Understanding Protein Structure

Proteins

• Organic macromolecules composed of amino acids.
• Contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
• Formed through peptide bonds that join amino acids in a specific sequence.
• Essential for structure, function, and regulation of body tissues and processes in living organisms.

Protein Structure

• There are four levels of structural organization, each vital for protein function.

Primary Structure

• Linear sequence of amino acids in a polypeptide chain.
• Peptide bonds are the bond type
• Significance: Determines all higher levels of protein structure.

Secondary Structure

• Local folding into specific patterns.
• Stabilized by hydrogen bonds between backbone atoms.
• Common forms included α-helix (spiral structure) and β-pleated sheet (zigzag sheet-like structure).

Tertiary Structure

• Overall 3D shape of a single polypeptide chain.
• Held together by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.
• Significance: Determines the protein's specific function.

Quaternary Structure

• Structure results when two or more polypeptide chains (subunits) combine.
• Example: Hemoglobin, which has four subunits.
• Not all proteins have quaternary structure.

Functions of Proteins

• Proteins are multi-functional molecules and are critical in many biological processes.
• Structural Support: Provides support to cells and tissues, like collagen in skin and keratin in hair.
• Enzymes: Accelerates biochemical reactions, such as amylase and DNA polymerase.
• Transport: Carries molecules throughout the body, for instance, hemoglobin transports oxygen.
• Hormonal Regulation: Acts as hormones to regulate body processes, such as insulin regulates blood sugar.
• Immunity: Forms antibodies to help fight infections, such as immunoglobulins.
• Storage: Stores substances, such as ferritin which stores iron in the liver.
• Movement: Enables muscle contraction, examples include actin and myosin.
• Cell Signaling: Involved in communication within and between cells, using receptor proteins.
• Buffering: Helps maintain pH balance in the body.
• Energy Source: Used when carbohydrates and fats are lacking.

Carbohydrates

• Organic compounds composed of carbon, hydrogen, and oxygen, typically with a 1:2:1 ratio (C:H:O).
• General formula: Cn(H2O)n.
• Sugars or saccharides.
• One of the four major biomolecules, along with lipids, proteins, and nucleic acids.
• Serve as major energy sources and play vital structural roles for living organisms.

Classification/Types of Carbohydrates

• Categorized based on the number of sugar units.

Monosaccharides (Simple Sugars)

• Single sugar unit, the most basic form.
• Soluble in water and sweet-tasting.
• General formula: (CH2O)n, where n = 3–7.
• Glucose is the main energy source in cells.
• Fructose is found in fruits.
• Galactose is a component of milk sugar (lactose).

Disaccharides

• Formed by condensation (dehydration) reaction between two monosaccharides.
• Bonded via a glycosidic bond.
• Sucrose = glucose + fructose (table sugar).
• Lactose = glucose + galactose (milk sugar).
• Maltose = glucose + glucose (malt sugar).

Oligosaccharides

• Contain 3-10 monosaccharide units.
• Involved in cell signaling and recognition.
• Located on the surface of cell membranes as glycoproteins/glycolipids.

Polysaccharides (Complex Carbohydrates)

• Composed of hundreds to thousands of monosaccharides linked together.
• Insoluble in water and not sweet-tasting.

Storage Polysaccharides

• Starch is an energy storage in plants.
• Glycogen is an energy storage in animals (mainly in the liver and muscles).

Structural Polysaccharides

• Cellulose forms plant cell walls and is indigestible to humans.
• Chitin is a component of the exoskeleton of insects and cell walls of fungi.

Functions of Carbohydrates

• Primary Energy Source: Glucose is the main fuel for cellular respiration.
• Energy Storage: Starch (plants) and glycogen (animals) store energy for later use.
• Structural Support: Cellulose provides rigidity to plant cell walls and chitin supports exoskeletons.
• Sparing Protein and Fat: Prevents use of proteins/fats for energy, preserving them for their main roles.
• Component of Nucleic Acids: Ribose and deoxyribose are components of RNA and DNA, respectively.
• Cell Signaling & Recognition: Oligosaccharides on cell surfaces aid in communication and immune response.
• Digestive Health: Dietary fiber (cellulose) helps regulate bowel movements and gut health.

Lipids

• Organic compounds that are insoluble in water but soluble in non-polar solvents like alcohol or ether.
• Composed mainly of carbon, hydrogen, and oxygen.
• Includes fats, oils, waxes, phospholipids, and steroids.
• Macromolecules important for energy storage and structural functions, but not polymers.

Types of Lipids

• Triglycerides (Fats and Oils): Made of 1 glycerol + 3 fatty acids, the main form of stored energy and are solid at room temperature (fats) or liquid (oils).
• Phospholipids: Composed of 1 glycerol, 2 fatty acids, and 1 phosphate group, being a major component of cell membranes with a hydrophilic head and hydrophobic tails (amphipathic).
• Steroids: Possess a four-ring structure, included are cholesterol, testosterone, estrogen, and cortisol and function as hormones while maintaining cell membrane stability.
• Waxes: Long-chain fatty acids bonded to alcohols, providing protective coatings in plants (leaves) and animals (fur, feathers).
• Glycolipids: Lipids with a carbohydrate group, found in cell membranes and involved in cell recognition.
• Lipoproteins: Complexes of lipids and proteins, transport lipids through the bloodstream (e.g., HDL, LDL).

Types of Fats (Triglycerides)

• Saturated Fats: Fatty acids with no double bonds between carbon atoms, are solid at room temperature (e.g., butter, lard), and excess intake is linked to heart disease.
• Unsaturated Fats: Fatty acids with one or more double bonds and usually liquid at room temperature (e.g., olive oil, fish oil).
  • Monounsaturated: One double bond (e.g., olive oil).
  • Polyunsaturated: Two or more double bonds (e.g., sunflower oil).
• Trans Fats: Unsaturated fats that have been hydrogenated to be more solid, found in some processed foods, and are harmful to heart health as they increase "bad" cholesterol (LDL).

Functions of Lipids

• Energy Storage: Provide long-term energy, more than twice the energy of carbohydrates.
• Structural Component: Form cell membranes (phospholipids) and maintain membrane integrity.
• Insulation and Protection: Fat cushions organs and provides thermal insulation under the skin.
• Hormone Production: Cholesterol is a precursor for steroid hormones like estrogen and testosterone.
• Vitamin Absorption: Help absorb fat-soluble vitamins (A, D, E, K).
• Waterproofing: Waxes help prevent water loss in plants and animals.
• Nerve Function: Myelin sheath made of lipids insulates neurons and speeds up nerve impulses.
• Buoyancy: in aquatic animals, lipids help maintain buoyancy.