substrate binds to the active site and physically blocks the substrate from attaching.
Competitive inhibition can sometimes be overcome by adding a high concentration of sub-
strate to outcompete the inhibitor. In noncompetitive inhibition(Figure 5.11), an inhibitor
molecule binds to a different part of the enzyme, causing a change in the shape of the active
site so that it can no longer interact with the substrate.
pH: Acids and Bases
The pH scale is used to indicate how acidic or basic a solution is. It ranges from 0 to 14;
7 is neutral. Anything less than 7 is acidic; anything greater than 7 is basic. The pH scale
is a logarithmic scale and as a result, a pH of 5 is 10 times more acidic than a pH of 6.
Following the same logic, a pH of 4 is 100 times more acidic than a pH of 6. Remember
that as the pH of a solution decreases, the concentration of hydrogen ions in the solution
increases, and vice versa. For the most part, chemical reactions in humans function at or
near a neutral pH. The exceptions to this rule are the chemical reactions involving some of
the enzymes of the digestive system. (See Chapter 15, Human Physiology.)
Reactions
There are five types of reactions you should know for this exam:
- Hydrolysis reaction. A reaction that breaks down compounds by the addition of H 2 O.
- Dehydration synthesis reaction. A reaction in which two compounds are brought
together with H 2 O released as a product.
- Endergonic reaction.A reaction that requires input of energy to occur.
A+B+energy→C
- Exergonic reaction.A reaction that gives off energy as a product.
A+B→energy+C
- Redox reaction.A reaction involving the transfer of electrons. Such reactions occur
along the electron transport chain of the mitochondria during respiration (Chapter 7).
Chemistry ❮ 47
Figure 5.11 Noncompetitive inhibition.
Enzyme Bound inhibitor Altered active site
Active site
Inhibitor binds elsewhere on enzyme,
altering the binding site so the substrate
cannot bind.
Substrate Inhibitor
Notice the substrate and the inhibitor are not identical.
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