In 1897 the German chemist Eduard Buchner ground yeast cells with sand, pressed out the juice and added sugar. The juice fermented the sugar, even though no living yeast cells were left. Many scientists had thought fermentation needed a living cell. Buchner showed it needed only the molecules inside one, and he won the 1907 Nobel Prize in Chemistry for it. Those molecules were enzymes, and they're the start of AP Biology Unit 3.
Activation energy
Many reactions in a cell release energy overall, but even these happen only very slowly on their own at body temperature. Bonds in the reactants must be strained and broken first, which takes an input of energy: the activation energy. On an energy diagram it's the hill between the reactants and the products. The top of the hill is the transition state.
An enzyme is a protein that acts as a biological catalyst. It lowers the hill. Look closer at what it doesn't do:
- It doesn't change the energy of the reactants or the products, so the energy released or absorbed is the same.
- It isn't used up. The same enzyme molecule can catalyze the reaction over and over.
This is the simple version. Some RNA molecules can also act as catalysts: the RNA at the core of the ribosome, which you met in Unit 2 and will see at work in Unit 6, is one. The AP course treats enzymes as proteins.
How enzymes lower the hill
The substrate binds the active site, forming an enzyme-substrate complex. The substrate's shape and charge have to be compatible with the active site, which is why enzymes are specific. As the substrate binds, the active site shifts slightly to grip it, called induced fit. In the active site, the enzyme can hold two substrates in the right orientation, strain a bond, or provide a chemical environment that favors the reaction.
Energy in the cell
Exergonic reactions release energy; endergonic reactions need it. Cells use energy coupling: an exergonic reaction, often ATP hydrolysis, drives an endergonic one. The course expects you to understand this idea, not to calculate it; the equation for Gibbs free energy is outside the scope of the AP Exam.
Energy is handled in metabolic pathways, where the product of one enzyme's step is the substrate for the next. Small steps release energy in a controlled way.
Temperature and pH
- Below the optimum, warming speeds molecules up, so enzymes and substrates collide more often and the rate rises.
- Above the optimum, the hydrogen bonds that hold the enzyme's shape break. The active site deforms and the rate falls. This is denaturation. Sometimes it's reversible, and the enzyme refolds when conditions return to normal.
- pH works the same way: each enzyme has an optimum. Pepsin in the stomach works best near pH 2; trypsin in the small intestine works best near pH 8.
Substrate, product and inhibitors
With a fixed amount of enzyme, adding substrate raises the rate until every active site is busy. Then the curve levels off: the enzyme is saturated. As product builds up and substrate runs low, the reaction slows.
- A competitive inhibitor binds the active site reversibly. Adding lots of substrate can outcompete it.
- A noncompetitive inhibitor binds an allosteric site, elsewhere on the enzyme, and changes its shape. More substrate doesn't help.
Cells use allosteric sites for control. In feedback inhibition, the end product of a pathway binds an enzyme near the start, so the cell stops making more than it needs.
Common mistakes students make
- "Enzymes provide energy for the reaction." They lower the activation energy; they don't add energy.
- "Enzymes die at high temperature." Enzymes aren't alive. They denature: their shape changes.
- "Cold denatures enzymes." Cold slows them because there are fewer collisions; the shape usually survives.
- "The rate levels off because the enzyme is used up." It levels off because the active sites are saturated.
Frequently asked questions
How do enzymes lower activation energy?
They bind substrates in the active site and hold them in the right orientation, strain their bonds, or provide a favorable chemical environment, so less energy is needed to reach the transition state.
Do enzymes change the amount of energy a reaction releases?
No. They change only how fast the reaction happens, not the energy of the reactants or products.
What is the difference between competitive and noncompetitive inhibition?
A competitive inhibitor binds the active site, so extra substrate can outcompete it. A noncompetitive inhibitor binds an allosteric site and changes the enzyme's shape, so extra substrate doesn't restore the rate.
Why does enzyme activity drop above the optimum temperature?
Heat breaks the hydrogen bonds holding the enzyme's shape, so the active site no longer fits the substrate.
Try it
Say each answer before you flip the card. Then sketch a graph of rate against substrate concentration. What would you expect to see if you repeated the experiment with twice as much enzyme?
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