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Of all the functions of proteins, one of the most important is that of catalysis In the absence of catalysis, most reactions in biological systems would take place far too slowly to provide products at an adequate pace for metabolising organisms The catalysts that serve this function in living organisms are called ENZYMES All enzymes are globular proteins and are the most efficient catalysts known Enzymes are able to increase the rate of reaction by a factor of up to over uncatalysed reactions

They are proteins of high molecular weight They are biological catalysts They are sensitive to temperature changes, being denatured at high temperatures They are sensitive to pH They are generally specific in the reactions they catalyse Enzymes possess an active site within which chemical reactions take place Substrate molecule in the ACTIVE SITE Enzyme molecule

Substrate molecules (complementary shape to active site) Product molecules diffuse away from the active site Substrate molecules bind with enzyme molecules at the active site as a consequence of their complementary shapes. This is the basis of the LOCK AND KEY MODEL of enzyme activity Enzyme remains unchanged Enzyme molecule Active site Reaction occurs

In an enzyme-catalysed reaction, the enzyme binds to the substrate to form a complex An enzyme-substrate complex forms Products diffuse away from the active site Enzyme molecule The lock & key model proposes that the substrate binds to the active site which it fits exactly, like a key in a lock S S A reaction occurs forming an enzyme-product complex

This model takes into account the fact that proteins (enzymes) have some three-dimensional flexibility SUBSTRATE Substrate binds to the enzyme at the active site Binding of the substrate induces the enzyme to change shape such that there is an exact fit once the substrate has bound Enzyme Molecule According to this model, reactions can only take place AFTER induced fit has occurred

Energy barrier with enzyme Energy barrier without enzyme Energy level of substrate Energy level of the products Enzymes are catalysts because they lower the ACTIVATION ENERGY needed to drive a reaction Substrates need to overcome an energy barrier before they will convert to products Lower activation energy

Temperature pH Substrate Concentration Enzyme Concentration Inhibitors Activators

As the temperature increases, molecular motion and thus molecular collisions increase More product molecules are formed in a given time and hence the reaction rate increases For many enzymes, the maximum rate of reaction is reached at a temperature between 37 ° C to 40 ° C This is the optimum temperature The reaction rate doubles for every 10 ° C rise in temperature As the temperature increases beyond the optimum, bonds that stabilise the enzyme’s tertiary structure are broken The enzyme loses its shapes and the active site is altered Substrate can no longer bind to the enzyme The enzyme has been DENATURED

Each specific enzyme can only work over a particular range of pH Each enzyme has its own optimum pH where the rate of reaction is maximum The effects of pH on the rate of enzyme controlled reactions display characteristically bell shaped curves ABC Enzyme A = amylase optimum pH = 7.2 Enzyme B = pepsin optimum pH = 2.0 Enzyme C = lipase optimum pH = 9.0 Changes in pH can affect the ionic and hydrogen bonds responsible for the specific tertiary shape of enzymes Extremes of pH break these bonds and denature the enzyme

Low Substrate Concentration Low product concentration per unit time Increased Substrate Concentration More product formation; increased reaction rate

Further increase in substrate concentration Excess substrate concentration Maximum product formation; maximum rate of reaction No further increase in product formation; maximum reaction rate maintained Enzyme concentration is the LIMITING FACTOR

Increasing concentration of substrateRate of reaction A Rate of reaction increases as the substrate concentration increases Rate of reaction reaches a maximum at substrate concentration A No further increase in the reaction rate despite the increasing substrate concentration All the active sites of the enzymes are occupied - enzyme concentration is the limiting factor

Rate of reaction Increasing concentration of enzyme The rate of reaction is directly proportional to the enzyme concentration As enzyme concentration increases, the rate of reaction increases In living cells, enzyme concentrations are usually much lower than substrate concentrations Substrate concentration is rarely a limiting factor

When the substrate concentration is low, the inhibitor competes successfully for the active site; fewer substrate molecules are converted into product and the rate of reaction is reduced Low substrate concentration Inhibitor molecule

The effect of the competitive inhibitor is overcome when the high concentration of substrate molecules compete successfully for the active sites of the enzymes; at high substrate concentration, maximum reaction rate is achieved High substrate concentration Inhibitor molecule

maximum rate At low substrate concentrations, the rate of reaction is reduced in the presence of the inhibitor without inhibitor present The effect of the inhibitor is overcome by very high substrate concentrations At high substrate concentrations, the inhibitor is out-competed by the substrate and the maximum rate of reaction is achieved

Substrate binds to the enzyme when a non-competitive inhibitor is present but cannot be converted to product; the rate of reaction is reduced Low substrate concentration Inhibitor molecule Substrate molecules not converted to product when inhibitor molecules are bound to the enzyme Substrate molecules converted into product when no inhibitor is attached to the enzyme

High substrate concentration Inhibitor molecule Substrate molecules converted into product when no inhibitor is attached to the enzyme At high substrate concentration all enzyme active sites are occupied Substrate molecules bound to enzymes with attached inhibitor are NOT converted into product - maximum reaction rates are never achieved The effect of the inhibitor is not overcome by increasing the substrate concentration. All the enzyme molecules with bound non-competitive inhibitor do NOT convert substrate to product; the effect is equivalent to lowering enzyme concentration X X X X X X X X X X X X

Non-competitive inhibitors act by preventing bound substrate being converted into product with inhibitor; maximum reaction rate never achieved - the effect of the inhibitor cannot be overcome by increasing the substrate concentration no inhibitor; maximum reaction rate achieved at high substrate concentration