Proteins: Structure, Function, and Nutrition

Proteins: Structure, Function, and Nutrition

Section 6.1: Defining Protein

Proteins are essential macromolecules in the human body, responsible for a wide range of structural and functional roles. They are polymers made up of amino acid monomers, which differ from carbohydrates and lipids in their complexity and nitrogen content.

• Proteins: Macromolecules composed of amino acids; make up about 20% of the human body.

• Amino acids: Simple monomers containing carbon, hydrogen, oxygen, and nitrogen; building blocks of proteins.

• There are over 100,000 different proteins in the human body.

Amino acids have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

• Amino acids are classified into four groups based on their side chains:

  • 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.

• Amino acids are also categorized by nutritional aspects:

  • Nonessential amino acids: Synthesized by the body.

  • Essential amino acids: Must be obtained from the diet.

  • Conditionally essential amino acids: Required from the diet only under certain conditions (e.g., growth).

Proteins are synthesized through three main steps:

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

• Translation: mRNA is decoded to synthesize a protein.

• Protein folding: The amino acid chain folds into its functional shape.

Amino acids are linked by peptide bonds to form proteins.

Proteins have four levels of structural organization:

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding (alpha helices, beta sheets).

• Tertiary structure: Overall 3D shape of a single polypeptide.

• Quaternary structure: Arrangement of multiple polypeptide chains.

Section 6.2: The Role of Proteins in Foods: Cooking and Denaturation

Proteins contribute to the structure and texture of foods. When exposed to external stresses, proteins can undergo denaturation, losing their functional shape.

• Denaturation: Physical changes in protein structure due to heat, acid, salt, alcohol, or mechanical agitation.

• Denatured proteins lose their complex structure and become long strands of amino acids.

Section 6.3: Protein Digestion and Absorption

Protein digestion begins in the mouth and continues in the stomach and small intestine, involving both mechanical and chemical processes.

• In the stomach, gastric juices (hydrochloric acid and pepsin) initiate protein breakdown.

• In the small intestine, pancreatic enzymes (trypsin, chymotrypsin) and intestinal enzymes further digest proteins into amino acids.

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

The body continually recycles amino acids through protein turnover, breaking down and rebuilding proteins daily.

Section 6.4: Protein’s Functions in the Body

Proteins perform a wide variety of functions essential for life, due to their diverse structures and properties.

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

• Enzymes: Catalyze biochemical reactions, lowering activation energy and increasing reaction rates.

• Hormones: Many hormones are proteins that regulate physiological processes.

• Fluid and acid-base balance: Proteins like albumin maintain fluid distribution and pH balance in the body.

• Transport: Transport proteins move molecules across cell membranes and in the blood.

• Protection: Antibodies and enzymes defend against pathogens.

• Wound healing and tissue regeneration: Proteins are required for cell growth and repair.

• Energy production: Some amino acids can be used for energy, especially during fasting or intense exercise.

Section 6.5: Diseases Involving Proteins

Both protein deficiency and excess can have significant health consequences.

• Protein deficiency can lead to:

  • Kwashiorkor: Severe protein and micronutrient deficiency, causing swelling, poor skin health, growth retardation, and liver malfunction.

  • Marasmus: Severe protein and energy deficiency, resulting in emaciation and growth retardation.

• Excess protein intake (over 30% of calories from protein) may increase the risk of cardiovascular disease, diabetes, and organ stress, and can lead to nutrient imbalances.

Section 6.6: Proteins, Diet, and Personal Choices

Protein requirements depend on age, activity level, and health status. The Recommended Dietary Allowance (RDA) for adults is 0.8 grams per kilogram of body weight per day.

• Nitrogen balance: Achieved when protein intake equals protein use and excretion.

• Equation for daily protein requirement:

$(\text{Weight in lbs.} \div 2.2) \times 0.8 = \text{grams of protein per day}$

• Dietary sources of protein include meat, seafood, poultry, eggs, soy, beans, peas, and seeds.

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

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

• Complementary foods: Combining different plant proteins to obtain all essential amino acids.

• Protein Digestibility Corrected Amino Acid Score (PDCAAS): Measures protein quality based on amino acid content and digestibility.

• Vegetarians and vegans can meet protein needs with careful planning and inclusion of fortified foods.

• Sarcopenia: Age-related muscle loss; adequate protein intake may help prevent it.

• Protein and amino acid supplements are generally unnecessary for most people and do not enhance exercise performance more than food protein.

Key Takeaways: Adequate protein intake is essential for health, but both deficiency and excess can be harmful. Most adults can meet their protein needs through a balanced diet without supplements.