4th year Exam.
Benha University
Faculty of Science
First Term 2012
Botany Department
Plant physiology
Answer the following questions:
Q1:
(20 Marks)
1-Define enzymes and describe its properties.
2- Mention five precautions must be taken during the preparation of
a crude enzyme solution.
Q2:
(20 Marks)
Answer two only of the following:
1- Write about the different explanations for mechanism of enzyme
action.
2- Write short notes on three only:
a- Amidases
b- Z-enzyme
d- Competitive inhibitors
c- α-amylase
e- Co-enzymes.
3- Explain about two of the important factors which affect on the rate of
enzyme reaction.
With My Best Wishes
Answer the Exam.
Answer the Q1:
1Enzymes: are organic catalysts produced by the protoplasm. They are nonliving
vompounds. Proteinic in nature and are able to accelerate the rate of many chemical
reactions of biological importance.
Properties of enzymes:
1- Enzymes are organic catalysts, active in extremely small quantities.
2- 2-Like any catalyst, enzymes remain unaffected by the reaction they catalyze.
3- 3-Some enzymes can initiate or start new reaction but all enzymes accelerate
the rate of thermodynamically possible reaction without affecting the
equilibrium of any reaction.
4- 4-Being proteinic in nature.
5- 5-Enzymes are affected by temperature, Ph, metal ions.
6- 6-Enzymes show specificity to reactions which they catalyze.
2Five precautions must be taken during the preparation of a crude enzyme
solution:
1- The cell must be kept cold during disruption.
2- 2-The medium must be buffered about neutrality.
3- 3-Enzymes which contain sulfhydryl group (-SH) are liable to oxidation
during extraction.
4- 4-Enzyme proteins may combine with cell phenolics by hydrogen bonding and
irreversibly following oxidation of the phenols to their corresponding quinines
resulting in a low in enzyme activity. To remove these phenolics with add
PVP.
5- 5-EDTA is also added to chelae heavy metal ions.
Answer the Q2:
1Arrhenius first pointed out that, all the molecules in a given population do not have
the same kinetic energy some molecules are energy poor and other are energy rich.
Higher is the energy barrier the grater is the inactiveness of reaction. This energy
barrier can be overcome by the enzymes and making the molecule active with
available energy level.
To explain the velocity of enzymatic reaction Leonor Michaels and Moud Menten
(1913) proposed following assumptions.
i.
ii.
iii.
iv.
v.
Only a single substrate and a single product are formed in enzymatic
reaction.
The process continued essentially to its completion.
Concentration of substrate is much greater than the enzyme in the system.
An intermediate enzyme substrate complex is formed.
The rate of decomposition of the substrate is proportional to the
concentration of the enzyme substrate complex. They proposed an
equation popularly accepted as Michaelis. Menten's equation, which
concerned the velocity of enzymatic reaction.
Where Km is the Michaelis constant 'S' is the substrate concentration, Vmax maximum velocity of the reaction and V0 is the initial velocity.Km value is constant
for all enzymes up to the half of the maximum velocity of reaction. Greater is the ES
complex period the lower is the Km value.
2A- Amidases:
An amidase an enzyme that catalyzes the hydrolysis of an amide:
Thus, the two substrates of this enzyme are monocarboxylic acid amide and H2O,
whereas its two products are monocarboxylate and NH3.
B- Z-enzyme:
Enzyme found associated with amylase, that attacks the few β-1, 3-links present in
amylose. Pure, crystalline β-amylase will convert only 70% of amylose to maltose; it
requires the presence of the Z-enzyme for complete conversion.
C- α-amylase:
Is present in germinated barely grains. It attacks the 1:4 α –linkages in either amylase
or amylopectine in random fashion to split the macromolecules into a number of low
molecular weight (short chain) α-dextine. It can attack the 1:4 α linkage in either the
free ends or chains between the branch points.
D- Competitive inhibitors:
A competitive inhibitor is any compound which closely resembles the chemical
structure and molecular geometry of the substrate. The inhibitor competes for the
same active site as the substrate molecule. The inhibitor may interact with the enzyme
at the active site, but no reaction takes place. The inhibitor is "stuck" on the enzyme
and prevents any substrate molecules from reacting with the enzyme. However, a
competitive inhibition is usually reversible if sufficient substrate molecules are
available to ultimately displace the inhibitor. Therefore, the amount of enzyme
inhibition depends upon the inhibitor concentration, substrate concentration, and the
relative affinities of the inhibitor and substrate for the active site.
E- Co-enzymes.
Coenzymes are small organic molecules that link to enzymes and whose presence is
essential to the activity of those enzymes. Coenzymes belong to the larger group
called cofactors, which also includes metal ions; Many coenzymes are derived from
vitamins .The coenzymes derived from them, the type of reactions in which they
participate, and the class of coenzyme. Prosthetic groups are tightly bound to enzymes
and participate in the catalytic cycles of enzymes. Like any catalyst, an enzyme–
prosthetic group complex undergoes changes during the reaction, but before it can
catalyze another reaction, it must return to its original state.
31- Temperature Effects
Like most chemical reactions, the rate of an enzyme-catalyzed reaction increases as
the temperature is raised. A ten degree Centigrade rise in temperature will increase the
activity of most enzymes by 50 to 100%. Variations in reaction temperature as small
as 1 or 2 degrees may introduce changes of 10 to 20% in the results. In the case of
enzymatic reactions, this is complicated by the fact that many enzymes are adversely
affected by high temperatures. As shown in Figure 13, the reaction rate increases with
temperature to a maximum level, then abruptly declines with further increase of
temperature. Because most animal enzymes rapidly become denatured at temperatures
above 40°C, most enzyme determinations are carried out somewhat below that
temperature.
Over a period of time, enzymes will be deactivated at even moderate temperatures.
Storage of enzymes at 5°C or below is generally the most suitable. Some enzymes
lose their activity when frozen.
2-Effects of pH
Enzymes are affected by changes in pH. The most favorable pH value - the point
where the enzyme is most active - is known as the optimum pH. This is graphically
illustrated in Figure 14.
Extremely high or low pH values generally result in complete loss of activity for most
enzymes. pH is also a factor in the stability of enzymes. As with activity, for each
enzyme there is also a region of pH optimal stability. The optimum pH value will vary
greatly from one enzyme to another