Proteins and Amino Acids

Protein Structure and Composition

Proteins are essential macromolecules composed of amino acids, which contain the elements carbon, hydrogen, oxygen, and nitrogen. The presence of nitrogen distinguishes amino acids from carbohydrates and fats.

• Amino acids are linked together by peptide bonds to form peptides and proteins.

• A peptide bond is a covalent bond formed between the amino group of one amino acid and the carboxyl group of another, releasing water in a condensation reaction.

• Proteins are long chains of amino acids folded into specific three-dimensional shapes.

Relationship between Amino Acids, Peptides, and Proteins:

• Amino acids: The building blocks of proteins.

• Peptides: Short chains of amino acids (typically 2–50).

• Proteins: Polypeptides with more than 50 amino acids, folded into functional structures.

The function of a protein is determined by its amino acid sequence and the chemical properties of the side chains (R groups), which influence the protein's shape and activity.

Amino Acids: Types and Essentiality

• There are 20 different amino acids used to build proteins in the human body.

• Essential amino acids (9): Cannot be synthesized by the body and must be obtained from the diet.

• Non-essential amino acids (11): Can be synthesized by the body from other compounds.

Protein Functions and Metabolism

• Proteins serve as enzymes, hormones, structural components (e.g., collagen), antibodies, and transporters (e.g., hemoglobin).

• They help maintain fluid balance and acid-base balance in the body.

• Excess protein is converted to energy or stored as fat after deamination (removal of the amino group).

Protein's Role in the Body:

• Enzyme production: Catalyze biochemical reactions.

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

• Transport: Carry substances in the blood (e.g., lipoproteins, hemoglobin).

• Collagen: Provides structure to skin, bones, and connective tissues.

• Antibodies: Defend against pathogens.

Protein Quality: Complete vs. Incomplete Proteins

• Complete proteins: Contain all essential amino acids in adequate amounts (e.g., animal-based foods like meat, eggs, dairy; plant-based sources like soy and quinoa).

• Incomplete proteins: Lack one or more essential amino acids (e.g., most plant-based proteins such as beans, nuts, grains).

• Complementary proteins: Combining two or more plant-based foods to provide all essential amino acids (e.g., rice and beans).

Protein Deficiency Disorders

• Kwashiorkor: Protein deficiency with adequate energy intake; symptoms include edema, fatty liver, and poor growth.

• Marasmus: Severe deficiency of both protein and energy; symptoms include severe wasting and loss of fat and muscle.

Protein Requirements

• The Recommended Dietary Allowance (RDA) for protein for healthy adults is 0.8 grams per kilogram of body weight per day.

• Protein needs increase for athletes, adolescents, pregnant/lactating women, and individuals with medical trauma or illness.

Protein Calculation Example:

• Convert weight from pounds to kilograms:

• Multiply by the appropriate factor (e.g., 0.8 g/kg for adults):

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 food denatures proteins, making them easier to digest.

• Stomach acid (hydrochloric acid) denatures dietary proteins and activates pepsin, an enzyme that begins protein digestion.

Protein Digestion and Absorption

• Digestion begins in the stomach and continues in the small intestine.

• Enzymes break proteins into amino acids, which are absorbed into the bloodstream and used to build new proteins in the body.

• The amino acid pool refers to the available amino acids in the body for protein synthesis and other functions.

Fats, Oils, and Other Lipids

Types of Lipids

Lipids are a diverse group of hydrophobic molecules that include triglycerides, phospholipids, and sterols.

• Triglycerides: The most abundant lipid in the body and diet; composed of three fatty acids attached to a glycerol backbone. Main form of stored energy.

• Phospholipids: Contain two fatty acids, a glycerol backbone, and a phosphate group; major component of cell membranes (e.g., lecithin).

• Sterols: Have a ring structure; cholesterol is the most well-known sterol, important for cell membranes and as a precursor for hormones.

Examples and Sources:

• Triglycerides: Found in butter, oils, meats.

• Phospholipids: Found in egg yolks, soybeans.

• Sterols: Cholesterol in animal products; plant sterols in vegetable oils and nuts.

Omega-3 Fatty Acids

• Omega-3 fatty acids are polyunsaturated fats with the first double bond at the third carbon from the omega end.

• They are beneficial for heart and brain health, reducing inflammation, and supporting cell membrane function.

• Rich sources include fatty fish (salmon, mackerel, sardines), flaxseeds, chia seeds, and walnuts.

Saturated vs. Unsaturated Fats

• Saturated fats: No double bonds between carbon atoms; solid at room temperature (e.g., butter, lard, coconut oil).

• Unsaturated fats: One or more double bonds; liquid at room temperature (e.g., olive oil, canola oil, avocados).

• Liquid oils are generally considered healthier because they contain more unsaturated fats, which can help lower LDL cholesterol and reduce heart disease risk.

Cholesterol and Lipoproteins

• LDL (Low-Density Lipoprotein): Transports cholesterol to tissues; high levels contribute to atherosclerosis ('bad' cholesterol).

• HDL (High-Density Lipoprotein): Removes cholesterol from tissues and transports it to the liver for excretion ('good' cholesterol).

• LDL contributes to plaque formation in arteries, increasing heart disease risk; HDL helps reduce this risk.

• Lifestyle choices such as regular exercise, healthy diet, and not smoking can increase HDL and lower LDL.

Atherosclerosis and Heart Disease

• Atherosclerosis: The buildup of fatty deposits (plaque) in the walls of arteries, leading to reduced blood flow and increased risk of heart attack and stroke.

• LDL promotes plaque formation; HDL helps remove cholesterol from arteries.

• Major risk factors for heart disease include high LDL, low HDL, high blood pressure, smoking, diabetes, obesity, physical inactivity, age, and family history.

• Controllable risk factors: diet, exercise, smoking, blood pressure, cholesterol levels.

• Non-controllable risk factors: age, genetics, family history.

Fat Digestion and Absorption

• The pancreas releases enzymes (lipases) that digest fats and proteins in the small intestine.

• The liver produces bile, which is stored in the gallbladder and released into the small intestine to emulsify fats, increasing their surface area for enzyme action.

• Bile is a substance that helps break down large fat globules into smaller droplets (emulsification).

• Most fat digestion and absorption 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.

• Flexitarian: Primarily plant-based but occasionally includes meat, poultry, or fish.

Foods Included/Excluded:

• Lacto-ovo: Includes dairy, eggs; excludes meat, poultry, fish.

• Lacto: Includes dairy; excludes eggs, meat, poultry, fish.

• Ovo: Includes eggs; excludes dairy, meat, poultry, fish.

• Vegan: Excludes all animal-derived foods.

Key Vocabulary

• Abundant: Present in large quantities.

• Adequate: Sufficient for a specific requirement.

• Convert / Converted: To change from one form to another.

• Element: A substance that cannot be broken down into simpler substances.

• 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 in which amino acids are arranged in a protein.

• Synthesis: The process of building complex molecules from simpler ones.

Practice Activities

• Calculate protein needs using body weight in kilograms and appropriate factors (e.g., 0.8 g/kg for adults).

• Explain protein digestion and absorption using examples such as chicken or soy milk, describing breakdown, absorption, and synthesis of new proteins.

• Develop dietary and lifestyle recommendations for individuals with heart disease, including frequency of fish consumption (e.g., 2 servings of fatty fish per week).

• Differentiate between types of vegetarian diets and identify included/excluded foods.

Table: Comparison of Protein and Lipid Types

Table: Types of Vegetarian Diets

Additional info: Academic context and examples have been added to expand on brief points and ensure the notes are self-contained and suitable for exam preparation.