Enzymes

There are a few things to remember about enzymes. First of all, all enzymes are proteins (just about - there are a few exceptions but you will not come across them in this class). Also, don't get confused and think that all proteins are enzymes, they aren't. You have already learned that the three dimensional structure of a protein affects it function, so it should not be surprise to you that those factors that denature proteins will reduce the activity of enzymes. These include changes in pH, temperature, and ionic strength.

The second thing to remember about enzymes is their function - they are catalysts. Catalysts act to reduce the amount of activation energy required for a reaction to occur. Many reactions that are thermodynamically possible do not occur very quickly (or seem not to occur at all) due to the requirement of energy to ‘get them started'. This required energy is called activation energy. For example, a very common reaction in your cells is the conversion of sugars such as glucose to carbon dioxide and water. The delta G for this reaction is very negative and so this reaction is exergonic and therefore a favorable one. However, sugars such as glucose are very stable and can be stored for years with no appreciable amount being converted to carbon dioxide and water. This is due to the activation energy required for this reaction. If you were to use a lit match to add energy to some glucose, you would be providing the activation energy required to start the reaction and glucose would quickly be converted to carbon dioxide and water. This is what happens when you use a match to light a piece of paper (remember, paper is made of cellulose which is a polymer made from glucose). The reduction in the amount of required activation energy allows for the reaction to occur more quickly (without raising the temperature) and thus increase the rate at which a reaction occurs. So the take home message is that enzymes increase the rate of a given reaction by reducing the need for activation energy.

Properties of Enzymes

There are several general properties of enzymes that you should remember when studying enzymes. Some of these are outline below:

In allosteric regulation, an enzyme has a second binding site that is not catalytic (it is not an active site). This second site, known as an allosteric site, is specific for a particular molecule. When this molecule binds to the allosteric site, the activity of the enzyme is altered.

In covalent modification, the enzyme structure is altered by the addition of another compound what is attached to the enzyme by a covalent bond. The most common modification entails the addition of a phosphate (PO4) group. Phosphate is negatively charged so the addition of such a group alters the structure of the enzyme and thus its activity. The addition of the phosphate group to the protein (known as phsophorylation) is regulated as well (and so gets very confusing - but that is for another course).

Try your had at a quiz on enzymes.

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