Proteins: Structure, Function, and Nutrition

Overview of Protein Functions

Proteins are essential macromolecules that perform a wide variety of functions in the human body. Their roles extend from structural support to regulation of physiological processes.

• Structural Support and Movement: Proteins such as collagen and actin provide structure to cells and tissues and enable muscle contraction.

• Catalysis: Enzymes are proteins that accelerate biochemical reactions.

• Chemical Messengers: Some hormones, like insulin, are proteins that regulate body processes.

• Fluid Balance: Proteins help maintain the proper distribution of fluids in body compartments.

• Acid-Base Balance: Proteins act as buffers to help maintain the body's pH within a narrow range.

• Transport: Proteins such as hemoglobin transport substances like oxygen throughout the body.

• Immune Function: Antibodies are proteins that defend against pathogens.

• Energy Source: Proteins can be used for energy, providing 4 calories per gram.

• Satiety: Protein-rich foods help promote a feeling of fullness after eating.

Protein Composition and Structure

• Monomers: Proteins are composed of amino acids.

• Number of Amino Acids: There are 20 different amino acids that combine to form proteins.

• Peptide Bonds: Amino acids are linked by peptide bonds to form polypeptide chains.

• Essential Amino Acids: Nine amino acids are essential, meaning they must be obtained from the diet.

• Conditionally Essential Amino Acids: These are normally nonessential but become essential under certain conditions when the body cannot synthesize them.

Levels of Protein Structure

• Primary Structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding into geometric shapes such as alpha-helices and beta-sheets.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain.

• Quaternary Structure: The association of two or more polypeptide chains to form a functional protein.

Note: During denaturation, the primary structure remains unchanged.

Protein Denaturation

Denaturation refers to the unfolding and loss of a protein's native structure, which can be caused by heat, acids, bases, salts, or mechanical agitation. This process disrupts secondary, tertiary, and quaternary structures but leaves the primary structure intact.

Protein Digestion and Absorption

• Site of Digestion: Protein digestion begins in the stomach.

• Stomach Enzymes: Gastrin stimulates the release of gastric juices, and pepsin breaks down proteins into smaller peptides.

• Pancreatic Enzymes: The pancreas releases chymotrypsin and carboxypeptidase into the small intestine for further digestion.

• Absorption: Amino acids are absorbed through the small intestine into the bloodstream.

• Trypsin: Trypsin stimulates the pancreas to release proteases into the small intestine.

Protein Utilization and Nitrogen Balance

• Protein Wasting: An amino acid is considered "wasted" when it is converted to energy rather than used for protein synthesis.

• Nitrogen Balance: Reflects the balance between nitrogen intake and loss.

Dietary Protein Recommendations

• Recommended Range: Protein should provide 10–35% of daily caloric intake.

• Caloric Value: Each gram of protein provides 4 calories.

Protein Quality and Limiting Amino Acids

• Limiting Amino Acid: The essential amino acid present in the lowest quantity in a food protein, limiting the body's ability to synthesize proteins.

• Incomplete Proteins: Proteins that lack one or more essential amino acids (often found in plant sources).

Health Implications of Protein Intake

• Excess Protein: May increase the risk of heart disease, kidney stones, and can displace other essential nutrients.

• Inadequate Protein: Can lead to protein-energy malnutrition, impairing essential bodily functions.

Vegetarian Diets: Pros and Cons

• Pros: Lower risk of heart disease, high blood pressure, diabetes, cancer, stroke, and obesity.

• Cons: Potential for low intake of essential nutrients such as vitamin B12, iron, and zinc.

Summary Table: Protein Concepts

Key Equations

• Protein Caloric Contribution:

• Nitrogen Balance:

Example: A person consuming 60 grams of protein per day receives calories from protein.

Additional info: Essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Protein-energy malnutrition includes conditions such as kwashiorkor and marasmus.