Many species of fungi,

such as the pin mold grow­ing on a carrot, obtain nutri­ents by secreting digestive enzymes into the foodstuff in which they are growing. The enzymes break down the large foodstuff mole­cules into smaller mole­cules. These are then ab­sorbed by the fungus. In a similar way, human diges­tive enzymes break down food into small molecules. These can then be absorbed through the wall of the ali­mentary canal.

There are several theories to explain why enzymes are specific and how they operate. They all center on the enzymes' three-dimen­sional structure. The simplest theory is the "Iock-and-key" hypothesis. It postulates that a substrate molecule (the "key") attaches itself to an active site (the "lock") of an enzyme mole­cule, forming a temporary complex. The active site has a particular shape. So only a substrate with the complementary shape can attach it­self to this site. In the same way, a lock only ac­cepts a key with the right shape. Hence, mole­cules with different shapes cannot attach to the active site. Also, if the active site is dis­torted by excessive heat, the molecule itself will no longer fit.

However, some other compounds may be close enough in shape to the substrate to fit into the active site of the enzyme. These alien substrates are not changed through contact with the enzyme. Rather, they compete with the true substrate for active sites. This inhibits the enzyme's activity.

There are two types of inhibition, in com­petitive inhibition, an alien molecule forms only a temporary bond with an enzyme. In noncompetitive inhibition, an alien inhibitor molecule either permanently blocks an en­zyme's active site or affects it by temporarily binding to a site elsewhere on the enzyme. Many inhibitors, particularly of the noncom­petitive type, act as poisons. Cyanide, which binds with an enzyme necessary for cellular respiration, is a good example. Cellular respi­ration is the process by which cells get oxy­gen.

More complicated theories of enzyme ac­tion hold that an enzyme exists in two separate shapes. A nonactive enzyme would then have one shape. When the same enzyme is active, it would have another shape. Some enzymes do not work without the presence of what are called cofactors. Some cofactors participate in the enzyme reaction. Others probably lock into the enzyme away from the active site. They hold the enzyme in the correct position

to receive the substrate.

Enzyme systems

Most enzymes work as part of a chain of reac­tions during metabolism. The product of one enzyme-induced reaction then becomes the substrate for another. To prevent the wasteful production of unnecessary amounts of a par­ticular substance, the whole reaction se­quence is controlled by the slowest step.

Enzymes may consist entirely of protein. Or they may consist of protein linked to a group that helps to maintain the shape of the mole­cule. The group may also participate in the re­action. Sometimes, such a group fulfills both these functions. Trace metal minerals (such as iron and cobalt) are often necessary in the diet. They are used by these groups. Water-soluble vitamins are often important because they are cofactors in an enzyme system.

Sexual reproductionin

many animals depends on enzymes. When the head of a sperm hits the outer mem­brane of an egg, a small sac of special enzymes at the front of the sperm's head breaks open. The released enzymes break down the egg's membrane. This al­lows the sperm to penetrate and fertilize the egg—as shown in the scanning elec­tron micrograph /above!. After the egg has been ferti­lized, its outer membrane undergoes little-understood changes. These changes make the egg resistant to the enzymes of subsequent sperm. This prevents multi­ple fertilization.

DNAis a giant nucleic acid molecule consisting of two nucleotide chains twisted into a double helix (above). The sequence of reactions by which it is formed is shown in the diagram fright). The first stage is the synthesis of individual nu­cleotides by a condensation reaction between phos­phoric acid, the five-carbon sugar deoxyribose, and an organic base. These bases may be thymine, adenine, guanine, or cytosine. Thy­mine is the example used in the diagram. Water is ex­pelled as a by-product. Many nucleotides then link by further condensation re­actions to form a nucleotide chain. Finally, two nucleo­tide chains link by forming hydrogen bonds (dotted blue lines in the diagram) between their organic bases. This linking produces the twisted, ladderlike DNA molecule.

Nucleotide of DNA fortned by condensation reaction of phosphoric] acid, deoxyribose and organic basej(thyrpine)

■ ■" i

Nucleotides link by condensation reactions to form a chain. Two chaifls then link by hydrogen bonds between the organic) bases to form the double helix of DNA H

Nucleic acids

Nucleic acids are the means by which informa­tion about the structure and function of a liv­ing organism is stored and passed on to the next generation. Nucleic acids consist of only two types of molecules, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are found in the cells of all living organisms, from viruses to humans.

Date: 2015-12-11; view: 1553