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Biology sem1- chap4

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lecture note for matriculation student



Energy Energy is capacity to do work A Criterion for spontaneous change Free energy is the portion of a systems energy that can perform work when temperature is uniform throughout the system.

Free energy

The change in free energy as a system goes from a starting state to a different stage is represented by G (free energy change)


EXERGONIC1. 2. 3. 4. 5. 6. Exergonic (energy-yielding) In exergonic reactions free energy is released The product have less energy than the reactant Exothermic (heat releasing) Spontaneously E.g., cellular respiration


Endergonic1. 2. 3. 4. 5. 6. Endergonic (energy-requiring) There is a net input of free energy The product contains more energy than was present in the reactants Endothermic reactions (absorb heat) Non-Spontaneously E.g., protein synthesis, photosyntensis



Activation energya typical chemical reaction may be represented as : A B+C in this case A represents the substrate and B and C are the products.


Enzymes and activation energy Activation energy (EA) is the amount of energy necessary to push the reactants over an energy barrier. Enzyme speed reactions by lowering EA. The transition state can then be reached even at moderate temperatures.7

At the summit the molecules are at an unstable point, the transition state. The difference between free energy of the products and the free energy of the reactants is the delta G.


Enzymes do not change delta G. It hastens reactions that would occur eventually. Because enzymes are so selective, they determine which chemical processes will occur at any time.




Enzyme1. All are globular protein 2. Being protein, they are coded by DNA


Properties of Enzyme1. They are catalysts 2. They are very efficient. Very small amount of catalysts brings about the change of large amount of substrates 3. They are highly specific 4. Enzyme lower the activation energy 5. The catalyzed reaction is reversible 6. Enzymes posses active sites where the reaction takes place 7. Their presence does not alter the nature of 12 properties of the end product of the reaction

Factors affecting the rate of enzyme reactions Enzyme Concentration Substrate Concentration Temperature pH


Enzyme ConcentrationRate of reaction is proportional to the enzyme concentration (pH and temperature kept constant) The rate of reaction increased by increasing an enzyme concentration14

3. At high substrate concentration, the active sites are virtually saturated with substrate 4. Any extra substrate has to wait the complex has released the product


Substrate Concentration1. The rate of enzyme reaction increases with increasing substrate concentration 2. The theoretical maximum rate (Vmax) is never quite obtained. This is because when any further increase in substrate concentration produce no significant.16

Temperature1. The temperature that promotes maximum activity is referred to as optimum temperature 2. Temperature increased above this level, a decreased of activity occurs 3. Optimum temperature of most mammalian is about 37 40 oC


pH1. Every enzyme functions most efficiently over a particular pH range 2. As pH decreases, acidity increases and the concentration of H+ ions increases. This increases the number of positive charges in the medium 3. Extreme pH are encountered by an enzyme, then it will be denaturized 4. Optimum pH values for some enzymes


Optimum pH values for some enzymesEnzyme Pepsin Sucrase Enterokinase Salivary Amylase Catalase Optimum pH 2.00 4.50 5.50 4.80 7.6019



Enzyme structure and function :1. enzymes are complex three dimensional globular proteins 2. some of enzyme have other associates molecules 3. enzyme molecule is normally larger than the substrate molecule 4. only a small part of the enzyme molecule actually comes into contact with the substrate - active site.


5. only a few of the amino acids which so called catalytic amino acids make up the active site 6. and they are often some distance apart in the protein chain but are brought into close proximity by the folding of that chain


Mechanism of enzyme action2 hypothesis Lock and Key Hypothesis Proposed by Emil Fisher, 1890

Induced Fit Hypothesis Proposed by Daniel Koshland, 1959


Lock and Key Hypothesis Active site of the enzyme is complementary to the structure of the substrate molecule.


Lock and Key Hypothesis Mechanism Substrate (the key) fits into a rigid active site of the enzyme (the lock), like a key into lock. Forming enzymes-substrate complex Reaction product molecules leaves active site of the enzyme


Mechanism of enzyme action

Enzymes are thought to operate on a lock and key mechanism26


In the same way that a key fits a lock very precisely, so the substrate fits accurately into the active site of the enzyme molecule.28

The two molecules form a temporary structure called the enzyme-substrate complex. The products have a different shape from the substrate and so, once formed, they escape from the active site, leaving it free to become attached to another substrate molecule.29

Modern interpretations of the lock and key mechanism suggest that in the presence of the substrate the active site may change in order to suit the substrate s shape.30


Modern interpretations of the lock and key mechanism suggest that in the presence of the substrate the active site may change in order to suit the substrate s shape. The enzyme is flexible and moulds to fit the substrate molecule in the same way that clothing is flexible and can mould itself to fit the shape of the wearer. The enzyme initially has a binding configuration which attracts the substrate.32

On binding to the enzyme, the substrate disturbs the shape of the enzyme and causes it to assume a new configuration. It is this new configuration that is catalytically active and affects the shape of the substrate, thus lowering its activation energy. This is referred to as an induced fit of the substrate of the enzyme.




COFACTORS1. non protein substance 2. essential for some enzymes to function efficiently. 3. may be bound tightly to the active site as permanent residents 4. or they may bind loosely and reversibly along with the substrate.35

Types of cofactorsThree types of cofactors 1. Activators 2. Coenzymes 3. Prosthetic group


Activators (metal ions)Substances which are necessary for the functioning of the certain enzymes. Enzyme thrombokinase,prothrombin thrombin during blood clotting activated by calcium (Ca2+) ions.


Salivary amylasestarch maltose activated by chloride ( Cl- ) ions

These activators assist in forming the enzyme- substrate complex by moulding either the enzyme or substrate molecule into a more suitable shape.38

Coenzymesnon-protein organic substances which are essential functioning of some enzymes, but are not themselves bound to the enzyme. many coenzymes are derived from vitamins.


e.g.nicotinamide adenine dinucleotide (NAD) derived from nicotinic acid a member of the vitamin B complex.

NAD acts as a coenzymes to dehydrogenases by acting as a hydrogen acceptor


Prosthetic groupsorganic molecules bound to the enzyme example: haem.a ring- shaped organic molecule with iron at its center an oxygen carrier in haemoglobin.


Function of CofactorsWork by binding briefly with the enzyme, they sometimes alter its shape so that it can bind more effectively with substrate Sometimes help the enzyme to transfer a particular group of atoms from one molecule to another Metal ions changing enzymes shape and making it easier for substrate molecules to fit into active site.42

Inhibitionreversible and irreversible43

Reversible Inhibitors The effect of this type of inhibitor is temporary Causes no permanent damage to the enzyme The association of the inhibitor and enzyme is a loose one It can easily be removed Removal of the inhibitor restores the activity of the enzyme to normal There are two types: competitive inhibitors and non-competitive inhibitors44

Competitive InhibitorsIt compete with the substrate for the active sites The inhibitor may have structure with substrate While it remains bound to the active site, it prevents a substrate molecule from occupying that site and so reduces the rate of the reaction


Competitive InhibitorsThe substrate continues to use any unaffected enzyme The same quantity of product is formed But take longer to make the products Substrate and inhibitor are in direct competition The greater the proportion of substrate, the greater their chance of finding the active sites If the concentration of the substrate is increased, less inhibition 46 occurs


Non-Competitive InhibitorsNot attach to the active site but elsewhere on the enzyme molecule They alter the shape of the enzyme


Non-Competitive InhibitorsInhibitors and substrate are not competing for the same sites An increase in substrate concentration will not therefore reduce the effect of the inhibitor.



Non-Reversible InhibitorsInhibitors leave the enzyme permanently damaged Enzyme unable to carry out its catalytic function Heavy metal ions such as mercury (Hg 2+) and silver (Ag + ) cause disulphide bonds to break These bonds help to maintain the shape of the enzyme molecule Once broken the enzyme moleculeds structure becomes irreversibly altered with the permanent loss of 51 its catalytic properties.

Classification Of Enzymes Oxidoreductases Transferases Hydrolases Lyases Isomerases Ligases52

The Classification Of EnzymesEnzyme group1. Oxidoreductases 2. Transferases

Type of reaction catalysedTransfer of O and H atoms between substances, i.e. all oxidation-reduction reactions. Transfer of a chemical group from one substance to another Hydrolysis reactions

Enzyme examplesDehydrogenases Oxidases Transaminases Phosphorylases Peptidases Lipases Phosphatases53

3. Hydrolases

Enzyme group4. Lyases

Type of reaction catalysedAddition or removal of a chemical group other than by hydrolysis The rearrangement of groups within a molecule Formation of bonds between two molecules using energy derived from the breakdown of ATP

Enzyme examplesDecarboxylases

5. Isomerases

Isomerases Mutases Synthetases

6. Ligases


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