Protein Structure, Function, and Nutrition: Study Guide for ANP College Students

Proteins: Structure and Classification

Defining Proteins and Amino Acids

Proteins are essential macromolecules in the human body, composed of monomeric subunits called amino acids. They account for approximately 20% of body mass and are crucial for structural and functional roles. Over 100,000 distinct proteins exist in the human body, each with unique functions.

• Amino acids are organic compounds containing carbon, hydrogen, oxygen, and nitrogen.

• Each amino acid has a central carbon atom, an amino group, a carboxyl group, and a variable side chain (R group).

• The side chain determines the chemical properties and classification of the amino acid.

Classification of Amino Acids

Amino acids are grouped based on their side chain properties:

• Nonpolar amino acids: Hydrophobic, with long or bulky side chains.

• Polar amino acids: Hydrophilic, not charged.

• Acidic amino acids: Hydrophilic, negatively charged.

• Basic amino acids: Hydrophilic, positively charged.

Nutritional Classification

Amino acids are also classified by their nutritional role:

• Nonessential amino acids: Synthesized by the body in sufficient amounts.

• Essential amino acids: Cannot be synthesized by the body; must be obtained from diet.

• Conditionally essential amino acids: Become essential during certain life stages (e.g., growth).

Essential Amino Acids	Nonessential Amino Acids
Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine	Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Proline, Serine, Tyrosine

Protein Synthesis and Structure

Building Proteins: Transcription, Translation, and Folding

Protein synthesis is a multi-step process:

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

• Translation: mRNA is decoded to synthesize a protein.

• Protein folding: The sequence of amino acids folds into a specific three-dimensional shape.

Peptide Bond Formation

Amino acids are linked by peptide bonds to form proteins. The reaction produces a peptide and water.

• Peptide bond: Covalent bond between the carboxyl group of one amino acid and the amino group of another.

$\text{Amino acid 1} + \text{Amino acid 2} \rightarrow \text{Peptide} + \text{Water}$

Levels of Protein Structure

Proteins are organized into four structural levels:

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding into alpha helices and beta sheets.

• Tertiary structure: Overall three-dimensional shape.

• Quaternary structure: Association of multiple polypeptide chains.

Protein Denaturation

Denaturation: Unraveling Protein Structure

Denaturation refers to physical changes in a protein’s structure due to external stress, such as heat, acid, salt, alcohol, or mechanical agitation. The protein unfolds, losing its functional shape.

• Denatured proteins cannot perform their biological functions.

• Examples: Cooking eggs (heat denatures egg proteins).

Protein Digestion and Absorption

Digestive Process

Protein digestion begins in the mouth and continues in the stomach and small intestine.

• Stomach releases gastric juices (hydrochloric acid and pepsin) to break down proteins.

• Pepsin dismantles protein chains into smaller fragments.

• Pancreatic enzymes (chymotrypsin, trypsin) and intestinal enzymes further break down proteins to amino acids.

• Amino acids are absorbed into the bloodstream in the small intestine.

Protein Turnover and Amino Acid Pool

Protein turnover is the continuous breakdown and synthesis of proteins in the body.

• Over 250 grams of protein are broken down and rebuilt daily.

• Amino acids are recycled for protein synthesis, DNA/RNA production, and energy.

• The body does not store protein; adequate dietary intake is essential.

Functions of Proteins in the Body

Structural and Motion Functions

Proteins provide structure and enable movement.

• Collagen: Major component of bone, tendons, ligaments, cartilage, skin, and muscle; provides strength and flexibility.

Enzymes

Enzymes are proteins that catalyze biochemical reactions.

• Lower activation energy and speed up reactions.

• Highly specific for their substrates.

Hormones

Many hormones are proteins, acting as chemical messengers produced by endocrine glands.

• Regulate enzyme activity and bodily functions.

Fluid and Acid-Base Balance

Proteins help maintain fluid distribution and pH balance.

• Albumin: Most abundant blood protein; regulates fluid and acid-base balance.

Molecular Transport

Transport proteins move molecules in and out of cells.

• Channels and carriers facilitate movement across membranes.

Protection

Proteins are vital for immune defense.

• Antibodies: Protect against pathogens.

• Lysozyme: Enzyme that destroys bacteria.

Wound Healing and Tissue Regeneration

Proteins are required for cell growth, repair, and regeneration.

• Enzymes, transport proteins, hormones, and collagen are involved in healing.

Energy Production

Amino acids can be catabolized for energy, especially when dietary intake is insufficient.

• About 10% of dietary protein is used for energy daily.

• The liver breaks down amino acids to carbon skeletons for the citric acid cycle.

Diseases Involving Proteins

Protein Deficiency

Severe protein deficiency leads to two main syndromes:

• Kwashiorkor: Swelling, poor skin health, growth retardation, low muscle mass, liver malfunction.

• Marasmus: Emaciation, poor skin health, growth retardation.

Excess Protein Intake

High-protein diets (>30% of calories from protein) may increase risk for cardiovascular disease, diabetes, and restrict other essential nutrients.

• Potential for vitamin/mineral deficiencies and organ stress.

• Moderation and variety are key for healthy protein intake.

Proteins, Diet, and Personal Choices

Recommended Dietary Allowance (RDA) for Protein

The RDA for protein is calculated based on body weight:

• RDA for adults: 0.8 grams per kilogram of body weight.

• Calculation: $\text{Protein (g)} = \left(\frac{\text{Weight in lbs}}{2.2}\right) \times 0.8$

Dietary Sources of Protein

Protein-rich foods include:

• Meat, seafood, poultry, eggs, soy, beans, peas, seeds.

• High-quality proteins contain all essential amino acids.

• Complete protein sources: All nine essential amino acids (e.g., animal products, soy).

• Incomplete protein sources: Missing one or more essential amino acids (e.g., most plant foods).

• Complementary foods: Combining plant foods to provide all essential amino acids.

Protein Quality: PDCAAS

Protein Digestibility Corrected Amino Acid Score (PDCAAS) measures protein quality:

• Score of 1: Highest quality (milk protein, egg whites, whey, soy).

Vegetarian and Vegan Diets

• High-quality plant-based proteins: Tofu, soy, quinoa, pistachios.

• Vegetarians should include fortified foods for vitamins B12, D, calcium, omega-3 fatty acids, and choline.

• Vegetarian diets are associated with reduced risk of diabetes, heart disease, and cancer.

Protein and Exercise

• Dietary protein helps prevent sarcopenia (age-related muscle loss).

• Protein supplements do not enhance exercise performance or muscle gain beyond food protein.

• Most people, including athletes, do not require high protein intake.

Summary Table: Complete vs. Incomplete Protein Sources

Complete Protein Sources	Incomplete Protein Sources
Eggs, milk, meat, fish, soy	Beans, peas, nuts, grains, vegetables

Key Takeaways

• Proteins are polymers of amino acids, essential for structure, function, and regulation in the body.

• Protein synthesis involves transcription, translation, and folding.

• Proteins are digested and absorbed through enzymatic and mechanical processes.

• Protein deficiency and excess can lead to health risks; moderation is essential.

• Dietary sources and quality of protein are important for health, especially for vegetarians and older adults.