AP Biology-UNIT III: CELLULAR ENERGETICS Study Notes

C. Enzymes
➢ Biological catalysts that speed up reactions

•  Accomplished by lowering activation energy and helping transition state form

➢ Lowers activation energy by:

•  Orienting substrate correctly
•  Straining substrate bonds
•  Providing favorable microenvironment
•  Bonding to substrate

➢ Do NOT change the energy of the starting point or the ending point of the reaction. Only lower activation energy
➢ Enzyme Specificity

•  Each enzyme catalyzes only one kind of reactions
•  Enzyme are usually named after the molecules they target
  ■ Replace suffix of substrate with -ase
            ● Ex. maltose catalyzed by maltase
• Substrates are the targeted molecules (reactant)
  
  

➢ Enzyme-Substrate Complex

•  Enzyme brings about transition state by helping the substrate(s) get into position
•  Accomplished through active site

•  Once the reaction has occurred, the enzyme is released from the complex and restored to its original state
  ■ Now the enzyme is free react with other substrates again 
• Induced fit

               ■ Enzyme slightly changes shape to accommodate the shape of substrates
              ■ Sometimes certain factors are necessary for this process
              ■ Cofactors sometimes aid induced fit and also help catalyze reactions
                             ● Nonprotein helpers of enzymes
                             ● Ex. vitamins
➢ Factors affecting reaction rates
○ Temperature
■ Although the rate of reaction increases with temperature, it only does so up to a point, because too much heat can denature an enzyme
■ $Q_10$
● Measure of sensitivity of a physiological process of enzymatic reaction rate

$Q_{10}=\left(\frac{R_2}{R_1}\right)^{\left(\frac{10}{T_2-T_1}\right)}$

• Temp must be celsius or kelvin
  ■ Same unit for $T_1 and T_2$
•  Two reaction rates $(k_1 and k_2) $must have same unit
• Reaction rates with$ Q_10=1$ are temperature independent
•  pH
  ■ Most enzyme’s ideal $\text pH$ is 7

➢ Enzyme Regulation

•  Cell can control enzymatic reactions by regulating the conditions that change the shape of the enzyme
• Can be turned off/on by things that bind to them
             ■ Some bind at active site
             ■ Some bin at allosteric sites (non-active sites)
• Competitive inhibition
          ■ If a substance has the exact complementary shape to the active site, it can compete with the substrate and block it from getting into the active site
        ■ If there is enough substrate available, it will out-compete the inhibitor and the reaction will occur
        ■ As substrate is used up, inhibitor out-competes the substrate and less reaction will occur
• Allosteric inhibitors/Non Competitive inhibition
        ■ Binds to an allosteric site
       ■ Distorts shape of enzyme so it cannot function until the inhibitor is removed
       ■ Substrate can still bind if active site is intact, but the enzyme will not be able to catalyze the reaction
       ■ Activators can also be used to stabilize the enzyme’s active state
      ■ Inhibitors stabilize the inactive state

• Enzymes can also be controlled by negative feedback mechanisms
  ■ Product of reaction the enzyme is helping is also an allosteric inhibitor
  ■ Prevents a cell from wasting resources by synthesizing more of a product than is needed