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Enzyme-controlled reaction Essay

Write an explanation of how any two factors affect the rate of an enzyme-controlled reaction. How do these factors affect the chemical structure and properties of the enzyme.

Many things can affect the rate of enzyme activity. The temperature of the enzyme, the pH of the solution, the concentration of the enzyme, substrate and the product. Also, another affector is the number of competitive and non-competitive inhibitors.

As I cannot explain them all, I have chosen to explain the effect of temperature and also the effect of inhibitors on enzyme activity.

Temperature :

As the temperature is increased the kinetic theory is applied. More energy is present and the molecules move fast. This increases the chance of collision between the enzymes and substrate and so increasing the rate of reaction. At these higher temperatures the active site of the enzyme is actually fairly flexible. The heat has broken a small number of Hydrogen bonds which hold the site to its shape. According to the lock and key theory the enzyme’s active site is specific to the substrate and so they fit together. The enzyme only catalases a single reaction. As the site is now flexible, there is an increased amount of induced fit and so the production of an enzyme-substrate complex is more likely. If the temperature is lower, the active site of the enzyme is much less flexible as there is the maximum number of bonds present holding it all together.

At the optimum temperature shown on the curve, the conditions are perfect for this enzyme to work in. Different enzymes have different optimums. For example, human enzymes work at body temperature whereas there are thermophilic/stable enzymes which are optimum at much higher temperatures such as 85C. Once the enzyme-substrate complex has been formed, the activation energy is lowered and the reaction is at its optimum. For mammalian enzymes, the rate of reaction doubles for every 10C change. Once the temperature goes beyond optimum, the rate of reaction lowers. At a certain point on this negative gradient, the enzyme becomes denatured. The tertiary structure is changed and the enzyme becomes permanently damaged.

Inhibitors:

There are two types of inhibitors to an enzyme. They are competitive and non-competitive. The competitive type are similar in shape to that of the substrate and so fit into the active site of the enzyme, not reacting, therefore inhibiting the production of an enzyme-substrate complex. An example of this is Malonate. It is similar in shape to that of succinate and competes for the active site of succinate dehydrogenase which is involved in the krebs cycle. Another example is that of Acetohydroxamic Acid which is similar in shape to urea. It competes for the active site of urease. Luckily, the effects are reversible. To counteract the effects of an inhibitor without being able to remove it itself one should increase the concentration of the substrate to increase the ratio of substrate to inhibitor.

The non-competitive inhibitor attaches itself somewhere else on the enzyme other than the active site and changes the shape of the active site thus preventing the substrate from fitting in. The effects of this can be either reversible or irreversible. Examples of non-competitive inhibitors are very well known as they have been used to harm humans or other lifeforms. These are things such as cyanide, heavy metals (which are reversible as they are loosely bound to the enzyme) and phosphate insecticides and nerve gasses (which are irreversible and cause death).

What do you understand by the following terms :

i)Enzyme-Substrate Complex

ii)Enzyme Specificity

iii)Prosthetic Group

i)I have mentioned this already in my earlier answers. This complex is what is produced when the substrate fits and binds to the active site of the enzyme. In this state, the conditions are favourable and the bonds are weak meaning the product can be easily formed.

ii)I have also mentioned this in my above answers. It simply means that the enzyme is specific to one substrate. The lock and key theory explains this where the enzyme is the lock and the key, being the substrate, fits only into that lock. The enzyme is totally specific and only catalases a single reaction.

iii)In the prosthetic group are non-protein molecules. They are covalently (strongly) bonded to the enzyme and must be there for the enzyme to function properly. If removed they will most probably cause the enzyme to denature. They are one type of an enzyme co-factor. Such examples of this are haemoglobin, chlorophyll and cytochromes.


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