Enzyme kinetics explains how enzymes speed up reactions and how reaction rates change when conditions change. Metabolism explains how cells build, break down, and regulate molecules to keep living systems working. This cheat sheet helps students connect graphs, formulas, and pathway logic in one printable reference.
It is especially useful for interpreting enzyme data, comparing inhibitors, and reviewing cellular energy flow.
The core ideas include activation energy, active sites, substrate concentration, Vmax, Km, and enzyme inhibition. A key relationship is the Michaelis-Menten equation, v = (Vmax[S])/(Km + [S]), which describes how reaction rate depends on substrate concentration. Metabolic pathways are controlled by ATP, NADH, feedback inhibition, and enzyme regulation.
Cells balance catabolic reactions that release energy with anabolic reactions that require energy.
Key Facts
- Enzymes lower activation energy, so reactions happen faster without changing the overall free energy change, ΔG.
- The Michaelis-Menten equation is v = (Vmax[S])/(Km + [S]), where v is reaction rate, [S] is substrate concentration, Vmax is maximum rate, and Km is the substrate concentration when v = Vmax/2.
- A lower Km usually means higher enzyme affinity for the substrate because half-maximal speed is reached at a lower substrate concentration.
- Competitive inhibition increases apparent Km but does not change Vmax because enough substrate can outcompete the inhibitor.
- Noncompetitive inhibition lowers Vmax but does not change Km when the inhibitor reduces active enzyme function without blocking substrate binding directly.
- ATP hydrolysis is ATP + H2O -> ADP + Pi + energy, and cells couple this energy release to reactions that need energy.
- Catabolism breaks molecules down and releases usable energy, while anabolism builds molecules and requires energy input.
- Feedback inhibition occurs when the final product of a pathway inhibits an earlier enzyme, helping prevent wasteful overproduction.
Vocabulary
- Activation energy
- The minimum energy needed for reactants to reach the transition state and begin forming products.
- Active site
- The specific region of an enzyme where the substrate binds and the reaction is catalyzed.
- Vmax
- The maximum reaction rate when all enzyme active sites are saturated with substrate.
- Km
- The substrate concentration at which the reaction rate is half of Vmax.
- Inhibitor
- A molecule that decreases enzyme activity by binding to the enzyme or enzyme-substrate complex.
- Metabolism
- The complete set of chemical reactions in a cell, including energy-releasing and energy-consuming pathways.
Common Mistakes to Avoid
- Confusing Km with Vmax is wrong because Km describes the substrate concentration at half-maximal rate, while Vmax describes the highest possible rate.
- Saying enzymes change ΔG is wrong because enzymes lower activation energy but do not change the overall energy difference between reactants and products.
- Assuming more substrate always increases rate is wrong because the rate levels off near Vmax when enzyme active sites are saturated.
- Mixing up competitive and noncompetitive inhibition is wrong because competitive inhibitors raise apparent Km, while noncompetitive inhibitors lower Vmax.
- Treating ATP as stored energy that never changes form is wrong because ATP releases usable energy mainly when it is hydrolyzed to ADP and Pi.
Practice Questions
- 1 An enzyme has Vmax = 120 micromol/min and Km = 5 mM. What is the reaction rate when [S] = 5 mM?
- 2 Using v = (Vmax[S])/(Km + [S]), calculate v when Vmax = 80 units/s, Km = 2 mM, and [S] = 6 mM.
- 3 A competitive inhibitor is added to an enzyme reaction. Predict what happens to apparent Km and Vmax.
- 4 Why does feedback inhibition help a cell conserve energy and materials in a metabolic pathway?
Understanding Enzyme Kinetics & Metabolism
An enzyme rate is usually measured at the start of a reaction. At this point, the substrate supply is still high and little product has built up. This matters because products can slow a reaction or run it in reverse.
On a rate versus substrate graph, the curve rises steeply at first because many active sites are available. It then flattens because most enzyme molecules are busy.
Adding more substrate at that stage has little effect. Raising the amount of enzyme can raise the maximum rate, provided enough substrate is present.
The value called Km is useful for comparing results, but it needs careful interpretation. It describes the substrate level needed to reach half of the maximum rate under particular conditions. A small Km often suggests that an enzyme works effectively when substrate is scarce.
It does not always prove a stronger physical attraction in every situation. Temperature, acidity, salt level, and the presence of other molecules can change enzyme shape or movement. Real cells are crowded environments, so measurements from a clean laboratory tube are simplified models of what happens inside an organism.
Inhibitors reveal that enzymes can be controlled in more than one way. Some molecules compete for the active site. Their effect can be reduced when substrate concentration increases.
Other molecules bind at a different location and alter the enzyme's working shape. These are often called allosteric effects. Some inhibitors bind permanently and remove enzyme molecules from use until the cell makes replacements.
Poisons and certain medicines work this way. Students should focus on what happens to the graph.
A change in the substrate amount needed for a given rate points to one kind of effect. A lower top speed points to a loss of working enzyme activity.
Metabolism depends on linking reactions rather than treating them as isolated steps. When a food molecule is broken down, some energy is captured in ATP and in electron carriers such as NADH. ATP can transfer a phosphate group to another molecule.
This often makes the receiving molecule more reactive. During cellular respiration, electrons move through a chain of proteins in the inner mitochondrial membrane. Their energy helps move hydrogen ions across the membrane.
As hydrogen ions flow back through ATP synthase, the enzyme produces ATP. This process explains why oxygen is important in aerobic respiration. Oxygen accepts electrons near the end of the chain, allowing the chain to keep operating.
Cells regulate pathways at key steps because running every reaction at full speed would waste materials. An early enzyme may respond to the amount of final product, ATP, or another signal about cell needs. Muscle cells, for example, increase glucose breakdown during exercise when ATP use rises.
Liver cells can store glucose after a meal or release it between meals. When studying pathway diagrams, track where carbon atoms go, where ATP is spent or made, and where electrons are transferred. These details make it easier to connect enzyme behavior with the energy needs of a whole cell.