Introduction to Proteins: Structure, Function, and Dietary Sources

Proteins: Crucial Components of all Body Tissues

Introduction to Proteins

Proteins are essential macromolecules that play a vital role in the structure and function of all living cells. They are composed of chains of amino acids and are involved in numerous biological processes, including tissue building, enzyme activity, and nutrient transport.

• Definition: Proteins are functionally diverse classes of molecules made of one or more chains of amino acids (AA).

• Structure: Proteins typically have 50-10,000 amino acids joined together by peptide bonds.

• Dietary Proteins: Foods can contain complete or incomplete proteins. Complete proteins provide all essential amino acids, while incomplete proteins lack one or more.

• Sources: Common protein sources include meat, poultry, seafood, dairy products, whole grains, legumes, and nuts/seeds.

Amino Acids and Peptide Bonds

Amino acids are the building blocks of proteins. They are linked together by peptide bonds to form polypeptide chains, which fold into functional proteins.

• Amino Acid: Organic compounds containing both amino (-NH2) and carboxyl (-COOH) groups.

• Peptide Bond: A covalent bond formed between the amino group of one amino acid and the carboxyl group of another.

• General Formula:

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + \text{H}_2\text{O}$

Functions of Proteins in the Body

Proteins serve a variety of functions essential for health and metabolism:

• Structural: Build and repair tissues (e.g., muscle, skin, hair).

• Enzymatic: Act as enzymes to catalyze biochemical reactions.

• Transport: Carry molecules such as oxygen (hemoglobin) and nutrients.

• Hormonal: Regulate physiological processes (e.g., insulin).

• Energy: Provide energy when carbohydrates and fats are insufficient.

Dietary Sources of Protein

Consuming a variety of protein-rich foods ensures adequate intake of essential amino acids.

• Animal Sources: Meat, poultry, fish, eggs, dairy products.

• Plant Sources: Legumes, nuts, seeds, whole grains.

Protein in Nutrition: Key Points

• Proteins are not a primary storage form of energy but can be used for energy if needed.

• Proteins are not composed of long chains of nucleic acids (that describes DNA/RNA).

• Proteins are an important source of nitrogen in the diet.

Examples and Applications

• Example: Chicken breast, lentils, and tofu are all considered good sources of dietary protein.

• Non-examples: Foods like cucumber and yogurt (in small amounts) are not considered significant sources of protein.

Table: Common Sources of Dietary Protein

Food	Type	Protein Content (per 100g)
Chicken Breast	Animal	31g
Lentils	Plant	9g
Eggs	Animal	13g
Almonds	Plant	21g
Cucumber	Plant	0.7g
Yogurt	Animal	3.5g
Tiger Shrimp	Animal	24g

Additional info: Protein requirements vary by age, sex, and activity level. The Recommended Dietary Allowance (RDA) for adults is 0.8g protein per kg body weight per day.