CHAPTER
CHAPTER 56
THE STRUCTURE
AND FUNCTION
AN INTRODUCTION
OFTO
MACROMOLECULES
METABOLISM
Section B: Enzymes
1.
2.
3.
4.
Enzymes speed up metabolic reactions by lowering energy barriers
Enzymes are substrate specific
The active site in an enzyme’s catalytic center
A cell’s physical and chemical environment affects enzyme activity
1
Pages 96 - 103
Hydrolysis of sucrose (table sugar)
Dehydration
Glucose + Fructose
Glucose + Fructose
Sucrose
Hydration (H2O)
Sucrase
Hydrolysis of sucrose in the presence of Sucrase results
in its two monosaccharide components.
This process consumes ‫ تستهلك‬energy (Activation Energy; EA)
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1. Enzymes speed up ‫تسرع‬reactions ‫التفاعالت‬by
lowering ‫ تقليل‬the activation energy ‫طاقة التنشيط‬
A catalyst ‫ ال ُمحفز‬is a chemical agent that changes the rate of a
reaction without being consumed ‫ دون أن يُستهلك‬by the reaction.
An enzyme is a catalytic protein.
Activation Energy: is the amount of energy needed for the
reaction (between enzyme & substrate) to complete (to break the bonds)
Raising the temperature for these reactions to complete
will either denature the compounds or kill the cell.
Thus, organisms must use a catalyst ‫عامل محفز‬.
3
Fig. 6.11, Page 96
Enzymes are substrate specific
• The substrate ‫ المادة المطلوب هضمها‬is a reactant which binds to an enzyme.
• When a substrate binds to an enzyme, the enzyme catalyzes ‫ يسهل‬the
conversion ‫ تحويل‬of the substrate to the product ‫مكوناتها البنائية‬.
– Sucrase (catalyst) is an enzyme that binds to sucrose (substrate) and
breaks the disaccharide into fructose and glucose (products).
Enzyme (a catalyst)
Substrate
Product (s)
Sucrase
Sucrose + H2O
Glucose + Fructose
Specificity of enzyme ‫ تخصصية اإلنزيم‬refers to its Active Site
‫ المركز النشط‬which fit to ‫ يـُناسب‬the surface of substrate.
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The active site is an enzyme’s catalytic center
The active site ‫ المكان النشط‬of an enzymes is the groove ‫ الجيب‬on the
surface of the enzyme into which the substrate fits.
The specificity of an enzyme is due to the fit between ‫ التناسب بين‬the
active site and that of the substrate.
5
Fig. 6.14
Active site of enzyme and Catalytic Cycle
Sucrose
Sucrase
Glucose
H 2O
Fructose
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Cellular factors affecting enzyme activity
1.
Temperature: has a major
impact on reaction rate.

As temperature increases,
reaction between substrate
and active sites occur faster.

However, at some point
thermal increase begins to
denature the substrate.

Each enzyme has an optimal
temperature ‫ درجة حرارة ُمثلى‬.
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Fig. 6.16a, Page 100
Cellular factors affecting enzyme activity
2. pH also influences the reaction rate, each enzyme has
an optimal pH falls between pH 6 - 8 for most
enzymes.
•
However, digestive enzymes in the
stomach are designed to work best
at pH 2 while those in the intestine
are optimal at pH 8, both matching
their working environments.
Fig. 6.16b, page 100
3. Cofactors (coenzymes): ‫العوامل المساعدة‬
A non-protein helpers for catalytic activity of enzymes. They bind
permanently ‫ دائما‬to the enzyme and include two types:a)- Inorganic cofactors, include zinc, iron, and copper.
b)- Organic cofactors, include vitamins or molecules derived
from vitamins.
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B)- Enzyme inhibitors: ‫ـثـبطات اإلنزيمات‬
ِّ ‫ُم‬
Enzyme inhibitors are molecules that prevent enzymes from
catalyzing reactions.
• Competitive inhibition ‫ تثبيط تنافسى‬: the inhibitor binds to the same
site as the substrate, then it called.
• Non-competitive inhibition: ‫تثبيط ال تنافسى‬
the inhibitor binds somewhere other
than the active site, then it called
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Fig. 6.17a, b & c, Page 101