Premed · Premed · Cell Biology
Lecture 3: Macromolecules and the Chemistry of Life (Review)
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the four major classes of biological macromolecules and their monomers
- Explain the role of non-covalent interactions in macromolecular structure
- Describe the four levels of protein structure and how proteins fold
- Distinguish between nucleic acid types and their roles
- Explain principles of thermodynamics and free energy as applied to biochemical reactions
Lecture Content
I. Water, Bonds, and Non-Covalent Interactions
Water serves as the solvent of life, and its unique properties arise from its polar nature. As a polar molecule, water is an excellent solvent for ionic and polar substances. Its high specific heat capacity stabilizes temperature in biological systems, while its cohesion and surface tension result from extensive hydrogen bonding between water molecules.
Covalent bonds, in which electron pairs are shared between atoms, are the strongest bonds in biological molecules, with bond energies of approximately 350 to 400 kJ/mol. These can be single, double, or triple bonds. When electrons are shared unequally, as in O-H and N-H bonds, the result is a polar covalent bond.
Non-covalent interactions are individually weak but collectively exert powerful effects on macromolecular structure and function. Hydrogen bonds (approximately 4 to 20 kJ/mol) form between a hydrogen atom bonded to an electronegative atom and a lone pair on another electronegative atom. Ionic interactions (approximately 20 kJ/mol) arise from the attraction between opposite charges. Van der Waals forces (approximately 2 to 4 kJ/mol) are transient interactions resulting from induced dipoles. Hydrophobic interactions, in which nonpolar molecules are excluded from water, are a major driving force behind protein folding and membrane assembly.
The concept of pH and buffers is critical for understanding biological chemistry. pH is defined as -log[H+], and physiological pH is maintained near 7.4. Buffers, such as the bicarbonate system in blood, resist changes in pH. The Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), provides the quantitative framework for understanding buffer behavior.
II. Proteins
Proteins are built from amino acids, of which there are 20 standard types. Each amino acid has a central (alpha) carbon bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain). The chemical properties of the side chain determine the character of each amino acid. Nonpolar, hydrophobic amino acids include alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, and methionine. Polar uncharged amino acids include serine, threonine, asparagine, glutamine, tyrosine, and cysteine. The positively charged (basic) amino acids are lysine, arginine, and histidine, while the negatively charged (acidic) amino acids are aspartate and glutamate. Amino acids are linked by peptide bonds, formed through a condensation reaction between the amino group of one amino acid and the carboxyl group of another. The peptide bond has partial double-bond character, making it planar and rigid.
Protein structure is organized across four hierarchical levels. Primary structure is the linear sequence of amino acids in the polypeptide chain. Secondary structure refers to local folding patterns stabilized by hydrogen bonds between backbone atoms. The alpha-helix is a right-handed coil in which hydrogen bonds form between the carbonyl oxygen of residue i and the amide nitrogen of residue i+4. Beta-sheets consist of extended strands running parallel or antiparallel to one another, connected laterally by hydrogen bonds. Turns and loops connect these elements of secondary structure. Tertiary structure is the overall three-dimensional shape of a single polypeptide chain, stabilized by a hydrophobic core, hydrogen bonds, ionic interactions, and disulfide bonds between cysteine residues. Proteins often contain independently folding functional units called domains. Quaternary structure describes the arrangement of multiple polypeptide subunits into a functional complex, as exemplified by hemoglobin with its alpha2-beta2 tetrameric arrangement.
Protein folding is determined by the primary amino acid sequence, as elegantly demonstrated by Anfinsen's experiment showing that RNase A refolds spontaneously after denaturation. In the cellular environment, molecular chaperones such as Hsp70 and the chaperonin GroEL-GroES assist the folding process. When folding goes awry, proteins can aggregate, a process associated with diseases such as Alzheimer's, Parkinson's, and prion diseases. Protein denaturation, the loss of three-dimensional structure, can be caused by heat, extreme pH, detergents, or chaotropic agents like urea.
<image>Diagram illustrating the four levels of protein structure. Panel A: Primary structure shown as a linear chain of amino acids with peptide bonds. Panel B: Secondary structure showing an alpha-helix with hydrogen bonds as dashed lines between backbone atoms, and a beta-sheet with parallel and antiparallel arrangements. Panel C: Tertiary structure showing a complete 3D protein fold with labeled hydrophobic core, disulfide bond, and distinct domains. Panel D: Quaternary structure showing hemoglobin with its four subunits (two alpha in blue, two beta in red) assembled together.</image>
III. Nucleic Acids
DNA (deoxyribonucleic acid) stores genetic information and serves as the template for its own replication and for transcription. Its monomer is the deoxyribonucleotide, consisting of a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). DNA adopts the iconic double helix, with two antiparallel strands held together by complementary base pairing -- adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. Nucleotides are linked by phosphodiester bonds, giving the polymer a 5' to 3' directionality. In B-form DNA, the most common conformation, major and minor grooves run along the helix, which has approximately 10 base pairs per turn and a pitch of 3.4 nanometers.
RNA (ribonucleic acid) uses ribose instead of deoxyribose and replaces thymine with uracil. RNA is typically single-stranded but can form secondary structures such as hairpins and stem-loops through intramolecular base pairing. The cell produces many types of RNA, including mRNA, tRNA, rRNA, snRNA, miRNA, lncRNA, and siRNA, each with distinct roles. RNA is more chemically labile than DNA because the 2'-hydroxyl group on ribose makes it susceptible to hydrolysis.
The Central Dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is translated into protein. Notable exceptions include reverse transcriptase, which copies RNA into DNA in retroviruses, and RNA replication in some RNA viruses.
IV. Carbohydrates
Monosaccharides are the simplest carbohydrates. Glucose (C6H12O6), an aldohexose, is the major energy source for most cells. Fructose, a ketohexose, is another common six-carbon sugar. Ribose and deoxyribose are five-carbon sugars (pentoses) that form the backbone of nucleic acids. At the anomeric carbon, monosaccharides can adopt alpha or beta configurations.
Disaccharides consist of two monosaccharides joined by glycosidic bonds. Common examples include sucrose (glucose plus fructose), lactose (galactose plus glucose), and maltose (two glucose molecules).
Polysaccharides serve either as energy stores or structural elements. Starch, composed of amylose and amylopectin, is the plant energy storage polysaccharide, featuring alpha-1,4 glycosidic linkages. Glycogen serves the same storage function in animals but is more highly branched, containing both alpha-1,4 and alpha-1,6 linkages. Cellulose, the structural polysaccharide of plant cell walls, consists of beta-1,4-linked glucose units that most animals cannot digest. Chitin performs a structural role in arthropod exoskeletons and fungal cell walls.
Glycoproteins and glycolipids, which carry oligosaccharide chains attached to proteins or lipids respectively, play important roles in cell-cell recognition, signaling, and protection.
V. Lipids
Fatty acids are long hydrocarbon chains with a terminal carboxyl group. Saturated fatty acids have no double bonds, form straight chains, and tend to be solid at room temperature. Unsaturated fatty acids contain one or more cis double bonds that introduce kinks in the chain, making them liquid at room temperature.
Triacylglycerols (triglycerides), consisting of glycerol esterified to three fatty acids, represent the primary form of energy storage in animals. Phospholipids contain glycerol bonded to two fatty acids, a phosphate group, and a variable head group. Their amphipathic nature -- with a hydrophilic head and hydrophobic tails -- causes them to spontaneously form bilayers in aqueous solution, the structural basis of biological membranes.
Sterols are characterized by four fused carbon rings. Cholesterol, the most important sterol in animal cells, modulates membrane fluidity and serves as the precursor for steroid hormones. Sphingolipids, built on a sphingosine backbone rather than glycerol, include sphingomyelin, cerebrosides, and gangliosides.
<image>Overview of the four major classes of macromolecules. Panel A: Amino acid structure and peptide bond formation. Panel B: Nucleotide structure showing phosphate, sugar, and base components for DNA and RNA. Panel C: Monosaccharide (glucose) ring structure and glycosidic bond formation between two glucose units. Panel D: Phospholipid structure with hydrophilic head and hydrophobic tails, showing spontaneous bilayer formation in water.</image>
VI. Thermodynamics and Free Energy in Biological Systems
The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. The second law holds that the entropy of the universe always increases in any spontaneous process.
Gibbs free energy (delta-G) provides the criterion for predicting whether a reaction will occur spontaneously. It is calculated as delta-G = delta-H - T(delta-S). When delta-G is negative, the reaction is exergonic and proceeds spontaneously. When delta-G is positive, the reaction is endergonic and requires an input of energy. At equilibrium, delta-G equals zero.
ATP serves as the universal energy currency of the cell. Hydrolysis of ATP to ADP and inorganic phosphate releases approximately -30.5 kJ/mol under standard conditions. Cells couple this exergonic hydrolysis to endergonic reactions that would not otherwise proceed. ATP is continually regenerated through oxidative phosphorylation and substrate-level phosphorylation.
Enzymes are biological catalysts that accelerate reactions by lowering the activation energy without altering the overall free energy change. Enzymes bind their substrates at a specific active site, forming an enzyme-substrate complex. The kinetics of many enzymes follow the Michaelis-Menten model: V = Vmax[S] / (Km + [S]), where Km represents the substrate concentration at which the reaction rate is half-maximal and reflects the enzyme's binding affinity for its substrate. Enzyme activity can be regulated through allosteric mechanisms, covalent modifications such as phosphorylation, and feedback inhibition.
