Download Review session for exam-I

Review session for exam-III
Lectures 10-12
The concept of
“induced fit” refers to the fact that:
A.
B.
C.
D.
E.
Enzyme specificity is induced by enzyme-substrate
binding.
Enzyme-substrate binding induces an increase in the
reaction entropy, thereby catalyzing the reaction.
Enzyme-substrate binding induces movement along the
reaction coordinate to the transition state.
Substrate binding may induce a conformational change
in the enzyme, which then brings catalytic groups into
proper orientation.
When a substrate binds to an enzyme, the enzyme
induces a loss of water from the substrate.
The steady state assumption,
as applied to enzyme kinetics, implies:
A.
B.
C.
D.
E.
Km = Ks.
The enzyme is regulated.
The ES complex is formed and broken down at
equivalent rates.
The Km is equivalent to the cellular substrate
concentration.
The maximum velocity occurs when the enzyme is
saturated.
The Lineweaver-Burk plot is used to:
A.
B.
C.
D.
E.
determine the equilibrium constant for an enzymatic
reaction.
extrapolate for the value of reaction rate at infinite
enzyme concentration.
illustrate the effect of temperature on an enzymatic
reaction.
solve, graphically, for the rate of an enzymatic
reaction at infinite substrate concentration.
solve, graphically, for the ratio of products to
reactants for any starting substrate concentration.
To determine Km from a double-reciprocal
plot, you would:
A.
B.
C.
D.
E.
multiply the reciprocal of the x-axis intercept
by -1.
multiply the reciprocal of the y-axis intercept
by -1.
take the reciprocal of the x-axis intercept.
take the reciprocal of the y-axis intercept.
take the x-axis intercept where V0 = 1/2
Vmax.
The number of substrate molecules
converted to product in a given unit of time
by a single enzyme molecule at saturation
is referred to as the _______________.
In competitive inhibition, an inhibitor:
A.
B.
C.
D.
E.
binds at several different sites on an
enzyme.
binds covalently to the enzyme.
binds only to the ES complex.
binds reversibly at the active site.
lowers the characteristic Vmax of the
enzyme.
A transition-state analog:
A.
B.
C.
D.
E.
is less stable when binding to an enzyme than the
normal substrate.
resembles the active site of general acid-base
enzymes.
resembles the transition-state structure of the
normal enzyme-substrate complex.
stabilizes the transition state for the normal
enzyme-substrate complex.
typically reacts more rapidly with an enzyme than
the normal substrate.
A small molecule that decreases the activity of
an enzyme by binding to a site other than the
catalytic site is termed a(n) _______________.
For the reaction E + S  ES  P, the
Michaelis-Menten constant, Km, is
actually a summary of three terms.


What are they?
How is Km determined graphically?
Fifteen mg of an enzyme of Mr 30,000
working at Vmax catalyzes the conversion
of 60 mmol of substrate into product in 3
min.
 What is the enzyme's turnover
number?
Answer:

The amount of enzyme present is



The rate of product formation is



15 X10-6 g, which is
(15 ´ 10-6 g)/(3 ´ 104 g/mol) = 5 X10-10 mol of
enzyme.
60 ´ 10-6 mol/3 min or
20 X10-6 mol of product per minute.
The turnover number is, therefore,
 (20 X10-6 mol/min)/(5 X10-10 mol of enzyme)
or
 4 X10-4 min-1.
Why does pH affect the activity of an
enzyme?
Answer:
The state of ionization of several amino
acid side chains is affected by pH and
the activity of many enzymes requires
that certain of the amino acid residue
side chains be in a specific ionization
state.
Chymotrypsin belongs to a group of proteolytic
enzymes called the “serine proteases,” many
of which have an Asp, His, and Ser residue
that are crucial to the catalytic mechanism.
The serine hydroxyl functions as a nucleophile.

What do the other two amino acids do to
support this nucleophilic reaction?
Answer:

In chymotrypsin,histidine functions as
a general base, accepting a proton
from the serine hydroxyl, thereby
increasing serine's reactivity as a
nucleophile.

The negatively charged Asp stabilizes
the positive charge that develops on
the His.
question
What is a zymogen (proenzyme)?
Explain briefly with an example.
Answer

A zymogen is an inactive form of an enzyme
that is activated by one or more proteolytic
cleavages in its sequence.

Chymotrypsinogen, trypsinogen, and
proelastase are all zymogens, becoming
chymotrypsin, trypsin, and elastase,
respectively, after proper cleavage.
What is the less active conformational
form of an allosteric enzyme?
The enzymes that catalyze protein
phosphorylation are:
Aspartyl transcarbamylase catalyzes the
first step in the synthesis of
______________.
____________ is the most common
donor molecule in phosphorylation
reactions catalyzed by protein kinases.
Protein kinases add phosphoryl groups to
serine, threonine, and _______________
residues in target proteins.
The relaxed form of an allosteric enzyme has
_________ affinity for the substrates.
A)
B)
C)
D)
E)
higher
equal
lower
no
None of the above.
The common activator of the pancreatic
zymogens is
A.
B.
C.
D.
E.
trypsinogen.
trypsin.
chymotrypsin.
elastase.
None of the above.
How is the clotting cascade initiated?
What is the final step in the clotting pathway?
Individuals in a royal family suffered from
hemophilia and often died early from
bleeding.
A.
B.
What is the cause of this disease?
How is it treated?
Answer
A.
The disease in the royal family was
caused by faulty factor VIII of the
intrinsic pathway.
B.
It is treated by the addition of
supplemental protein, originally
isolated from serum but now a
recombinant product.