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Enzyme
Enzymes are globular proteins. Like all globular proteins, enzyme molecules are coiled
into a precise three – dimensional shape, with hydrophilic R groups (side – chains) on
the outside of the molecule ensuring that they are soluble.
An enzyme is a protein that acts as a biological catalyst – that is, it speeds up a metabolic
reaction without itself being permanently charged.
The substance present at the start of an enzyme – catalysed reaction is called the
substrate. The product is the new substance or substances formed.
Explain the mode of action of enzymes in terms of an active site,
enzyme/substrate complex, lowering of activation energy and enzyme
specificity;
The active site of an enzyme is a region, usually a cleft or depression, to which
another molecule or molecules can bind. The shape of the active sit allows the
substrate to fit perfectly. The idea that the enzyme has a particular shape into which
the substrate fit exactly is known as the lock and key hypothesis. The substrate is
the key whose 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. This combined structure is termed the enzyme
- substrate complex.
Each type of enzyme will usually act on only one type of substrate molecule. This is
because the shape of the active site will only allow one shape of the molecule to fit.
The enzyme is said to be specific for this substrate.
The modern hypothesis for enzyme action is known as the induced fir hypothesis. It
is basically the same as the lock and key hypothesis, but adds the idea that the
enzyme, and sometimes the substrate, can change shape slightly as the substrate
molecule enters the enzyme, in order to ensure a perfect fit. This makes the catalysis
even more efficient. An enzyme may catalyse a reaction in which the substrate
molecule is split into two or more molecules. Alternatively, it may catalyse the joining
together of two molecules, as when making a dipeptide. Interactions between the R
groups of the enzyme and the atoms of the substrate can break, or encourage
formation of, bonds in the substrate molecule, forming one, two or more products.
When the reaction is complete, the product leave the active site. The enzyme is
unchanged by this process, so it is now available to receive another substrate
molecule. The rate at which substrate molecules can bind to the enzyme’s active
site, be formed into products and leave can be very rapid.
Activation energy
Substrates generally need to be supplied with energy to cause them to change into
products. The energy required to do this is called activation energy. In a laboratory,
you might supply energy by heating to cause two substances to react together.
Enzymes are able to make substances react even at low temperatures. They reduce
the activation energy needed to make the reaction take place. They do this by
distorting the shape of the substrate molecule when it binds at the enzyme’s active
site.
Follow the progress of an enzyme-catalysed reaction by measuring
rates of formation of products (for example, using catalase) or rates of
disappearance of substrate (for example, using amylase);
Investigate and explain the effects of temperature, pH, enzyme
concentration and substrate concentration on the rate of enzymecatalysed reactions;
Temperature
At low temperatures, enzyme and substrate molecules have little kinetic energy.
They move slowly, and so collide infrequently. This means that the rate of reaction is
low. If the temperature is increased, then the kinetic energy of the molecules
increases. Collision frequency therefore increases, causing an increase in the rate of
reaction.
Above a certain temperature, however, hydrogen bonds holding the enzyme
molecule in shape begin to break. This causes the tertiary structure of the enzyme to
change, an effect called denaturation. This affects the shape of its active site. It
becomes less likely that the substrate molecule will be able to bind with the enzyme,
and the rate of reaction slows down.
The temperature, at which an enzyme works most rapidly, just below, that at which
denaturation begins, is called its optimum temperature. Enzymes in the human body
generally have an optimum temperature of about 37cel, but enzymes from
organisms that have evolved to live in much higher or lower temperature may have
much higher it lower optimum temperatures.
pH
pH affects ionic bonds that hold protein molecules in shape. Because enzymes are
proteins, their molecules are affected by changes in pH. Most enzymes molecules
only maintain their correct tertiary structure within a very narrow pH range, generally
around pH 7. Some, however, require a very pH; one example is the protein –
digesting enzyme pepsin found in the human stomach, which has an optimum pH of
2.
Enzyme Concentration
The greater the concentration of enzyme, the more frequent the collisions between
enzyme and substrate, and therefore the faster the rate of the reaction. However, at
very high enzyme concentrations, the concentration of substrate may become a
limiting factor, so the rate does not continue to increase if the enzyme concentration
is increased.
Substrate concentration
The greater the concentration of substrate, the more frequent the collisions between
enzyme and substrate, and therefore the faster the rate of the reaction. However, at
high substrate concentrations, the concentration of enzyme may become a limiting
factor, so the rate does not continue to increase if the substrate concentration is
increased.
Explain the effects of competitive and non-competitive inhibitors on
the rate of enzyme activity;
An inhibitor is a substance that slow done the rate at which an enzyme works.