Ch. 20 Enzymes and Vitamins
In the lock-and-key model of enzyme action, the enzyme active site is thought of as
a rigid, nonflexible shape that fits the substrate exactly
When a substance bonds to an enzyme for reaction, its place of binding is the
active site
In the induced-fit model of enzyme action, the enzyme active site
adjusts shape to adapt to the shape of the substrate
The purpose of the many chemical reactions in our bodies is to
all of the above.
A noncompetitive inhibitor
alters the three-dimensional structure of the enzyme.
A competitive inhibitor is one that
binds to the active site in place of the substrate
The presence of enzymes to catalyze bioreactions in our bodies allows
bioreactions to take place under mild conditions
The general function of an enzyme in the body is to
catalyze chemical reactions
The active site of an enzyme
catalyzes the reaction
When a cofactor is a small organic molecule, it is known as a/an
coenzyme
The water-soluble B and C vitamins supply
coenzymes required by some enzymes.
Metal ions such as Zn²+ and Fe³+ are often needed by enzymes as
cofactors
The function of the enzyme-substrate complex is to provide an alternative reaction pathway that
decreases the activation energy for the reaction
The formation of an enzyme-substrate complex is the __________ step in enzyme action.
first
Most enzymes are
globular proteins
Hexokinase catalyzes only the addition of phosphate to any hexose sugar. This type of activity is called
group specificity
To what main class of enzymes does the enzyme that catalyzes the conversion of lactose to galactose and glucose belong?
hydrolase
To what main class of enzymes does the enzyme that catalyzes the following reaction belong? ser-ala → ser + ala
hydrolase
In an enzyme-substrate reaction, when excess substrate is present, increasing the concentration of the enzyme wil
increase the amount of reaction occuring
Penicillin functions as an antibiotic by
inhibiting the enzymes for cell wall formation in bacteria
Substances that react under the influence of an enzyme are usually held to the enzyme by
side chains of amino acids in the enzyme protein.
In any reaction catalyzed by an enzyme, the reacting molecule is called the
substrate
The B vitamins are examples of
water-soluble vitamins
Compared to an uncatalyzed reaction, an enzyme-catalyzed reaction
occurs at a faster rate
The optimum temperature for sucrase activity is 37 °C. The hydrolysis of sucrose is slowest at which temperature in the choices below?
0°C
Most enzymes are deactivated permanently above a temperature of about
50°C
"Physiological pH", the pH for optimum activity for most enzymes, is a pH equal to
7.4
Which of the following is NOT true for a competitive inhibitor?
It binds to the enzyme at a site remote from the active site.
Urea is converted to ammonia and carbon dioxide by the action of urease. What will be the effect on the rate if the temperature of the reaction is lowered from 37 °C (the optimum temperature) to 27 °C?
The rate will slow down
Which of the following is NOT a step in the enzyme-catalyzed conversion of a substrate to product?
The substrate changes its shape so it can bind at the active site.
A noncompetitive inhibitor has a structure that
does not resemble the substrate structure
Coenzymes such as water-soluble vitamins are needed in only small amounts because
each vitamin molecule can be reused many times as a cofactor
A compound that binds to the surface of an enzyme, and changes its shape so that a substrate cannot enter the active site, is called a/an
noncompetitive inhibitor
The hydrolysis of ester bonds in triglycerides is catalyzed by a/an
lipase
Small molecules that make up the repeat unit in polymers are called
monomers
To what main class of enzymes does the enzyme that catalyzes the following reaction belong?
oxidoreductase
"Physiological conditions" for reactions within the body are approximately
pH 7 and 37°C