Proteins: Structure, Function, and Nutrition

Proteins: Structure, Function, and Nutrition

Section 6.1: Defining Protein

Proteins are essential macromolecules in the human body, composed of amino acid monomers. They are crucial for structure, function, and regulation of the body's tissues and organs. More than 100,000 different proteins exist in the human body, making up about 20% of body mass.

• Proteins: Polymers made of amino acids, containing carbon, hydrogen, oxygen, and nitrogen.

• Amino acids: The building blocks of proteins, each with a central carbon, an amino group, a carboxyl group, and a unique side chain (R group).

Classification of Amino Acids:

• 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.

Amino acids are also categorized by nutritional aspects:

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

• Essential amino acids: Must be obtained from the diet; the body cannot synthesize them.

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

Proteins are synthesized in three main steps:

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

2. Translation: mRNA is decoded to synthesize a protein.

3. 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 helix, beta sheet).

• 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 stress, proteins can undergo denaturation, where their complex structure unravels into a simple strand of amino acids.

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

• Denatured proteins lose their functional shape and properties.

Section 6.3: Protein Digestion and Absorption

Protein digestion begins in the mouth and continues in the stomach and small intestine. Enzymes and acids break proteins into amino acids, which are absorbed into the bloodstream.

• Stomach: Gastric juices (hydrochloric acid and pepsin) initiate protein breakdown.

• Small intestine: Pancreatic enzymes (trypsin, chymotrypsin) and intestinal enzymes complete digestion to amino acids.

• Amino acids are absorbed and transported to the liver and other tissues.

The body continually recycles amino acids through protein turnover, breaking down and synthesizing proteins as needed.

Section 6.4: Protein’s Functions in the Body

Proteins perform a wide variety of functions essential for life:

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

• 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, repair, and regeneration.

• Energy production: 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 growth, and liver malfunction.

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

• Excess protein intake (over 30% of calories from protein) may increase the risk of cardiovascular disease, diabetes, and kidney problems, and can displace other essential nutrients.

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.

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

Dietary protein sources include meat, seafood, poultry, eggs, soy, beans, peas, and seeds. Proteins are classified as:

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

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

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

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 scores.

Vegetarians and vegans can meet protein needs with careful planning and by including high-quality plant proteins and fortified foods. Older adults and athletes may require slightly more protein, but supplements are generally unnecessary for most people.

Key Takeaways:

• Proteins are vital for structure, function, and regulation in the body.

• Amino acids are classified by chemical properties and nutritional needs.

• Protein digestion involves multiple enzymes and organs, and amino acids are continually recycled.

• Both deficiency and excess of protein can cause health problems; moderation and variety are essential.

• Most people can meet protein needs through a balanced diet without supplements.