Proteins: Structure, Function, Digestion, and Dietary Considerations

Proteins in Nutrition

Introduction to Proteins

Proteins are essential macronutrients present in every cell of the human body. They play a critical role in cellular activities, supporting bodily functions, and are the building blocks of enzymes and some hormones.

• Definition: Proteins are large, complex molecules made up of amino acids and are vital for growth, repair, and maintenance of tissues.

• Role in the Body: Proteins participate in every cellular activity, including catalyzing biochemical reactions (as enzymes) and regulating physiological processes (as hormones).

Protein Structure and Comparison with Other Macronutrients

Structural Differences

Proteins differ from carbohydrates and lipids in their chemical composition and structure.

• Proteins: Composed of chains of amino acids, which are assembled based on genetic information (DNA).

• Carbohydrates: Made of monosaccharides (simple sugars) forming polysaccharides (e.g., glucose units).

• Lipids: Consist of fatty acids forming triglycerides.

• Unique Elements: Proteins contain nitrogen (and sometimes sulfur), unlike carbohydrates and lipids, which are composed mainly of carbon, hydrogen, and oxygen.

• Storage: Excess dietary protein cannot be stored in the body.

Table: Macronutrient Composition and Examples

Macronutrient	Composed of	Example
Carbohydrates	Monosaccharides	Polysaccharide (Glucose units)
Lipids	Fatty acids	Triglyceride (Fatty acids)
Proteins	Amino acids	Peptide (Amino acids)

Amino Acids: The Building Blocks of Proteins

Classification and Structure

Proteins are polymers of amino acids. There are 20 amino acids used to make proteins, classified by the number of amino acids in the chain.

• Peptides: Fewer than 50 amino acids

• Dipeptides: 2 amino acids

• Tripeptides: 3 amino acids

• Polypeptides: More than 10 amino acids

• Proteins: More than 50 amino acids (typically 100 to 10,000 linked together)

Anatomy of an Amino Acid: Each amino acid contains an amine group (NH2), a carboxylic acid group (COOH), and a unique side chain (R group) that determines its properties and function.

Peptide Bonds

• Formation: Peptide bonds form between the carboxyl group of one amino acid and the amine group of another via condensation reactions.

• Breakdown: Peptide bonds are broken by hydrolysis.

Types of Amino Acids

Essential, Nonessential, and Conditionally Essential Amino Acids

• Essential Amino Acids: Must be consumed in the diet (9 total).

• Nonessential Amino Acids: Can be synthesized by the body (11 total).

• Conditionally Essential Amino Acids: Normally nonessential but become essential under certain conditions (e.g., illness, inadequate synthesis).

Table: Classification of Amino Acids

Essential Amino Acids	Nonessential Amino Acids	Conditionally Essential
Histidine (His), Isoleucine (Ile), Leucine (Leu), Lysine (Lys), Methionine (Met), Phenylalanine (Phe), Threonine (Thr), Tryptophan (Trp), Valine (Val)	Alanine (Ala), Arginine (Arg), Asparagine (Asn), Aspartic acid (Asp), Cysteine (Cys), Glutamic acid (Glu), Glutamine (Gln), Glycine (Gly), Proline (Pro), Serine (Ser), Tyrosine (Tyr)	Tyrosine, Glycine (under specific conditions)

Protein Structure and Function

Levels of Protein Structure

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Geometric folding (e.g., alpha helix, beta sheet).

• Tertiary Structure: Three-dimensional globular shape.

• Quaternary Structure: Multiple polypeptide chains bonded together.

Denaturation

• Definition: Denaturation is the unfolding of proteins due to heat, acids, bases, salts, or mechanical agitation.

• Effect: Alters protein function but does not change the primary structure.

Protein Digestion and Absorption

Digestive Process

• Stomach: Gastrin stimulates HCl release, which denatures proteins and activates pepsinogen to pepsin. Pepsin breaks polypeptides into shorter chains.

• Small Intestine: Cholecystokinin stimulates pancreatic proteases, which further break down polypeptides into tripeptides and dipeptides. Dipeptidases and tripeptidases complete digestion to amino acids.

Table: Enzymes Involved in Protein Digestion

Enzyme	Location	Function
Pepsin	Stomach	Breaks polypeptides into peptide chains
Proteases	Pancreas/Small intestine	Break peptide bonds
Dipeptidase/Tripeptidase	Brush border of small intestine	Break dipeptides/tripeptides into amino acids

Amino Acid Absorption

• Amino acids are absorbed in the small intestine and transported to the liver via the portal vein.

• In the liver, amino acids are used for protein synthesis, converted to energy, glucose, or fat, or released into the bloodstream for use by cells.

Metabolism of Amino Acids

Amino Acid Pool and Protein Turnover

• The amino acid pool supplies the body's ongoing need for protein synthesis.

• Protein turnover refers to the continuous breakdown and synthesis of proteins (over 200 grams daily).

• Extra protein aids healing and tissue repair.

Protein Synthesis

• Transcription: DNA information is copied to messenger RNA (mRNA).

• Translation: mRNA binds to ribosomes, and transfer RNA (tRNA) brings specific amino acids to build the polypeptide chain.

• Elongation: The chain grows until the protein is complete.

Deamination and Transamination

• Deamination: Removal of the amine group from amino acids, forming ammonia, which is converted to urea and excreted in urine.

• Transamination: Formation of nonessential amino acids by transferring nitrogen from one amino acid to a keto acid.

Metabolic Fate

• Glucogenic amino acids can be converted to glucose via gluconeogenesis.

• Excess protein is converted to fatty acids and stored as triglycerides.

• Protein can be oxidized for energy if caloric intake is insufficient.

Functions of Protein in the Body

Major Functions

• Structural support and movement (muscles, tissues)

• Catalysis (enzymes)

• Chemical messaging (hormones)

• Fluid and acid-base balance

• Transport (oxygen, waste, lipids, vitamins, minerals)

• Immune function (antibodies)

• Energy source (4 kcal/gram)

• Satiety and appetite control

Protein Requirements and Nitrogen Balance

Daily Protein Needs

• Healthy adults should maintain nitrogen balance (intake equals loss).

• Positive nitrogen balance: Growth, pregnancy, recovery, muscle building.

• Negative nitrogen balance: Illness, trauma, starvation.

Recommended Intake

• RDA: 0.8 g/kg body weight per day for adults over 18.

• AMDR: 10–35% of total daily kilocalories.

• Athletes, pregnant women, and burn victims may require higher intake.

Dietary Sources and Quality of Protein

High-Quality vs. Low-Quality Protein

• High-Quality Protein: Easily digestible, contains all essential amino acids, supports synthesis of nonessential amino acids.

• Complete Proteins: Contain all nine essential amino acids (usually animal sources; soy is a plant-based exception).

• Incomplete Proteins: Lacking one or more essential amino acids (usually plant sources).

• Complementary Proteins: Combining foods to provide all essential amino acids (e.g., grains and legumes).

Food Sources

• Eggs, meat, fish, soy, and dairy are significant sources.

• 3-ounce serving of cooked meat, poultry, or fish provides 21–25 grams of protein.

• Eating a variety of foods is recommended; supplements are generally unnecessary.

Health Consequences of Protein Intake

Excess Protein

• May increase risk for heart disease (especially with high saturated fat intake).

• Increases risk for kidney stones (high animal protein, low carbohydrate diet).

• May increase risk for osteoporosis if calcium intake is low.

• Can displace other important foods (whole grains, fruits, vegetables).

Protein Deficiency

• Kwashiorkor: Severe protein deficiency, often with adequate energy intake. Symptoms: Edema, muscle wasting, brittle hair, increased infection risk.

• Marasmus: Severe deficiency of both energy and protein. Symptoms: Emaciation, weakness, thin hair, low body temperature, dehydration.

Vegetarian Diets: Benefits and Risks

Types of Vegetarian Diets

• Motivations: Ethical, religious, environmental, health.

• Types: Vegan, lacto-vegetarian, ovo-vegetarian, lacto-ovo vegetarian, semi-vegetarian.

Table: Types of Vegetarian Diets

Type	Included Foods	Excluded Foods
Vegan	Grains, vegetables, fruits, legumes, seeds, nuts	All animal foods (meat, fish, poultry, dairy, eggs)
Lacto-vegetarian	Grains, vegetables, fruits, legumes, seeds, nuts, dairy foods	Meat, fish, poultry, eggs
Ovo-vegetarian	Grains, vegetables, fruits, legumes, seeds, nuts, eggs	Meat, fish, poultry, dairy foods
Lacto-ovo vegetarian	Grains, vegetables, fruits, legumes, seeds, nuts, dairy foods, eggs	Meat, fish, poultry
Semi-vegetarian	Grains, vegetables, fruits, legumes, seeds, nuts, dairy foods, eggs, fish, poultry (occasionally)	Meat (occasionally)

Benefits

• Reduced risk of heart disease, high blood pressure, diabetes, cancer, stroke, and obesity.

Risks

• Potential for low intake of protein, calcium, iron, vitamin B12, zinc, vitamins A and D, and omega-3 fatty acids.

• Requires careful meal planning to ensure nutritional adequacy.

Additional info:

• Suggested servings for a healthy vegetarian diet include a variety of whole grains, legumes, nuts, seeds, fruits, and vegetables to meet nutrient needs.