2.5 - Enzymes

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2.5 - Enzymes

Active sites and enzymes

Enzymes:

globular proteins that work as catalysts

speed up biochemical reactions

aren't altered themselves in reactions

found in all living cells

also secreted by some cells to work outside

the substances that enzymes turn into products are substrates

enzyme-substrate specificity: enzymes need to fit with the substrate

active site:

a region on the surface on the enzyme

where substrates bind

the shape and chemical properties of the enzyme and substrate match

this allows the substrate to bind, but not other substances

substrates are converted into products when bound to the active site

the products are then released,

freeing the active site to catalyze another reaction

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Enzyme activity

is the catalysis of a reaction by an enzyme.

three stages:

the substrate binds to the active site of the enzyme, sometimes 2 substrates

substrates change into different chemical substances, which are the products

products separate from the active site, leaving it free for substrates to bind

a substrate molecule can only bind to the active site if it moves very close to it

the coming together of a substrate and an active site is called collision

with most reactions, substrates are dissolved in water around the enzyme

particles dissolved in water are in contact with each other and in continuous motion

both substrates and enzymes with an active site are able to move

but most substrates are smaller than the enzyme so they move faster

collisions between substrate molecules and the active site occur because of random movements of both substrate and enzyme

successful collisions are ones where the substrate and active site are successfully aligned to allow binding to take place

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Factors affecting enzyme activity

temperature, pH and substrate concentration affect the rate of activity in enzymes

Temperature:

1st way

in liquids, the particles are in continual motion

higher temperatures increase the kinetic energy

so both enzyme and substrate move faster

therefore, enzyme activity increases

2nd way

when heated, enzyme bonds vibrate more

higher chance of the breaking of the bonds

when the bonds break, the structure is changed

this permanent change is called denaturation

the active site also changes, so no binding

as more enzymes are denatured, enzyme activity falls

pH

the lower the pH, the higher the hydrogen ion concentration

most enzymes have an optimum pH where their activity is highest

if the pH is increased or decreased, enzyme activity decreases then falls

because the structure of the enzyme is altered, including the active site

beyond a certain pH, the structure of the enzyme is irreversibly altered, (denaturation)

enzymes don't all have the same pH optimum

this reflects the wide range of pH environments in which enzymes work

for example, the protease secreted by Bacillus licheniformis has a pH optimum between 9 to 10. This bacterium is cultured to produce its alkaline-tolerant protease for use in biological laundry detergents, which are alkaline

Substrate concentration

1st way

more substrates, more frequent collisions

enzyme activity increases

2nd way

after binding, the active site is occupied and unavailable

more substrates, more occupied active sites

a greater proportion of substrate-active site collisions are blocked

the increases in the rate of reactions get smaller

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Denaturation

like other proteins, enzymes' structure can be irreversibly altered by certain conditions

can be caused by both high temperatures and either high or low pH

when an enzyme is denatured,

the active site is altered and a substrate cannot bind

f it binds, the reaction that the enzyme normally catalyzes doesn't occur

in many cases, denaturation causes enzymes that were dissolved in water to become insoluble and form a precipitate

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Immobilized enzymes

in 1897, the Buchner brothers showed that an extract of yeast, containing no yeast cells, would convert sucrose into alcohol

Louis Pasteur had claimed that fermentation of sugars to alcohol could only occur if living cells were present

part of the vitalism theory (stated that substances in animals and plants can only be made under the influence of a "vital spirit" or "vital force")

the artificial synthesis of urea disproved this theory

the Buchners' research provided a clearer falsification of the theory

more than 500 enzymes now have commercial uses

the enzymes used in industry are usually immobilized

this is the attachment of the enzymes to another material or into aggregations so that the movement of the enzyme is restricted

there are many ways of doing this, such as

attaching the enzymes into a glass surface

trapping them in an alginate gel

bonding them together to form enzyme aggregates

enzyme immobilization has several advantages:

the enzyme can be easily separated from the products of the reaction, stopping the reaction at the ideal time and preventing contamination of the products

after being retrieved from the enzyme mixture, the enzyme can be recycled, giving useful cost savings, especially as many enzymes are very expensive

immobilization increases the stability of enzymes to changes in temperature and pH, reducing the rate at which they are degraded and have to be replaced

substrates can be exposed to higher enzyme concentration than with dissolved enzymes, speeding up reaction rates

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FLASHCARDS: 2.5 - Enzymes