3.1Cellular Energetics
Enzymes — AP BIOLOGY Revision Notes & Topical Questions
5 min read 8 parts Lesson 1 of 5 2 quizzes
Lesson summary
3.1 Enzymes — Notes — AP BIOLOGY notes: Explain how enzymes affect the rate of biological reactions.Enzymes — globular proteins that act as biological catalysts, and virtually every metabolic reaction in living organisms is catalysed by one, so they are essential for lifeNames — …
- Specificity
- Active site
- Cofactors and coenzymes
- Induced fit
- Activation energy
- How enzymes work
Section 1 of 8Enzyme structure and properties
Enzyme structure and properties
Enzymes
- Enzymes globular proteins that act as biological catalysts, and virtually every metabolic reaction in living organisms is catalysed by one,
- so they are essential for life
Names
- Names enzyme names usually end in -ase, and a few enzymes are RNA (ribozymes)
Specificity
- Specificity each enzyme has high specificity for the reaction it catalyses and the substrate it binds
Active site
- Active site a region with a specific shape and chemical environment complementary to a specific substrate
- (charge, polarity of R groups)
- The shape and charge of the substrate must be compatible with the active site.
- The tertiary (and quaternary) structure determines the shape of the active site, and the 3-D shape depends on the amino acid sequence.
Cofactors and coenzymes
- Cofactors and coenzymes inorganic ions such as Mg²⁺, Zn²⁺ and Fe²⁺ (cofactors) and organic molecules such as NAD⁺, FAD and vitamins (coenzymes) may be needed for activity
Section 2 of 8Enzyme–substrate complex
Complex
- Complex when a substrate binds the active site an enzyme–substrate complex forms, only temporarily, before the enzyme catalyses the reaction and releases the product(s)
- (a few nanoseconds)
Collision
- Collision substrates must collide with the active site at the correct orientation and speed
Induced fit
- Induced fit the active site changes shape slightly as the substrate binds
Recycling
- Recycling enzymes are not used up in the reaction and can be used again
1
The substrate collides with the enzyme in the correct orientation.
2
The substrate binds the complementary active site, forming an enzyme–substrate complex (induced fit).
3
The enzyme stabilises the transition state, lowering the activation energy.
4
The substrate is converted to product(s).
5
The products are released and the unchanged enzyme is free to bind another substrate.
Section 3 of 8Enzymes and activation energy
Catalysts
- Catalysts enzymes are biological because they work in living systems and catalysts because they speed up reactions without being used up or changed
Activation energy
Activation energy the energy the substrate needs to become just unstable enough for a reaction to occur and products to form
How enzymes work
- How enzymes work they lower the activation energy and so provide an alternative energy pathway
- They destabilise bonds in the substrate, making it more reactive.
- Ways include orienting substrates, straining bonds, providing a favourable microenvironment and temporary covalent bonding.
Free energy
- Free energy enzymes do not change ΔG or the equilibrium position, only the rate at which equilibrium is reached
Reaction rate
- Reaction rate amount of product formed ÷ time, for example in µmol min⁻¹
- (or substrate used)
Section 4 of 8Measuring enzyme activity
Measure
- Measure product formed or substrate used at timed intervals,
- such as oxygen released by catalase from hydrogen peroxide or the time for starch to disappear with iodine and amylase
Initial rate
- Initial rate the gradient of the curve at the start, when substrate is not yet limiting
Rate from time
- Rate from time rate = 1 ÷ t,
- so a longer time means a slower reactionWhat the symbols mean: rate is inversely proportional to the time taken (t, in s) · a shorter time means a faster rate
Units
- Units rates are given per second or per minute, and per cm³ or per unit of enzyme when comparing different volumes
Section 5 of 8Enzyme inhibitors
Inhibitor
Inhibitor a molecule that slows or stops enzyme activity, so increasing its concentration lowers the rate and eventually stops the reaction
Reversible inhibitors
- Reversible inhibitors they do not form covalent bonds,
- so their effect can be reduced by removing the inhibitor or, for competitive inhibitors, by raising substrate concentration
Irreversible inhibitors
- Irreversible inhibitors they permanently inactivate the enzyme, usually by forming a covalent bond in the active site,
- and removing the inhibitor or adding substrate does not reverse this
Use
- Use many drugs and poisons are inhibitors, and cells use inhibitors naturally to control metabolism
Mechanism
- Mechanism a competitive inhibitor has a shape similar to the substrate and competes for the active site, binding it reversibly or irreversibly
Overcoming it
- Overcoming it raising substrate concentration makes it more likely that substrate occupies the active site, so inhibition is reduced
Rate
- Rate the initial rate is lowered because some active sites are occupied,
- but the same maximum rate (Vmax) is eventually reached, only at a higher substrate concentration
Examples
- Malonate resembles succinate and inhibits succinate dehydrogenase in the Krebs cycle.
- Sulfonamide competes with a precursor for a bacterial enzyme that makes folic acid.
- Statins inhibit HMG-CoA reductase and reduce cholesterol synthesis.
Section 6 of 8More from the specification
More from the specification
Enzyme active site catalytic function
- Enzyme active site catalytic function tertiary conformational folding creates unique catalytic clefts that bind specific substrates to fulfill essential biological catalytic function.
Optimum
- The temperature or pH at which activity is highest.
Denaturation
- Permanent loss of enzyme shape, including the active site.
Enzyme-substrate complex
- The temporary structure formed when a substrate binds to an enzyme's active site.
Successful collision
- A collision with enough energy and correct orientation.
Optimum temperature
- The temperature at which an enzyme works fastest, 37 °C in humans.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Enzyme | A protein catalyst that speeds up a reaction without being used up. |
| Active site | Region of the enzyme where the substrate binds. |
| Substrate | The molecule an enzyme acts on. |
| Activation energy | The energy barrier of a reaction, lowered by enzymes. |
| Denaturation | Loss of 3-D shape, often irreversible. |
| Optimum | The temperature or pH at which activity is greatest. |
| Competitive inhibitor | Binds the active site and competes with the substrate. |
| Allosteric site | A binding site away from the active site. |
Section 8 of 8Exam technique
1
Saying high temperature kills enzymes — enzymes are denatured, not killed.
2
Believing a cold enzyme is denatured — at low temperature it is only less active and activity returns on warming.
3
Writing that the substrate plateau is caused by denaturation — it is caused by saturated active sites.
4
Saying a non-competitive inhibitor binds in the active site — it binds at an allosteric site.
5
Stating that adding more substrate overcomes non-competitive inhibition — it does not.
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