1

CHAPTER 5THE STRUCTURE AND FUNCTION

OF MACROMOLECULES

Pages 96 - 103

CHAPTER 6 AN INTRODUCTION

TO METABOLISMSection B: Enzymes

1. Enzymes speed up metabolic reactions by lowering energy barriers2. Enzymes are substrate specific3. The active site in an enzyme’s catalytic center4. A cell’s physical and chemical environment affects enzyme activity

2

Hydrolysis of sucrose (table sugar)

Glucose + Fructose SucroseDehydration

Hydration (H2O)

SucraseGlucose + Fructose

Hydrolysis of sucrose in the presence of Sucrase results in its two monosaccharide components.

This process consumes تستهلك energy (Activation Energy; EA)

3

A catalyst الُمحفز is a chemical agent that changes the rate of a reaction without being consumed يُستهلك أن .by the reaction دون

An enzyme is a catalytic protein.

1. Enzymes speed up تسرعreactions التفاعالتby loweringتقليل the activation energy طاقةالتنشيط

Fig. 6.11, Page 96

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 محفز .عامل

4

• The substrate هضمها المطلوب is a reactant which binds to an المادةenzyme.

• When a substrate binds to an enzyme, the enzyme catalyzes يسهل the conversion تحويل of the substrate to the product البنائية .مكوناتها– Sucrase (catalyst) is an enzyme that binds to sucrose (substrate) and

breaks the disaccharide into fructose and glucose (products).

Enzymes are substrate specific

Substrate Product (s)Enzyme (a catalyst)

Sucrose + H2OSucrase

Glucose + Fructose

Specificity of enzyme اإلنزيم refers to its Active Site تخصصيةالنشط .the surface of substrate يُـناسب which fit to المركز

5

The active site النشط on the الجيبof an enzymes is the grooveالمكانsurface of the enzyme into which the substrate fits.The specificity of an enzyme is due to the fit between بين the التناسبactive site and that of the substrate.

Fig. 6.14

The active site is an enzyme’s catalytic center

6

Active site of enzyme and Catalytic Cycle

SucraseSucrose

Glucose

FructoseH2O

7

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 ُمثلى حرارة . درجة

Fig. 6.16a, Page 100

8

3. Cofactors (coenzymes): المساعدة العوامل A non-protein helpers for catalytic activity of enzymes. They bind

permanently دائما to the enzyme and include two types:- a)- Inorganic cofactors, include zinc, iron, and copper.

b)- Organic cofactors, include vitamins or molecules derived from vitamins.

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

Cellular factors affecting enzyme activity

9

Enzyme inhibitors are molecules that prevent enzymes from catalyzing reactions.

• Competitive inhibition تنافسى the inhibitor binds to the : تثبيطsame site as the substrate, then it called.

Fig. 6.17a, b & c, Page 101

B)- Enzyme inhibitors: ِّطات ُمـثـباإلنزيمات

• Non-competitive inhibition: ال تثبيط the inhibitor binds تنافسىsomewhere other than the active site, then it called