Proteins and Lipids: Structure, Function, and Nutrition

Proteins and Amino Acids

Structure and Types of Amino Acids

Proteins are essential macromolecules composed of amino acids, which serve as their building blocks. The unique presence of nitrogen in amino acids distinguishes them from carbohydrates and fats. Amino acids are linked by peptide bonds to form peptides and proteins.

• Amino acid: Single building block of protein.

• Dipeptide: Two amino acids linked together.

• Polypeptide: Long chain of amino acids.

• Protein: One or more folded polypeptide chains.

The sequence and structure of amino acid side chains (R groups) determine the protein’s shape and function in the body.

• Essential amino acids (9): Must be obtained from food.

• Nonessential amino acids (11): Can be synthesized by the body.

Functions of Protein in the Body

Proteins perform a wide range of vital functions:

• Enzymes: Catalyze biochemical reactions (e.g., digestive enzymes).

• Antibodies: Support immune defense.

• Fluid balance: Maintain proper distribution of fluids in body compartments.

• Transport: Carry substances (e.g., hemoglobin transports oxygen).

• Structural: Build collagen and connective tissues.

• Hormones: Regulate physiological processes (e.g., insulin).

• Muscle structure and repair: Support growth and healing.

Protein Digestion and Absorption

Protein digestion begins in the stomach and is completed in the small intestine:

• Stomach: Hydrochloric acid (HCl) denatures proteins and activates pepsin, which breaks proteins into smaller peptides.

• Small intestine: Pancreatic enzymes further break peptides into individual amino acids, which are absorbed into the bloodstream.

Denaturation of Proteins

Denaturation is the process by which a protein loses its natural shape due to external factors such as heat, acid, alcohol, or physical agitation. This process is often irreversible and affects protein function.

• Example: Cooking an egg causes the clear albumin to turn white due to denaturation.

Protein Quality: Complete vs Incomplete Proteins

Proteins are classified based on their amino acid composition:

• Complete proteins: Contain all 9 essential amino acids. Examples: Meat, poultry, fish, eggs, dairy, soy, quinoa.

• Incomplete proteins: Lack one or more essential amino acids. Examples: Grains, legumes, nuts, seeds, vegetables.

Complementary Proteins

Combining two or more incomplete protein sources can provide all essential amino acids. This is important in vegetarian and vegan diets.

• Examples: Rice and beans, peanut butter and whole wheat bread, hummus and pita bread.

Protein Deficiency Disorders

• Kwashiorkor: Caused by very low protein intake with adequate calories. Symptoms: Edema, swollen belly, fatty liver, skin changes.

• Marasmus: Caused by severe deficiency of both protein and calories. Symptoms: Extreme weight loss, muscle wasting, very low body fat.

Recommended Protein Intake

The recommended dietary allowance (RDA) for protein in adults is:

• 0.8 grams per kilogram of body weight per day

Calculation steps:

1. Convert weight from pounds to kilograms: $\text{kg} = \frac{\text{lb}}{2.2}$

2. Multiply kilograms by 0.8: $\text{Protein (g)} = \text{kg} \times 0.8$

Example: 150 lb person: $150 \div 2.2 = 68 \text{ kg}$; $68 \times 0.8 = 54 \text{ g protein/day}$

Protein needs are higher for athletes, adolescents, and those recovering from illness or injury.

Fate of Excess Protein

The body cannot store excess protein. When intake exceeds needs:

1. Deamination: Nitrogen is removed from amino acids.

2. Nitrogen is converted to urea and excreted in urine.

3. The remaining carbon skeleton is used for energy, converted to glucose, or stored as fat.

Fats, Oils, and Other Lipids

Types of Lipids

Lipids are a diverse group of hydrophobic molecules. The three main types are:

• Triglycerides: Composed of glycerol and three fatty acids; main form of fat in food and body; used for energy storage.

• Phospholipids: Composed of glycerol, two fatty acids, and a phosphate group; essential for cell membrane structure.

• Sterols: Lipids with a ring structure; example: cholesterol, which is used to make hormones and vitamin D.

Omega-3 Fatty Acids

Omega-3 fatty acids are polyunsaturated fats with health benefits:

• Reduce inflammation

• Lower triglyceride levels

• Support heart and brain health

Food sources: Salmon, sardines, mackerel, flaxseed, chia seeds, walnuts.

Recommendation: Eat fish about two times per week.

Saturated vs Unsaturated Fats

• Saturated fats: No double bonds; usually solid at room temperature; found in butter, cheese, fatty meats, coconut oil.

• Unsaturated fats: One or more double bonds; usually liquid at room temperature; found in olive oil, nuts, seeds, avocados, fish.

Unsaturated fats are generally healthier and can improve cholesterol levels and support heart health.

Cholesterol and Lipoproteins

Cholesterol is transported in the blood by lipoproteins:

• LDL (Low-Density Lipoprotein): "Bad cholesterol"; delivers cholesterol to tissues and can contribute to artery plaque.

• HDL (High-Density Lipoprotein): "Good cholesterol"; removes cholesterol from tissues and transports it to the liver for excretion.

Atherosclerosis

Atherosclerosis is the buildup of plaque in arteries, leading to narrowed vessels and reduced blood flow. Plaque consists of cholesterol, fat, calcium, and inflammatory cells. This condition increases the risk of heart disease, heart attacks, and strokes.

Heart Disease Risk Factors

Controllable Risk Factors	Uncontrollable Risk Factors
Diet	Age
Smoking	Sex
Physical inactivity	Genetics
High cholesterol	Family history
High blood pressure
Obesity

Fat Digestion and Absorption

Fat digestion involves several organs and enzymes:

• Liver: Produces bile, which emulsifies fats.

• Gallbladder: Stores and releases bile into the small intestine.

• Pancreas: Releases lipase, which breaks triglycerides into fatty acids.

• Most fat digestion and absorption occurs in the small intestine.

Vegetarian Diet Types

Diet Type	Included Foods	Excluded Foods
Vegan	Plant foods only	All animal products
Lacto-vegetarian	Plant foods, dairy	Meat, eggs
Ovo-vegetarian	Plant foods, eggs	Meat, dairy
Lacto-ovo vegetarian	Plant foods, eggs, dairy	Meat
Pescatarian	Plant foods, fish	Other meats

Key Vocabulary

• Abundant: Present in large amounts

• Adequate: Enough to meet needs

• Convert: To change from one form to another

• Element: A basic chemical substance

• Emulsify: To break fat into smaller droplets

• Excluded: Left out

• Risk factors: Conditions or behaviors that increase disease risk

• Sequence: The specific order of components

• Synthesis: The process of building molecules