Proteins and Lipids: Structure, Function, and Health Implications

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Proteins and Amino Acids

Protein Structure and Composition

Proteins are essential macromolecules composed of amino acids, which contain the element nitrogen—a distinguishing feature not found in carbohydrates or fats. The sequence and structure of amino acids determine the protein's function in the body.

Amino Acids: The building blocks of proteins. There are 20 different amino acids, of which 9 are essential (must be obtained from the diet) and 11 are non-essential (can be synthesized by the body).
Peptide Bonds: Amino acids are linked together by peptide bonds through a dehydration synthesis reaction, forming polypeptides and, ultimately, functional proteins.
Hierarchy: Amino acids form peptides (short chains), which combine to form proteins (long, functional chains).
Protein Function: Determined by the sequence and chemical properties of amino acid side chains (R groups), which influence the protein's three-dimensional structure.

Example: Hemoglobin is a protein whose function (oxygen transport) is determined by its specific amino acid sequence and structure.

Protein Functions in the Body

Enzyme Production: Proteins act as biological catalysts, speeding up chemical reactions.
Fluid Balance: Proteins help maintain osmotic pressure and fluid distribution between body compartments.
Transport: Proteins transport substances such as oxygen (hemoglobin), lipids (lipoproteins), and minerals.
Structural Support: Collagen provides structure to skin, bones, and connective tissues.
Immune Function: Antibodies are proteins that help defend against pathogens.
Excess Protein: Surplus protein is deaminated; the amino group is converted to urea and excreted, while the carbon skeleton is used for energy or stored as fat.

Protein Quality and Dietary Sources

Complete Proteins: Contain all essential amino acids in adequate amounts. Found in animal products (meat, eggs, dairy) and some plant sources (soy, quinoa).
Incomplete Proteins: Lack one or more essential amino acids. Most plant proteins (beans, grains, nuts) are incomplete.
Complementary Proteins: Combining two or more plant-based foods (e.g., rice and beans) to provide all essential amino acids.

Example: Eating lentils (low in methionine) with rice (low in lysine) provides a complete amino acid profile.

Protein-Energy Malnutrition

Kwashiorkor: Caused by inadequate protein intake with sufficient calories; symptoms include edema, fatty liver, and poor growth.
Marasmus: Caused by severe deficiency of both protein and calories; symptoms include severe wasting, loss of fat and muscle mass.
Difference: Kwashiorkor presents with edema and a swollen belly, while marasmus is characterized by extreme thinness.

Protein Requirements

Recommended Dietary Allowance (RDA): For average adults, of protein per kilogram of body weight per day.
Calculation: To convert pounds to kilograms, divide by 2.2.
Increased Needs: Athletes, adolescents, and individuals with medical trauma may require higher protein intake (e.g., ).

Example: A 150 lb (68 kg) adult needs of protein per day.

Protein Denaturation and Digestion

Denaturation: The process by which a protein loses its native structure due to heat, acid, or mechanical agitation, resulting in loss of function.
Cooking: Heat denatures proteins, making them easier to digest.
Stomach Acid: Hydrochloric acid in the stomach denatures dietary proteins and activates pepsin, an enzyme that begins protein digestion.

Fats, Oils, and Other Lipids

Types of Lipids

Lipids are a diverse group of hydrophobic molecules essential for energy storage, cell structure, and signaling. The three main types are:

Triglycerides: Composed of three fatty acids attached to a glycerol backbone; main form of stored fat in the body and diet.
Phospholipids: Contain two fatty acids, a glycerol backbone, and a phosphate group; major component of cell membranes.
Sterols: Complex ring structures; cholesterol is the most well-known sterol, important for cell membranes and hormone synthesis.

Examples: Triglycerides (butter, oils), phospholipids (soy lecithin), sterols (cholesterol in eggs).

Omega-3 Fatty Acids

Definition: Polyunsaturated fatty acids with the first double bond at the third carbon from the omega end.
Health Benefits: Reduce inflammation, lower risk of heart disease, support brain function.
Sources: Fatty fish (salmon, mackerel), flaxseeds, walnuts, chia seeds.

Saturated vs. Unsaturated Fats

Saturated Fats: No double bonds; solid at room temperature; found in animal fats, butter, coconut oil.
Unsaturated Fats: One or more double bonds; liquid at room temperature; found in olive oil, canola oil, nuts, seeds.
Health Implications: Unsaturated fats are generally considered healthier due to their positive effects on blood cholesterol levels.

Example: Olive oil (unsaturated) is healthier than butter (saturated) because it helps reduce LDL cholesterol.

Cholesterol and Lipoproteins

LDL (Low-Density Lipoprotein): Transports cholesterol to tissues; high levels increase risk of atherosclerosis.
HDL (High-Density Lipoprotein): Removes cholesterol from tissues and transports it to the liver for excretion; high levels are protective against heart disease.
Lifestyle Factors: Regular exercise, healthy diet, and avoiding trans fats can increase HDL and lower LDL.

Atherosclerosis and Heart Disease

Atherosclerosis: The buildup of fatty deposits (plaque) in arterial walls, leading to reduced blood flow and increased risk of heart attack and stroke.
Role of Lipoproteins: High LDL promotes plaque formation; high HDL helps remove cholesterol from arteries.
Heart Disease: Atherosclerosis is a major underlying cause of coronary heart disease.

Risk Factors for Heart Disease

Controllable: Diet, physical activity, smoking, blood pressure, cholesterol levels, obesity, diabetes.
Non-Controllable: Age, gender, family history, genetic predisposition.

Fat Digestion and Absorption

Pancreas: Releases lipase (for fat digestion) and proteases (for protein digestion) into the small intestine.
Liver: Produces bile, which emulsifies fats, increasing their surface area for enzyme action.
Gallbladder: Stores and releases bile into the small intestine.
Bile: An emulsifier that helps disperse fat droplets for digestion.
Site of Digestion: Most fat digestion occurs in the small intestine.

Vegetarian Diets

Types of Vegetarian Diets

Lacto-ovo vegetarian: Includes dairy and eggs; excludes meat, poultry, and fish.
Lacto vegetarian: Includes dairy; excludes eggs, meat, poultry, and fish.
Ovo vegetarian: Includes eggs; excludes dairy, meat, poultry, and fish.
Vegan: Excludes all animal products.
Pescatarian: Includes fish; excludes other meats.

Example: A lacto-ovo vegetarian may eat cheese omelets but not chicken or fish.

Key Vocabulary

Abundant: Present in large quantities.
Adequate: Sufficient to meet needs.
Convert / Converted: To change from one form to another.
Element: A pure substance consisting of one type of atom.
Emulsify: To mix two substances that normally do not combine easily, such as oil and water.
Excluded: Left out or not included.
Risk Factors: Characteristics or behaviors that increase the likelihood of developing a disease.
Sequence: The order of amino acids in a protein or nucleotides in DNA/RNA.
Synthesis: The process of building complex molecules from simpler ones.

Practice Activities

Protein Needs Calculation: Divide weight in pounds by 2.2 to get kilograms, then multiply by the appropriate protein factor (e.g., 0.8 g/kg for adults).
Protein Digestion Scenario: When you eat chicken or drink soy milk, proteins are denatured in the stomach, broken down into amino acids in the small intestine, absorbed into the bloodstream, and used to synthesize new proteins as needed by the body (utilizing the amino acid pool).
Heart Disease Recommendations: For someone with high blood pressure and heart disease, recommend a diet low in saturated fat and cholesterol, rich in fruits, vegetables, whole grains, and omega-3 fatty acids (e.g., eat fatty fish at least twice a week), along with regular physical activity and smoking cessation.
Vegetarian Diets: Be able to identify which foods are included or excluded in different types of vegetarian diets.

Table: Comparison of Protein and Lipid Types

Type	Structure	Sources	Function
Complete Protein	All essential amino acids	Meat, eggs, dairy, soy	Growth, repair, enzymes
Incomplete Protein	Missing one or more essential amino acids	Most plant foods	Must be combined for adequacy
Triglyceride	Glycerol + 3 fatty acids	Butter, oils, animal fat	Energy storage
Phospholipid	Glycerol + 2 fatty acids + phosphate	Soy lecithin, egg yolk	Cell membranes
Sterol	Four-ring structure	Cholesterol (animal foods)	Hormones, membranes

Key Equations

Protein Needs:

Weight Conversion:

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