Enzymes are specific proteins that combine with other substances called substrates (reactants) . Where they bind together is called the active site. There is one enzyme for one substrate, and they fit together like pieces of a puzzle. The joining of the two forms an enzyme-substrate complex , which is where the chemical reaction takes place. The chemical reaction only changes the reactants, and never the enzyme.
Because enzymes do not change during a reaction, they can be used over and over again.
ENZYMES AS CATALYSTS
A catalyst is a substance that speeds up a chemical reaction without being chemically changed by the reaction.
Catalysts decrease the amount of activation energy required for the reactions to occur.
Activation energy is the amount of energy required in order for the reactant molecules to begin a chemical reaction.
Our bodies use catalysts called enzymes to break down food and convert it to energy. Every cellular activity is a result of many biochemical reactions that take place at a cellular level. Substances hat speed these reactions are called enzymes.
Metabolism cannot occur unless the energy of activation has been reached. These biological reactions would eventually take place on their own, but in the presence of enzymes, the reactions take place about a million times faster. Enzymes are proteins
(catalysts) that speed up chemical reactions. Enzymes work by lowering the energy of activation, making some chemical processes occur with greater frequency.
Factors that influence the rate at which enzymes act include such things as temperature, pH, and amount of substrate present. Most enzymes have an optimum temperature and pH. Their optimum temperature or pH is the range at which the enzyme functions best.
Enzymes vary from one organism to another. Some bacteria have enzymes that have an optimum temperature of 70°C r higher; this temperature would destroy most human enzymes.
Recall that a pH of 7 is considered neutral . Water has a pH of about 7. Substances with a pH less than 7 are acids and substances with a pH greater than 7 are bases . With a few exceptions, most enzymes have an optimum pH of between 6 and 8. Table 29.1 contains

several enzymes and their optimal pH. One exception is pepsin, an acidic enzyme found in the human stomach. Pepsin has an optimum pH of 1-2.
Table 29.1
pH of Optimum Activity
Enzyme Optimum pH Temperature
Lipase- hydrolyzes glycerides (pancreas) 8.0 35 - 40°C
Protease-decomposition of proteins
Urease-hydrolysis of urea
Maltase-hydrolysis of maltose to glucose
Amylase (pancreas)-hydrolysis of starch
Catalase-decomposition of hydrogen peroxide into water and oxygen
EcoRI-restriction enzyme that cuts DNA
Taq Enzyme
1.5-1.6
7.0
6.1-6.8
6.7-7.0
7.0
7.0 - 7.5
7.5 – 9.5
35 - 40°C
60°C
35 - 40°C
35 - 40°C
35 - 40°C
35 - 40°C
72°C
A. Define the following: catalyst activation energy enzyme neutral acids bases substrate (reactant) enzyme-substrate complex
B. Select the best answer.
1. Which of the following is true of enzymes?
A. enzymes have an optimal pH at which they function
B. enzymes have an optimal temperature at which they function
C. enzymes are proteins
D. All of the above are true
2. Most enzymes found in the human body have an optimum temperature of
A. 35 - 40°C
B. 60°C
A. reactant
B. activation
C. 72°C
D. 60°C - 72°C
3. The amount of energy needed for a chemical reaction to begin is called ___ energy.
C. substrate
D. enzyme
C. Complete the following exercises
1. Explain how enzymes work.