3 ENZYMES

LO

• explain that enzymes are globular proteins that catalyse metabolic reactions;

• explain the mode of action of enzymes in terms of an active site, enzyme-substrate complex, lowering of activation energy and enzyme specificity

ENZYMES

• Protein molecules; globular proteins

• Biological catalysts – speed up chemical reactions but remains unchanged at the end of the reaction

• Name ends with –ase

• Coiled into precise three dimensional shape with hydrophilic groups on the outside of the molecule = soluble

Active site

• Region to which another molecules (substrate) can bind

• Lock and key hypothesis

– Substrate is the key whose shape fits the lock of the enzyme

– The substrate is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme’s amino acids = enzyme-substrate complex

• Induced fit hypothesis

– Same as lock and key but adds the idea that:

– Substrate and enzyme can change shape slightly as the substrate molecule enters the enzyme in order to ensure a perfect fit = more efficient catalysis

3

• Each type of enzyme will act on only one type of substrate molecule = enzymes are specific

• Enzymes can catalyse the joining of two substrates (synthesis) or braking down the substrate (lysis) = products

• Enzyme remains unchanged

• Rate can be very high

• Lyzozyme

– Enzyme in saliva, tears and other secretions

– Natural defence against bacteria

– Breaks the polysaccharide chains that form the cell walls of bacteria

Activation energy

• Extra energy given to the substrate so that the reaction can happen

• Energy of the reactant can be increased by heating (benedict’s test)

• Enzymes decrease the activation energy to the reaction

Factors effecting the enzyme activity

• The effect of enzyme concentration

– The more enzyme present, the more active sites will be available for the substrate to slot into

– As long as there is plenty of substrate available, the initial rate of a reaction increases linearly with enzyme concentration

– P. 59. fig. 3.6

• The effect of substrate concentration

– As substrate concentration increases, the initial rate of reaction also increases

– The more molecules are around the more often an enzyme’s active site can bind with one

– If the enzyme reaches its maximum possible rate = Vmax (maximum velocity)

• The effect of temperature

– Low temperatures – reaction takes place very slowly as the molecules move relatively slowly

– As temperature increases – molecules move faster and therefore collide with the active site of an enzyme more frequently; more energy is involved = easier for bonds to be formed or broken

– At some point – bonds within the enzyme break and enzyme looses its shape and activity = it is DENATURATED

– Optimum temperature = temperature at which an enzyme catalyses a reaction at the maximum rate (mostly 40 degrees)

• The effect of pH

– Most enzymes work fastest at a pH of around 7; some work better in acidic conditions (pepsin)

– Too different pH can cause denaturation

– The lower the pH the higher the hydrogen ion concentration – hydrogen ions can interact with the R-groups of amino acids – this affects the ionic bonding between the groups which affects the 3D arrangement of the enzyme

Enzyme inhibitors

• If a different molecule (very similar in shape to the enzyme’s substrate) binds with the active site of an enzyme – this inhibits the enzyme’s function = this molecule is called INHIBITOR

• Competitive inhibition

– If an inhibitor molecule binds only briefly to the site = competition between it an the substrate for the site

– If there is much more substrate than the inhibitor – substrate molecules can easily bind to the active site in the usual way = no effect on the enzyme’s function

– If the concentration of inhibitor is rises = less likely that the substrate will collide with an empty site

– This process is reversible

– Example:

– ethylene glycol – used as antifreeze; if drunk by somebody, it is rapidly converted to oxalic acid which causes irreversible kidney damage

– The active site of the enzyme will accept ethanol

– If the person is given ethanol – it acts as a competitive inhibitor = results in slowing down the reaction for long enough to allow the ethylene glycol to be excreted

• Non-competitive inhibition

– Inhibitor can remain permanently bonded with the active site and therefore cause an irreversible block to the substrate

– No competition occurs

– Example:

– Penicillin – permanently occupies the active site of an enzyme that is essential for the synthesis of bacterial cell wall

• Second type of non-competitive inhibition

– Molecule binds to another part of the enzyme (not the active site) – this disrupts the normal arrangement of hydrogen bonds and hydrophobic interactions holding the enzyme molecule in its 3D shape = results in unsuitability of the enzyme for the substrate; the enzyme is blocked

– This inhibition can be reversible

– Example:

– Digitalis – binds with ATPase = increase in the contraction of heart muscle

Inhibition of enzyme function can be lethal, but in many situations inhibition is essential!

• End product inhibition– Using end product of a chain of reactions as an

enzyme inhibitor

– As the enzyme converts substrate to product, it is slowed down because the end-product binds to another part of the enzyme and prevents more substrate binding

– As product levels fall, the enzyme is able to top them up again

– non-competitive reversible inhibition