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CHAPTER 4
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Energy• Energy
– is capacity to do work
• Free energy– A Criterion for spontaneous change– Free energy is the portion of a system’s
energy that can perform work when temperature is uniform throughout the system.
• The change in free energy as a system goes from a starting state to a different stage is represented by ∆G (free energy change)
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EXERGONIC 1. Exergonic (energy-yielding)2. In exergonic reactions free energy is released3. The product have less energy than the
reactant4. Exothermic (heat – releasing)5. Spontaneously6. E.g., cellular respiration
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Endergonic1. Endergonic (energy-requiring)2. There is a net input of free energy3. The product contains more energy than was present
in the reactants4. Endothermic reactions (absorb heat)5. Non-Spontaneously6. E.g., protein synthesis, photosyntensis
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Activation energy
• a typical chemical reaction may be represented as :
A→ B + C• in this case A represents the
substrate and B and C are the products.
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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.
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– 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.
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• 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.
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ENZYMES
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Enzyme
1. All are globular protein2. Being protein, they are coded by
DNA
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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 properties of the end product of the reaction
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Factors affecting the rate of enzyme reactions
• Enzyme Concentration• Substrate Concentration• Temperature• pH
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Enzyme Concentration
• Rate of reaction is proportional to the enzyme concentration (pH and temperature kept constant)
• The rate of reaction increased by increasing an enzyme concentration
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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
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Substrate Concentration
1. 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.
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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
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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
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Optimum pH values for some enzymes
Enzyme Optimum pH
Pepsin 2.00
Sucrase 4.50
Enterokinase 5.50
Salivary Amylase 4.80
Catalase 7.60
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ENZYME ACTION MECHANISM
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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.
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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
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Mechanism of enzyme action
2 hypothesis
• Lock and Key Hypothesis– Proposed by Emil Fisher, 1890
• Induced Fit Hypothesis– Proposed by Daniel Koshland, 1959
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Lock and Key Hypothesis
• Active site of the enzyme is complementary to the structure of the substrate molecule.
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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
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Mechanism of enzyme action
• Enzymes are thought to operate on a lock and key mechanism
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• 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.
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• 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.
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• 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.
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• 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.
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• 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.
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COFACTOR
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COFACTORS
1. non protein substance2. essential for some enzymes to
function efficiently.3. may be bound tightly to the active
site as permanent residents4. or they may bind loosely and
reversibly along with the substrate.
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Types of cofactors
Three types of cofactors1. Activators2. Coenzymes3. Prosthetic group
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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.
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• Salivary amylase – starch 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.
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Coenzymes
• non-protein organic substances which are essential functioning of some enzymes, but are not themselves bound to the enzyme.
• many coenzymes are derived from vitamins.
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• 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
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Prosthetic groups
• organic molecules• bound to the enzyme• example: haem.
– a ring- shaped organic molecule with iron at its center
– an oxygen carrier in haemoglobin.
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Function of Cofactors
• Work 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.
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Inhibition
reversibleand
irreversible
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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 inhibitors
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Competitive Inhibitors• It 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
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Competitive Inhibitors
• The 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 occurs
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Non-Competitive Inhibitors
• Not attach to the active site but elsewhere on the enzyme molecule
• They alter the shape of the enzyme
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Non-Competitive Inhibitors
• Inhibitors and substrate are not competing for the same sites
• An increase in substrate concentration will not therefore reduce the effect of the inhibitor.
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Non-Reversible Inhibitors
• Inhibitors 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 molecules structure becomes irreversibly altered with the permanent loss of its catalytic properties.
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Classification Of Enzymes
• Oxidoreductases • Transferases • Hydrolases • Lyases • Isomerases • Ligases
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The Classification Of Enzymes
Enzyme group
Type of reaction catalysed
Enzyme examples
1. Oxidoreductases
Transfer of O and H atoms between substances, i.e. all oxidation-reduction reactions.
Dehydrogenases
Oxidases
2. Transferases Transfer of a chemical group from one substance to another
Transaminases
Phosphorylases
3. Hydrolases Hydrolysis reactions Peptidases
Lipases
Phosphatases
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Enzyme group
Type of reaction catalysed
Enzyme examples
4. Lyases Addition or removal of a chemical group other than by hydrolysis
Decarboxylases
5. Isomerases The rearrangement of groups within a molecule
Isomerases
Mutases
6. Ligases Formation of bonds between two molecules using energy derived from the breakdown of ATP
Synthetases
