Proteins, Carbohydrates, and Lipids

Proteins, Carbohydrates, and Lipids Objectives

• Learning objectives include identifying macromolecules, learning functional groups, amino acids and protein structure, protein structure and function, carbohydrate monomers/polymers, and lipid components/roles.

Molecules Characteristic of Living Things

• Four kinds of molecules are characteristic of living things: proteins, carbohydrates, lipids, and nucleic acids.
• Proteins are polymers of 20 amino acids.
• Carbohydrates are formed by linking together monosaccharides.
• Nucleic acids are polymers of four kinds of nucleotides.
• Polymers are constructed by covalent bonding of smaller molecules, called monomers.

Functional Groups

• Functional groups are small groups of atoms occurring frequently in biological molecules.
• Functional groups confer specific chemical properties when attached to larger molecules.
• Hydroxyl groups (-OH) are polar, form hydrogen bonds with water, and enable linkage via condensation.
• Aldehyde groups (C=O at the end) are polar and very reactive, important in building molecules and energy release.
• Keto groups (C=O within the chain) are polar and crucial in carbohydrates and energy reactions.
• Carboxyl groups (-COOH) are charged, acidic, ionize, and are involved in condensation by giving up -OH.
• Amino groups (-NH2) are charged, basic, accept H+ to form -NH3+, and participate in condensation reactions by giving up H+.
• Phosphate groups (-OPO3^2-) are charged, acidic, enter condensation reactions by releasing -OH, and release much energy upon hydrolysis when bonded to another phosphate.
• Sulfhydryl groups (-SH) in thiols can react to form a disulfide bridge by giving up H, stabilizing protein structure.
• Methyl groups (-CH3) in alkyl compounds are nonpolar and important for interactions with nonpolar molecules and energy transfer.

Macromolecules Formation and Breakdown

• Macromolecules form through condensation reactions and are broken down by hydrolysis.
• Condensation reactions produce water.
• Hydrolysis reactions consume water.

Proteins

• Proteins are polymers, composed of 20 amino acids, and vary in proportion and sequence.
• The size of proteins can range from small, like insulin (51 amino acids), to huge, like titin (24,000-36,000 amino acids.
• Proteins consist of one or more polypeptide chains.
• Polypeptide chains are unbranched polymers of covalently bonded amino acids.
• Each chain folds into a particular 3D shape.
• Amino acids can exist as optical isomers: D (dextro, right) and L (levo, left).
• Only L-amino acids are found in proteins of most organisms.
• Isomers are molecules with the same chemical formula but different arrangements.
• At cellular pH levels (7.0–7.4), both the carboxyl and amino groups of amino acids are ionized.
• Amino acids show both acidic and basic properties.
• Side chains (R groups) contain functional groups vital for determining a protein's 3D structure and function.

Amino Acids and Their Properties

• Five amino acids have electrically charged side chains at pH levels typical of living cells.
• Electrically charged side chains attract water (are hydrophilic) and oppositely charged ions.
• Arginine, histidine, and lysine have a +1 charge.
• Aspartic acid and glutamic acid have a -1 charge.
• Five amino acids have polar (δ+ and δ-) side chains, which attract water (are hydrophilic) and form hydrogen bonds with water and other polar substances. The polar amino acids are Serine, Threonine, Asparagine, Glutamine and Tyrosine
• Seven amino acids have nonpolar hydrocarbon side chains, are hydrophobic and cluster together in aqueous solutions
• Alanine, Isoleucine, Leucine, Methionine, Phenylalanine, Tryptophan and Valine are the nonpolar amino acids

Special Case Amino Acids

• Cysteine contains an -SH group that can react with another cysteine side chain in an oxidation reaction to form a covalent bond, called a disulfide bridge.
• Glycine has a single H atom as its side chain, allowing it to fit into tight corners of the interior of a protein, lending flexibility. Glycine is generally hydrophobic
• Proline's amino group is modified, lacks an H atom, and covalently bonds to a hydrocarbon side chain via a generally hydrophobic ring structure, this limits hydrogen bonding capability and rotation around the alpha carbon, and is often found where a protein bends or loops
• The primary structure of a protein refers to the sequence of amino acids joined by peptide bonds forming a polypeptide.
• Short polymers of fewer than 20 amino acids are called oligopeptides or peptides.
• Linking monomers involves a reaction between carboxyl group of one amino acid and amino groups attached to the alfa carbon of another amino acid.

Protein Structure

• Primary Structure: Amino acid monomers joined by peptide bonds forming polypeptide chains.
• Secondary Structure: Polypeptide chains may have α helices or β pleated sheets stabilized by hydrogen bonds.
• Tertiary Structure: Polypeptides fold into specific 3D shapes, stabilized by hydrogen bonds, disulfide bridges, and hydrophobic interactions.
• Quaternary Structure: Two or more polypeptides assemble, forming larger protein molecules through hydrogen bonds, hydrophobic interactions, and ionic bonds.
• Tertiary structure is the definitive 3D shape of a protein
• Heat energy disrupts secondary and tertiary structures by breaking down weak interactions.
• A denatured protein is larger in volume, can take many shapes, and has exterior hydrogen bonds. In contrast, a native protein is compact, has a single shape, and has internal hydrogen bonds.
• A protein's quaternary structure results from the ways in which its subunits bind together and interact.

Protein Function

• Shape and surface chemistry contribute to protein function.
• A given molecule's shape will allow it to bind to a protein if their shapes have a general "fit".
• Surface chemistry includes exposed R groups on the surface, which permit chemical reactions with other substances, such as ionic, hydrophobic, or hydrogen bonds.
• Environmental conditions that can affect protein structure include temperature, pH, and concentrations of polar or nonpolar substances.
• Protein shapes can change due to interactions with other molecules or covalent modifications.

Protein Functions by Category

• Enzymes: Catalyze (speed up) biochemical reactions.
• Structural Proteins: Provide physical stability and movement.
• Defensive Proteins: Recognize and respond to nonself substances (e.g., antibodies).
• Signaling Proteins: Control physiological processes (e.g., hormones).
• Receptor Proteins: Receive and respond to chemical signals.
• Membrane Transporters: Regulate passage of substances across cellular membranes.
• Storage Proteins: Store amino acids for later use.
• Transport Proteins: Bind and carry substances within the organism.
• Gene Regulatory Proteins: Determine the rate of gene expression.
• Motor Proteins: Cause movement of structures in the cell.

Carbohydrates

• The general formula for carbohydrates is Cn(H2O)m, where n and m are numbers.
• Linked carbon atoms are bonded with hydrogen atoms (-H) and hydroxyl groups (-OH).
• Carbohydrates serve several major biochemical roles of energy storage, energy transport, carbon skeletons, and forming extracellular structures,.
• Monosaccharides (simple sugars) are monomers (e.g., glucose).
• Disaccharides consist of two monosaccharides linked by a covalent bond (e.g., sucrose).
• Oligosaccharides consist of several (3-20) monosaccharides.
• Polysaccharides are made up of hundreds or thousands of monosaccharides (e.g., starch, glycogen, cellulose).
• Monosaccharides can exist in straight chain and ring forms.
• Common monosaccharides include ribose, deoxyribose, glucose, mannose, and fructose.
• Disaccharides, oligosaccharides, and polysaccharides are constructed from monosaccharides bonded by glycosidic bonds via condensation reactions.
• Polysaccharides are large polymers of monosaccharides connected by glycosidic bonds.
• Polysaccharides are not necessarily linear chains of monomers; these branched molecules are possible

Polysaccharides and their functions

• Cellulose is an unbranched polymer of glucose with β-1,4 glycosidic bonds, and is chemically very stable.
• Cellulose serves as the main component of plant cell walls and is the most abundant organic compound on Earth and functions as an important structural material.
• Starch and glycogen are polymers of glucose with α-1,4 glycosidic bonds, with α-1,6 glycosidic bonds causing branching at carbon 6.
• Starch serves as the principal plant energy storage compound.
• Glycogen serves as the principal animal energy storage compound, which stores glucose in liver and muscle.
• Branching limits the number of hydrogen bonds, making starch less compact than cellulose, while glycogen is structured to be even more highly branched to increase the density of energy stockpiles.

Lipids

• Lipids, also known as fats, are hydrocarbons that are insoluble due to many nonpolar covalent bonds.
• Lipids are hydrophobic tending to aggregate away from water.
• Lipids are aggregates rather than true polymers because individual lipids are not covalently bonded.
• Fats, phospholipids, carotenoids, chlorophylls, and steroids lipids play roles in animal bodies (thermal/electrical insulation), plant cell membranes, plant photosynthesis, and regulatory roles for animals+plants
• Fats and oils store energy, are triglycerides (simple lipids), and are solid (fat) or liquid (oil) at room temperature.

Triglyceride Composition

• Triglycerides are composed of fatty acids and glycerol.
• Each glycerol molecule (an alcohol) has three hydroxyl (-OH) groups.
• Each fatty acid has a long, nonpolar hydrocarbon chain and an acidic carboxyl (-COOH) group
• Each hydroxyl group bonds to the carboxyl group of a fatty acid, forming an ester linkage, with each synthesis releases a single water molecule.
• The three fatty acids in a triglyceride don't have to be the same.
• Saturated fatty acids have no double bonds in the hydrocarbon chain.
• Unsaturated fatty acids have one or more double bonds between carbon atoms.
• Unsaturated fatty acids have kinks, while saturated fatty acids do not and this prevents close packing so they tend to be liquid e.g. Plant oils at room temperature
• Plant oils have low melting points and are liquid at room temperature; animal (meat) fats have high meeting points and are solid at room temperature

Phospholipids

• Phospholipids play structural roles in cell membranes.
• Phospholipids have a polar hydrophilic "head" comprising a phosphate group and a choline, glycerol
• Have long non-polar hydrophobic "tails" comprising hydrocarbons
• Therefore, they are amphipathic, having both hydrophilic and hydrophobic parts, and can form a phospholipid bilayer when in an aqueous solution.

Explore the essential molecules of life: proteins, carbohydrates, and lipids. This lesson covers the structure and function of these macromolecules, including amino acids, functional groups, monomers, and polymers. Understand their roles in living organisms.