amino Acids

Introduction

Proteins are complex macromolecules made up of amino acids linked by peptide bonds. They perform various structural, enzymatic, and regulatory functions in living organisms. Proteins are essential for cell growth, repair, and metabolic processes.

Amino Acids: Building Blocks of Proteins

Definition

Amino acids are organic molecules containing an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R-group), all attached to a central carbon atom (α-carbon).

General Structure of an Amino Acid

mathematica

CopyEdit

   H

   |

H₂N—C—COOH

   |

   R

• Amino group (-NH₂): Basic in nature.
• Carboxyl group (-COOH): Acidic in nature.
• R-group (Side chain): Determines the chemical properties of the amino acid.

Classification of Amino Acids

Amino acids are classified based on their properties and dietary requirements.

1. Based on Nutritional Requirement

Essential Amino Acids: Cannot be synthesized by the body and must be obtained from the diet.

Examples: Lysine, Leucine, Isoleucine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine, and Histidine (for infants).

Non-Essential Amino Acids: Can be synthesized by the body.

Examples: Alanine, Glycine, Serine, Asparagine, Glutamine.

2. Based on Chemical Properties

1. Polar Amino Acids (Hydrophilic): Serine, Threonine, Asparagine, Glutamine.
2. Non-Polar Amino Acids (Hydrophobic): Glycine, Alanine, Valine, Leucine, Isoleucine.
3. Acidic Amino Acids: Aspartic acid, Glutamic acid.
4. Basic Amino Acids: Lysine, Arginine, Histidine.

Peptide Bond Formation

Amino acids link together by peptide bonds to form proteins. This occurs through a condensation reaction, where a water molecule is removed.

Proteins: Structure and Function

Definition

Proteins are macromolecules composed of one or more long chains of amino acids, folded into specific three-dimensional structures.

Levels of Protein Structure

Proteins have four levels of structural organization:

Primary Structure: Linear sequence of amino acids linked by peptide bonds. Determines the protein’s function. Example: Insulin.

Secondary Structure: Folding of the polypeptide chain due to hydrogen bonding. Forms α-helix and β-pleated sheets. Example: Keratin (α-helix), Silk fibroin (β-sheet).

Tertiary Structure: Three-dimensional folding of the polypeptide chain due to interactions between R-groups (hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bonds). Determines the protein's biological activity. Example: Enzymes, Myoglobin.

Quaternary Structure: Association of two or more polypeptide chains. Example: Hemoglobin (4 polypeptide subunits).

Types of Proteins

1. Based on Composition

1. Simple Proteins: Contain only amino acids (e.g., Albumin, Globulin).
2. Conjugated Proteins: Contain amino acids and non-protein components (e.g., Hemoglobin - contains iron).

2. Based on Function

1. Structural Proteins: Provide support and strength (e.g., Collagen, Keratin).
2. Enzymes: Catalyze biochemical reactions (e.g., Amylase, DNA polymerase).
3. Transport Proteins: Carry molecules (e.g., Hemoglobin transports oxygen).
4. Hormonal Proteins: Regulate body functions (e.g., Insulin).
5. Defensive Proteins: Protect the body (e.g., Antibodies).
6. Contractile Proteins: Involved in muscle movement (e.g., Actin, Myosin).

 

Protein Metabolism

Protein Digestion: Begins in the stomach with the enzyme pepsin. In the small intestine, enzymes trypsin and chymotrypsin break proteins into amino acids.

1. Amino Acid Absorption: Absorbed into the bloodstream and transported to cells for protein synthesis.
2. Protein Synthesis: DNA codes for mRNA, which is translated into proteins by ribosomes.
3. Protein Degradation: Deamination: Excess amino acids are converted into ammonia (NH_₃) and excreted as urea.

Sources of Proteins

Animal Sources: Meat, fish, eggs, milk, cheese.

Plant Sources: Legumes, beans, nuts, grains, soy products.

Biological Importance of Proteins

1. Growth and Repair: Essential for tissue growth and repair.
2. Enzyme Production: Enzymes are proteins that speed up biochemical reactions.
3. Immune Function: Antibodies are proteins that protect against infections.
4. Transport of Molecules: Hemoglobin transports oxygen, and albumin transports nutrients.
5. Energy Source: Can be broken down for energy when carbohydrate and fat stores are low.

Protein Deficiency and Disorders

1.Kwashiorkor: Severe protein deficiency, common in children.

Symptoms: Edema, muscle wasting, swollen belly.

2. Marasmus: Deficiency of both proteins and calories.

Symptoms: Extreme thinness, weakness.

3.Phenylketonuria (PKU): Genetic disorder where the body cannot break down phenylalanine which can cause brain damage if untreated.

4.Albinism: Deficiency in melanin production due to a lack of tyrosinase enzyme.

References

1. Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry. W. H. Freeman.
2. Berg, J. M., Tymoczko, J. L., & Stryer, L. (2015). Biochemistry. W. H. Freeman.
3. Voet, D., Voet, J. G., & Pratt, C. W. (2016). Fundamentals of Biochemistry: Life at the Molecular Level. Wiley.
4. Guyton, A. C., & Hall, J. E. (2021). Textbook of Medical Physiology.