Protein Structure, Function, and Nutrition: Study Notes 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 different proteins exist in the human body.

• Amino acids: Simple monomers made of carbon, oxygen, hydrogen, and nitrogen. They are the building blocks of proteins.

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

Classification of Amino Acids

Amino acids are grouped based on their chemical properties and nutritional aspects.

• Chemical classification:

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

  • Polar amino acids: Hydrophilic, not charged.

  • Acidic amino acids: Hydrophilic, negatively charged.

  • Basic amino acids: Hydrophilic, positively charged.

• Nutritional classification:

  • 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).

Protein Synthesis and Structure

Building Proteins: Transcription, Translation, and Folding

Protein synthesis is a multi-step process involving genetic information and chemical reactions.

• 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 Bonds and Protein Complexity

Amino acids are linked by peptide bonds to form proteins. Proteins are structurally more complex than carbohydrates and lipids, organized into four structural levels:

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding (alpha helix, beta sheet).

• Tertiary structure: Overall three-dimensional shape.

• Quaternary structure: Association of multiple polypeptide chains.

Protein Denaturation and Its Consequences

Denaturation: Unraveling Protein Structure

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

• Example: Cooking an egg causes the proteins to denature, changing its texture.

Protein Digestion and Absorption

Digestive Process: From Mouth to Small Intestine

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.

• Small intestine: Pancreatic enzymes (chymotrypsin, trypsin) and intestinal enzymes further digest proteins into amino acids.

• Amino acids are absorbed into the blood for use by the body.

Protein Turnover and Amino Acid Recycling

Protein turnover is the continual breakdown and synthesis of proteins.

• Over 250 grams of protein are dismantled and rebuilt daily.

• Amino acids are used to build proteins, DNA, RNA, and produce energy.

Functions of Proteins in the Body

Structural and Functional Roles

Proteins perform a vast array of functions due to their diverse shapes and sizes.

• Structure and motion: Collagen provides strength and flexibility to bones, tendons, ligaments, cartilage, skin, and muscle.

• Enzymes: Catalyze specific chemical reactions, lowering activation energy.

• Hormones: Chemical messengers produced by endocrine glands; regulate enzyme activity.

• Fluid and acid-base balance: Albumin maintains water distribution and pH balance.

• Molecular transport: Transport proteins move molecules in and out of cells.

• Protection: Enzymes and antibodies defend against pathogens.

• Wound healing and tissue regeneration: Proteins are essential for cell creation and repair.

• Energy production: Some amino acids are catabolized for energy, especially during fasting or intense exercise.

Diseases Involving Proteins

Protein Deficiency Syndromes

• Kwashiorkor: Severe protein and micronutrient deficiency; symptoms include swelling, poor skin health, growth retardation, low muscle mass, and liver malfunction.

• Marasmus: Severe protein and energy deficiency; symptoms include emaciation, poor skin health, and growth retardation.

Excess Protein Intake

Diets high in protein (>30% of calories) may increase risk of cardiovascular disease, diabetes, and restrict other essential nutrients. Moderation and variety are key to a healthy diet.

Proteins, Diet, and Personal Choices

Recommended Dietary Allowance (RDA) for Protein

The RDA for protein for adults is 0.8 grams per kilogram of body weight.

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

• Acceptable Macronutrient Distribution Range (AMDR): 10–35% of calories from protein.

Dietary Sources of Protein

• Protein food group includes meat, seafood, poultry, eggs, soy, beans, peas, and seeds.

• Complete protein sources: Contain all nine essential amino acids (e.g., animal proteins, soy).

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

• Complementary foods: Combining foods to provide all essential amino acids (e.g., beans and rice).

Protein Quality: PDCAAS

The Protein Digestibility Corrected Amino Acid Score (PDCAAS) measures protein quality based on amino acid content and digestibility.

• Milk protein, egg whites, whey, and soy have the highest PDCAAS score (1).

Vegetarian and Vegan Diets

• High-quality plant-based protein sources include tofu, soy, quinoa, and pistachios.

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

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

Protein Requirements for Special Populations

• Sarcopenia: Age-related decline in muscle mass and strength; dietary protein may help prevent this.

• Intense exercise increases protein needs for muscle repair, but supplements are not proven to enhance performance or strength.

Summary of Key Takeaways

• Amino acids are classified by chemical and nutritional properties.

• Proteins are synthesized through transcription, translation, and folding, and have four structural levels.

• Denaturation alters protein structure and function.

• Protein digestion involves mechanical and chemical processes; amino acids are recycled for various functions.

• Proteins serve structural, enzymatic, hormonal, transport, protective, and energy roles.

• Protein deficiency causes serious health issues; excess intake may pose risks.

• RDA for protein is 0.8 g/kg body weight; quality and source of protein are important for health.