Premed · Premed · General Biology 1
Lecture 9: Bioenergetics and Enzymes
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Define free energy and predict whether a reaction is exergonic or endergonic
- Explain the laws of thermodynamics as they apply to biological systems
- Describe the role of ATP in energy coupling
- Explain how enzymes catalyze reactions by lowering activation energy
- Describe the factors that affect enzyme activity (temperature, pH, concentration)
- Distinguish between competitive and noncompetitive inhibition and explain allosteric regulation
Lecture Content
I. Thermodynamics and Living Systems
Thermodynamics is the study of energy transformations, and its principles govern every chemical reaction in every living cell. The first law of thermodynamics (the law of conservation of energy) states that energy can be converted from one form to another but cannot be created or destroyed. The total energy of the universe is constant. For biology, this means that organisms transform energy--converting light energy into chemical energy during photosynthesis, or chemical energy into mechanical work during muscle contraction--but they do not create energy from nothing.
The second law of thermodynamics states that every energy transformation increases the total entropy (disorder) of the universe. No energy conversion is perfectly efficient; some energy is always dissipated as heat. Living organisms appear to defy this law because they maintain a remarkably high degree of internal order, but they do so only by consuming energy and exporting entropy to their surroundings. Organisms are open systems--they exchange both matter and energy with their environment--and while they decrease entropy locally, they increase it globally, fully consistent with the second law.
Three thermodynamic quantities are essential for understanding biological energy flow. Entropy (S) measures disorder or randomness. Enthalpy (H) represents the total energy content of a system. Free energy (G) is the portion of a system's energy that is available to do useful work.
II. Gibbs Free Energy
The change in free energy for a reaction is given by the equation Delta G = Delta H - T(Delta S), where Delta H is the change in enthalpy, T is the absolute temperature in Kelvin, and Delta S is the change in entropy. This single number tells us whether a reaction will release or require energy.
Exergonic reactions have a negative Delta G: they release free energy and are thermodynamically spontaneous. The products contain less free energy than the reactants, and the difference is available to do work. Cellular respiration is a dramatic example, with a Delta G of -686 kcal/mol for the complete oxidation of glucose. Endergonic reactions have a positive Delta G: they require an input of free energy and are not spontaneous. The products contain more free energy than the reactants. Photosynthesis, which converts carbon dioxide and water into glucose, is endergonic. At equilibrium (Delta G = 0), there is no net change and no capacity to do work. Living cells are maintained far from equilibrium by a constant throughput of energy--a cell at equilibrium is a dead cell.
A crucial nuance: "spontaneous" in thermodynamics means energetically favorable, not fast. Many spontaneous reactions proceed at negligible rates without a catalyst. Glucose sitting on a tabletop in the presence of oxygen is thermodynamically poised to oxidize, but at room temperature and without enzymes, the reaction is immeasurably slow.
III. ATP and Energy Coupling
ATP (adenosine triphosphate) is the universal energy currency of the cell. The hydrolysis of ATP to ADP and inorganic phosphate releases approximately 7.3 kcal/mol of free energy under standard conditions. Cells use this energy to drive otherwise unfavorable endergonic reactions through a strategy called energy coupling, in which an exergonic reaction (usually ATP hydrolysis) is mechanistically linked to an endergonic reaction so that the overall process is thermodynamically favorable.
Energy coupling often involves phosphorylation--the transfer of a phosphate group from ATP to a substrate molecule. The resulting phosphorylated intermediate has higher free energy than the original substrate, enabling it to undergo a reaction that would otherwise not proceed. The human body turns over an impressive 200 to 300 moles of ATP per day--equivalent to roughly 40 to 75 kilograms--yet the total amount of ATP present at any moment is quite small. ATP is not stockpiled; it is rapidly recycled from ADP and Pi by the processes of cellular respiration and, in photosynthetic organisms, photosynthesis.
<image>An energy diagram showing energy coupling. Left graph: An exergonic reaction (ATP hydrolysis) with free energy decreasing from reactants to products, releasing Delta G = -7.3 kcal/mol. Right graph: An endergonic reaction (e.g., amino acid joining) with free energy increasing, requiring Delta G = +3.4 kcal/mol. Center: The two reactions are coupled — ATP hydrolysis provides the energy to drive the endergonic reaction, with the overall Delta G being negative. Arrows connect the two diagrams to show energy flow.</image>
IV. Enzymes — Biological Catalysts
Enzymes are biological catalysts--most are proteins, though some catalytic RNA molecules (ribozymes) also qualify. Enzymes accelerate chemical reactions by lowering the activation energy (Ea), the energy barrier that must be overcome for reactants to be converted into products. Crucially, enzymes do not change the Delta G of the reaction; they do not alter whether a reaction is thermodynamically favorable. What they do is make favorable reactions happen fast enough to sustain life, often increasing reaction rates by factors of 10^6 to 10^12.
Enzymes emerge from the reaction unchanged and are recycled indefinitely. Most are highly specific, catalyzing only a particular reaction or a narrow set of related reactions. This specificity arises from the precise geometry and chemistry of the enzyme's active site. Enzyme names conventionally end in "-ase"--lactase cleaves lactose, DNA polymerase synthesizes DNA, and hexokinase phosphorylates hexose sugars.
V. Enzyme-Substrate Interactions
The molecule upon which an enzyme acts is called its substrate. Catalysis occurs at the active site, a specific region on the enzyme surface formed by a small number of amino acid residues arranged to be complementary in shape, charge, and hydrophobicity to the substrate. The enzyme and substrate form an enzyme-substrate complex (ES), which then converts to an enzyme-product complex (EP) before the product is released and the enzyme is free to act again.
Two models describe how enzymes recognize their substrates. The older lock-and-key model envisioned a rigid active site perfectly shaped to fit the substrate, like a key fitting a lock. The modern induced fit model, supported by extensive structural evidence, recognizes that the active site is flexible: upon substrate binding, the enzyme undergoes a conformational change that tightens its grip on the substrate and brings catalytic residues into optimal alignment. This induced fit not only enhances specificity but also contributes to catalysis by straining substrate bonds, orienting reactants precisely, providing a favorable microenvironment (such as a hydrophobic pocket or an acidic niche), or directly participating in the reaction through acid-base or covalent catalysis.
<image>A diagram illustrating the induced fit model of enzyme action. Step 1: Free enzyme with an active site shown as a slightly open cleft. Step 2: Substrate approaches and binds, and the enzyme changes shape to enclose the substrate more tightly (induced fit). Step 3: The substrate is converted to product(s) within the active site. Step 4: Product(s) are released, and the enzyme returns to its original shape. An energy diagram is shown alongside, comparing uncatalyzed (high Ea) and enzyme-catalyzed (lower Ea) reaction pathways, both reaching the same product energy level (same Delta G).</image>
VI. Factors Affecting Enzyme Activity
Substrate concentration has a characteristic effect on reaction rate. As substrate concentration increases, the rate rises as more active sites encounter substrates. Eventually, all active sites become saturated, and the rate plateaus at Vmax, the maximum velocity. The Michaelis constant (Km) is defined as the substrate concentration at which the reaction proceeds at half of Vmax. A low Km indicates high affinity between the enzyme and substrate (less substrate is needed to reach half-maximal rate), while a high Km indicates lower affinity. The relationship is described by the Michaelis-Menten equation: v = Vmax[S] / (Km + [S]).
Increasing enzyme concentration raises Vmax by providing more active sites. Temperature increases the kinetic energy of molecules and thus the reaction rate--up to a point. Each enzyme has an optimal temperature at which it functions most efficiently (around 37 degrees Celsius for most human enzymes). Above the optimum, the enzyme denatures--its three-dimensional structure unravels and activity plummets. Some extremophile enzymes, such as Taq polymerase from Thermus aquaticus, function optimally at 72 degrees Celsius and are exploited in PCR.
Each enzyme also has an optimal pH. Changes in pH alter the ionization states of amino acid R groups, disrupting the precise charge distribution in the active site and potentially the overall enzyme structure. Pepsin, a protease in the stomach, works best at pH 2, while trypsin in the small intestine is most active at pH 8. Cofactors are inorganic ions (Zn2+, Mg2+, Fe2+, Cu2+) required by many enzymes for catalytic activity. Coenzymes are organic molecules that serve a similar role; many are derived from vitamins (NAD+ from niacin, FAD from riboflavin, coenzyme A from pantothenic acid). A prosthetic group is a cofactor permanently bound to the enzyme, such as the heme group in cytochrome c. The complete, catalytically active enzyme--protein plus cofactor--is called the holoenzyme, while the protein portion alone is the apoenzyme.
VII. Enzyme Inhibition
Competitive inhibitors resemble the substrate structurally and bind to the active site, physically blocking the substrate from entering. Because the inhibitor and substrate compete for the same site, the inhibition can be overcome by flooding the system with substrate. In kinetic terms, competitive inhibition increases the apparent Km (the enzyme "appears" to have lower affinity for its substrate) while leaving Vmax unchanged. Malonate, which competes with succinate for succinate dehydrogenase, is a classic example. Many pharmaceutical drugs are competitive inhibitors--statins, for instance, inhibit HMG-CoA reductase by mimicking the enzyme's natural substrate.
Noncompetitive inhibitors bind to an allosteric site (a location other than the active site), inducing a conformational change that reduces the enzyme's catalytic efficiency without preventing substrate binding. Because the inhibitor and substrate do not compete for the same site, increasing substrate concentration cannot overcome the inhibition. Noncompetitive inhibition leaves Km unchanged but decreases Vmax. Heavy metals such as lead and mercury, which bind to sulfhydryl groups on enzymes, often act as noncompetitive inhibitors.
Uncompetitive inhibitors bind only to the enzyme-substrate complex, decreasing both Vmax and Km. Irreversible inhibitors form permanent, often covalent, modifications to the enzyme. Nerve agents like sarin irreversibly inactivate acetylcholinesterase, and aspirin irreversibly inhibits cyclooxygenase (COX) enzymes.
VIII. Allosteric Regulation
Many enzymes, particularly those with quaternary structure, possess allosteric sites--regulatory binding sites distinct from the active site. When a molecule binds to an allosteric site, it induces a conformational change that shifts the enzyme between active and inactive states. Allosteric activators stabilize the active conformation, increasing catalytic activity, while allosteric inhibitors stabilize the inactive conformation. This mechanism allows for exquisitely sensitive, rapid, and reversible regulation of enzyme activity.
Cooperativity is a related phenomenon seen in multi-subunit proteins: binding of a substrate (or ligand) to one subunit induces a conformational change that enhances binding at the remaining subunits. Hemoglobin provides the classic example--the binding of the first oxygen molecule makes subsequent oxygen molecules bind more readily, producing the characteristic sigmoidal binding curve.
Feedback inhibition (also called end-product inhibition) is one of the most important regulatory strategies in metabolism. The final product of a multi-step metabolic pathway allosterically inhibits an enzyme early in the pathway, preventing the wasteful accumulation of intermediates and end products. For example, isoleucine inhibits threonine deaminase, the first enzyme in the isoleucine biosynthetic pathway. When isoleucine levels are sufficient, the pathway shuts itself down; when levels drop, the inhibition is relieved and synthesis resumes.
<image>A three-panel figure on enzyme regulation. Panel A: Competitive vs. noncompetitive inhibition — two diagrams showing a competitive inhibitor blocking the active site (substrate cannot bind) and a noncompetitive inhibitor binding to an allosteric site (enzyme shape changes, substrate can still bind but catalysis is impaired). Panel B: Lineweaver-Burk (double reciprocal) plots for no inhibitor, competitive inhibition (same y-intercept, different x-intercept), and noncompetitive inhibition (same x-intercept, different y-intercept). Panel C: Feedback inhibition — a metabolic pathway (A -> B -> C -> D -> E) where the final product E inhibits enzyme 1 (A -> B), shown with a dashed inhibitory arrow.</image>
