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Enzymes????? ??????????? ??????????? The

majority of the reactions that occur in living organisms are enzyme-controlled.

Without them, the rate of the reactions would be so slow as to cause serious,

if not fatal, damage. Without enzymes toxins would soon build up and the supply

of respiratory substrate would decrease.?

Enzymes are proteins and thus

have a specific shape. They are therefore specific in the reactions that they

catalyse ? one enzyme will react with molecules of one substrate.?? The site of the

reaction occurs in an area on the surface of the protein called the active site. Since the active site for

all molecules of one enzyme will be made up of the same arrangement of amino

acids, it has a highly specific shape.??

Generally, there is only one active site on each enzyme molecule and

only one type of substrate molecule will fit into it.? Chymotrypsin and trypsin both catalyse the hydrolysis of peptide

bonds but due to their shapes, the active site of chymotrypsin only splits

bonds after an aromatic amino acid (one containing a ring of atoms) whereas

trypsin only splits bonds after a basic or straight chain amino acid. This

specificity leads to the lock and key

hypothesis. However, it has been discovered that competitors for an active

site (similar in shape to the substrate) could fit even though they are larger

than the substrate. This means that the substrate and active site are a little

flexible.? This has lead to the induced fit model.????? Induced fit model????????? ??????????? ??????????? ??????????? ??????????? ???? When the enzyme and substrate form a

complex, structural changes occur so that the active site fits precisely around

the substrate (the substrate induces the active site to change shape).? The reaction will take place and the

product, being a different shape to the substrate, moves away from the active site.

The active site then returns to its original shape.?????? Enzyme controlled

reactions????? ??????????? ??????????? ??????????? ??????????? Reactions

proceed because the products have less energy than the substrates.? However, most substrates require an input of

energy to get the reaction going, (the reaction is not spontaneous).? The energy required to initiate the reaction

is called the activation energy.? When the substrate(s) react, they need to

form a complex called the transition state before the reaction actually occurs.

This transition state has a higher energy level than either the substrates or

the product.?? Outside the body, high

temperatures often supply the energy required for a reaction. This clearly

would be hazardous inside the body though!! Fortunately we have enzymes that

provide an alternative way with a different transition state and lower

activation energy.????? ??The

rate of the reaction without any external means of providing the activation

energy continues at a much faster rate with an appropriate enzyme than without

it. The maximum rate that any reaction can proceed at will depend, among other

things, upon the number of enzyme molecules and therefore the number of active

sits available.??????????? Factors affecting the

rate of reaction????? ??????????? ??????????? ??????????? ??????????? Temperature ? enzymes work best at an optimum temperature.?? Below this, an increase in temperature

provides more kinetic energy to the molecules involved. The numbers of

collisions between enzyme and substrate will increase so the rate will too.?? Above the optimum temperature, and the

enzymes are denatured. Bonds holding the structure together will be broken and

the active site loses its shape and will no longer work.? pH

? as with temperature, enzymes have an optimum pH. If the pH changes much from

the optimum, the chemical nature of the amino acids can change.?? This may result in a change in the bonds

and so the tertiary structure may break down. The active site will be disrupted

and the enzyme will be denatured.? Enzyme concentration ? at low enzyme

concentration there is great competition for the active sites and the rate of

reaction is low. As the enzyme concentration increases, there are more active

sites and the reaction can proceed at a faster rate.? Eventually, increasing the enzyme concentration beyond a certain

point has no effect because the substrate concentration becomes the limiting

factor.? Substrate concentration ? at a low substrate concentration there

are many active sites that are not occupied. This means that the reaction rate

is low.?? When more substrate molecules

are added, more enzyme-substrate complexes can be formed. As there are more

active sites, and the rate of reaction increases.? Eventually, increasing the substrate concentration yet further

will have no effect. The active sites will be saturated so no more

enzyme-substrate complexes can be formed.?

?????????????? Cofactors???????? ??????????? ??????????? ??????????? ??????????? Most

enzymes require additional help from cofactors, of which there are 2 main

types;? Coenzymes ? these are organic compounds, often containing a vitamin

molecule as part of their structure.??

Coenzymes are not permanently bound to the enzyme but may be temporarily

and loosely bound for the duration of the reaction and then move away once it

is completed. For example NAD, which transfers hydrogen away from one molecule

in a dehydrogenase reaction and takes it to another molecule. (see respiration QuickLearn) Metal ions ? most speed up the formation of the enzyme-substrate

complex by altering the charge in the active site e.g. amylase requires

chloride ions, catalase requires iron.????????? Inhibitors????????? ??????????? ??????????? ??????????? ??????????? Inhibitors

slow down the rate of a reaction. Sometimes this is a necessary way of making

sure that the reaction does not proceed too fast, at other times, it is

undesirable.? Reversible Inhibitors Competitive reversible inhibitors ? these molecules have a similar structure to the actual

substrate and so will bind temporarily with the active site. The rate of

reaction will be closer to the maximum when there is more ?real? substrate,

(e.g. arabinose competes with glucose for the active sites on glucose oxidase

enzyme). Non-competitive reversible

inhibitors ? these molecules are not necessarily anything like the

substrate in shape. They bind with the enzyme, but not at the active site. This binding does change the shape of the

enzyme though, so the reaction rate decreases Irreversible Inhibitors These molecules bind permanently with the

enzyme molecule and so effectively reduce the enzyme concentration, thus

limiting the rate of reaction, for example, cyanide irreversibly inhibits the

enzyme cytochrome oxidase found in the electron transport chain used in

respiration. If this cannot be used, death will occur.???????

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