Enzymes

Enzymes are a special group of complex proteins found in all living organisms. Most cells contain hundreds of enzymes and are continuously synthesizing enzymes. Enzymes act as catalysts of biochemical reactions. Enzymes accelerate the rate reactions as much as 1,000,000 times the rate of uncatalyzed reactions and permit the occurrence of biochemical reactions at temperatures that living cells can tolerate.

Some enzymes contain nonprotein groups or cofactors. Cofactors include coenzymes or organic molecules such as biotin (Figure 7.1) and metallic activators or inorganic ions such as cobalt (Co2+), copper (Cu2+), magnesium (Mg2+), potassium (K+), and zinc (Zn2+). Cofactors improve the efficiency and rate of enzymatic activity. Because enzymes are not consumed and do not undergo structural damage during biochemical reactions, they can be used repeatedly.

Enzymes provide a surface or active site on which biochemical reactions can occur. The active site is a binding site; that is, it forms a weak bond with its substrates, the molecules that the enzyme acts upon. However, enzymes are specific with respect to the substrates that they can degrade or compounds they can synthesize.

When a substrate comes in contact with an appropriate site of an enzyme, an enzyme-substrate complex is formed. Once the complex is formed, chemical bonds in the substrate are weakened, and the substrate may be degraded (catabolism) to simpler molecules (Figure 7.2) or assimilated (anabolism) to more complex molecules (Figure 7.3). Catabolic reactions result in a decrease in sludge production— for example, the degradation of stored food in bacterial cells. Anabolic reactions result in an increase in sludge production—for example, the transformation and assimilation of sugars into new bacterial cells or sludge.

Enzymes usually have a high degree of specificity; that is, they are very specific with respect to (a) the substrates that they can degrade and (b) the compounds and

FIGURE 7.1 Biotin. Biotin is a water soluble, organic vitamin B complex that is required for bacterial growth.

Enzyme

(a)

t tU

Substrate A

Enzyme

FIGURE 7.2 Catabolism. During catabolism, a large substrate molecule (A) bonds to an enzyme (enzyme-substrate complex) according to the shapes and charges of the substrate and enzyme (b). Once bonded, the enzyme degrades or breaks the substrate (c). Catabolism results in the production of smaller substrates and the release of energy.

Product B

FIGURE 7.2 Catabolism. During catabolism, a large substrate molecule (A) bonds to an enzyme (enzyme-substrate complex) according to the shapes and charges of the substrate and enzyme (b). Once bonded, the enzyme degrades or breaks the substrate (c). Catabolism results in the production of smaller substrates and the release of energy.

(a)
(b)

Enzyme

New product

FIGURE 7.3 Anabolism. During anabolism, small substrate molecules (A and B) bond to an enzyme (enzyme-substrate complex) according to the shapes and charges of the substrates and enzyme (b). Once bonded, the enzyme joins the substrates together to form a larger substrate. Anabolism results in the production of new products such as those used in the synthesis of cellular material.

ions that they can assimilate. For example, some enzymes can degrade a large number of carbohydrates, while other enzymes can degrade only a small number of carbohydrates or a specific carbohydrate.

The specificity of the biochemical reactions that an enzyme may catalyze is due to the shape and electrical charge of the active site of the enzyme. The shape of an enzyme is produced through the bonding of thiol groups (—SH), while its electrical charge is due to the ionization of hydrogen bonds. If an enzyme is capable of acting on more than one substrate, it usually acts on substrates with the same functional group [e.g., carboxyl (—COOH) or hydroxyl (—OH)], or the same kind of chemical bond (Table 7.1). For example, proteolytic (protein-splitting) enzymes break peptide bonds in proteins. Enzymes are usually named by adding the suffix

TABLE 7.1 Example of Enzymes

Enzyme

Function

Hydrolase

Isomerase

Ligase

Lipase

Lyase

Oxidoreductase Peptidase Phosphatase Sucrase

Adds water and breaks large molecules into smaller molecules Rearranges atoms on a molecule Joins two molecules together Degrades lipids

Removes chemical group from a molecule without adding water Oxidizes one molecule while its reduces another molecule Degrades peptide bonds

Transfers phosphate group from molecule to another Degrades the sugar sucrose

"-ase" to the name of the substrate that they act upon. Hydrolases are very important in the degradation of particulate BOD and colloidal BOD that enter wastewater treatment plants. These enzymes permit the solubilization of complex molecules to simplistic molecules that can be absorbed by bacterial cells where they are degraded.

Although all enzymes are produced intracellularly, enzymes are placed into two groups depending on where they perform their biochemical reactions. Endoenzymes (intracellular enzymes) act within the cell. Exoenzymes (extracellular enzymes) cross the cell membrane to act in the cell's immediate environment. Significant time often is required for cells to produce exoenzymes.

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