Proteins: Structure, Function, and Nutrition

Proteins: Structure, Function, and Nutrition

Section 1: Defining Protein

Proteins are essential macromolecules that play a critical role in the structure and function of the human body. They are composed of amino acids, which are organic compounds containing carbon, hydrogen, oxygen, and nitrogen. Proteins differ from carbohydrates and lipids in their complexity and the presence of nitrogen.

• Proteins: Polymers made up of amino acid monomers; constitute about 20% of the human body.

• Amino acids: Building blocks of proteins, each containing a central carbon atom, an amino group, a carboxyl group, and a unique 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 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.

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

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

Section 2: Protein Synthesis and Structure

Proteins are synthesized through a series of steps involving genetic information and chemical reactions:

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

• Translation: mRNA is decoded to assemble amino acids into a polypeptide chain.

• Protein folding: The polypeptide chain folds into its functional three-dimensional 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 into alpha helices or beta sheets.

• Tertiary structure: Overall three-dimensional shape.

• Quaternary structure: Association of multiple polypeptide chains.

Section 3: Protein Denaturation

Denaturation is the process by which proteins lose their native structure due to external stress, such as heat, acid, high salt, alcohol, or mechanical agitation. This process unravels the protein, rendering it nonfunctional.

• Denaturation: Physical change in protein structure, resulting in loss of function.

• Common in cooking (e.g., egg whites turning solid when heated).

Section 4: 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 then absorbed into the bloodstream.

• Stomach: Hydrochloric acid and pepsin initiate protein breakdown.

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

• Absorption: Amino acids are transported into the blood for use by the body.

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

Section 5: Functions of Proteins in the Body

Proteins perform a wide variety of functions essential for life:

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

• Enzymes: Catalyze biochemical reactions, lowering activation energy.

• Hormones: Chemical messengers that regulate physiological processes.

• Fluid and acid-base balance: Maintain water distribution and pH balance (e.g., albumin).

• Transport: Move molecules across cell membranes.

• Protection: Antibodies and enzymes defend against pathogens.

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

• Energy production: Amino acids can be used for energy when necessary.

Section 6: Diseases Involving Proteins

Both protein deficiency and excess can have significant health consequences:

• Kwashiorkor: Severe protein and micronutrient deficiency, leading to swelling, poor growth, and liver problems.

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

• Excess protein intake may increase the risk of cardiovascular disease, diabetes, and kidney problems.

Section 7: 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.

• High-quality proteins contain all essential amino acids; most animal proteins are complete, while most plant proteins are incomplete (except soy and quinoa).

• Vegetarians and vegans can meet protein needs by combining complementary plant proteins.

• Protein supplements are generally unnecessary for most people.

Equation for calculating daily protein needs:

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

Protein Digestibility Corrected Amino Acid Score (PDCAAS): A measure of protein quality based on amino acid content and digestibility. Milk protein, egg whites, whey, and soy have the highest scores (1.0).

Sarcopenia: Age-related loss of muscle mass; adequate protein intake and exercise can help prevent it.

Key Takeaways

• Proteins are vital for structure, function, and regulation of the body’s tissues and organs.

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

• Protein synthesis involves transcription, translation, and folding.

• Denaturation alters protein structure and function.

• Protein digestion and absorption are essential for providing amino acids to the body.

• Both deficiency and excess of protein can lead to health problems.

• Dietary protein should be balanced and sourced from a variety of foods to ensure all essential amino acids are consumed.