Enzymes


Mechanisms of Enzyme action

Enzymes are essential proteins that are found in all living cells. They have two main functions:

1) they act as catalysts ( a catalyst is a chemical which speeds up a chemical reaction )
2) they control all the chemical reactions going on inside the cell.

Every cell has many different enzymes. Each enzyme is specific : it only affects one particular chemical. This chemical that the enzyme alters is called the substrate. An enzyme can either break apart one substrate into two different products, or it can join two substrates into a single product. One example is shown below :

			Sucrase
	Sucrose   ----------------->     Glucose    +  Fructose
		
	Substrate	Enzyme		 Product       Product

The reason that enzymes are specific is that each enzyme has an area called the "active site" that fits around the substrate. You can imagine the enzyme fitting the substrate like a key fitting a lock. In this diagram, the "functional shape" is the active site.
One enzyme will therefore carry out one particular chemical reaction : for example lyases add or remove double bonds in molecules.

Before a chemical reaction can occur, energy has to be added. This energy is needed to activate, or start, the reaction. For example, gasoline burns very well in air, but if you just expose gasoline to the air nothing happens - you have to put a match to it to start the chemical reaction ( burning ). Similarly in a chemistry class, you often mix chemicals together, and then have to heat them to actually get them to react. Increasing the temperature is a common way to overcome the barrier of activation energy; another way is by increasing the pressure. In both cases the enzyme and substrate come together more quickly.

Typically, every time the temperature rises by 10o C, the rate of an enzyme controlled reaction will double. This holds true only if the temperature remains below about 50o C. Unlike an inorganic catalyst (such as the catalytic converter in a car exhaust ) which works well at very high temperatures, enzymes are held together by fragile hydrogen bonds, and so enzymes denature above about 50o C. Normal human body temperature of 37o C provides a good internal environment for enzymes to work efficiently.

There are a few exceptions to this rule that enzymes work best at higher temperatures : for example fish that live in arctic or antarctic waters often have enzymes that work very well even at -2o C, below the freezing point of freshwater.

One example is the antarctic fish Trematomus which is shown here. Like many antarctic fish, this one has large, bulging eyes to collect as much light as possible from the dim sea underneath the ice. The enzymes from these fish are so well adapted to cold environments that they fall apart (and the fish dies) if the temperature reaches only 5o C ( 41 o F ). As well as having enzymes that are adapted to the cold, these fish also have special glycoproteins that act as an antifreeze in their blood. This natural antifreeze is 300 times more effective than the regular antifreeze in your car, although exactly how it works is still a bit of a mystery. More information on antarctic fish is available at : http://tea.rice.edu/atwood/12.2.1998.html


Another environmetal factor that is important in controlling the rate of an enzyme controlled reaction is the pH. Acids have a pH of less than 7, bases (alkalis) have a pH greater than 7. Enzymes in the stomach, such as pepsin ( which digests protein ), work best in very acid conditions ( pH 1 - 2 ), but most enzymes in the body work best close to pH 7. Incidentally, the stomach lining is not digested by pepsin because it is protected by a layer of mucus.

When a plant or animal dies, the body is usually decomposed by bacteria or fungi. If however conditions prevent enzymes in the bacteria from working, the body will be preserved.

This photo ( Tollund man) shows a body that was discovered in Demark in a peat bog in 1950. The person had been strangled, and at first the police thought it was a recent murder. But peat bogs are very acid, and it turned out that the body was 2,000 years old, and had been very well preserved in the peat. Archaeologists believe the body is from a ritual murder, but they are not sure if the person was killed as a punishment, or whether the body was a sacrifice to the gods.
For more details of how bogs preserve bodies and other archaeological evidence, see Jenny Dente's site below:
University of Texas

Similar bodies from hundreds, or even thousands of years ago have been found in the Alps, and in the Andes, where the cold temperatures have prevented deterioration.

Inhibitors are chemicals that slow down the action of enzymes. This inhibition can be of two types:

A) Competitive inhibitors are similar to the true substrate, and actually fit into part of the active site

B) Noncompetitive inhibitors are chemically different from the substrate, and fit onto a different part of the enzyme.

The diagram below shows an example of an enzyme, substrate, and both types of inhibitor. Competitive inhibitors are affected by substrate concentration - if there is a lot of substrate, the active site of the enzyme will usually have a substrate in it, so the inhibitor cannot attach there. However non-competitive inhibitors attach to a different part of the enzyme, so the amount of substrate present has no effect on these inhibitors. This diagram is from Western Michigan University, which has more detailed information about inhibitors (see below ).
Enzyme inhibitors are important commercially in many ways. For example pesticides kill bugs by inhibiting essential enzymes that are present in insects ( these enzymes are not found in humans). Similarly many medications, such as aspirin and antibiotics are inhibitors. The enzyme substilin digests proteins, and is used in laundry detergent. Rennin, an enzyme extracted from calves, is used in curdling milk to make cheese. Glucose oxidase detects glucose in the urine ( for example in diabetics).

One final chemical that can affect enzymes is a coenzyme. Coenzymes help enzymes to function. Some coenzymes bond covalently to the enzyme, to form a "prosthetic group". Many coenzymes are vitamins, and generally cells need only small amounts of coenzyme. A typical diet gives you all the vitamins you need, so there is little or no benefit from taking additional vitamin pills. The main exception to this is the vitamin folate, or folic acid, which is mainly found in dark green vegetables like spinach or collard greens. Not surprizingly, this is often deficient in the diet, and so in January 1999 the US government required companies making basic products like flour to add folate to the flour. So now when you eat bread, or pizza, or other common foods you are getting the folate your body needs. So pizza really is health food!

This site : http://www.wmich.edu/bios150/enzymes.html (Western Michigan University) has more information about enzymes

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