Info

Coenzyme

Vitamin from Which It Is Derived

Substance Transferred

Example of Use

Nicotinamide adenine dinudeotide (NAD+)

Niacin

Hydride ions (2 electrons and 1 proton)

Carrier of reducing power

Flavin adenine dinudeotide (FAD)

Riboflavin

Hydrogen atoms (2 electrons and 2 protons)

Carrier of reducing power

Coenzyme A

Pantothenic acid

Acyl groups

Carries the acetyl group that enters the TCA cycle

Thiamin pyrophosphate

Thiamine

Aldehydes

Facilitates the removal of CO2 from pyruvate in the transition step

Pyridoxal phosphate

Pyridoxine

Amino groups

Transfers amino groups in amino acid synthesis

Tetrahydrofolate

Folic acid

1-carbon molecules

Used in nucleotide synthesis

140 Chapter 6 Metabolism: Fueling Cell Growth activity requires the corresponding coenzyme are impaired. Thus, a single vitamin deficiency has serious consequences.

Environmental Factors that Influence Enzyme Activity

The growth of any organism depends on the proper functioning of its enzymes. Several features of the environment influence how well enzymes function and in this way determine how rapidly bacteria multiply (figure 6.12). Each enzyme has a narrow range of environmental factors—including temperature, pH, and salt concentration—at which it operates optimally. A 10°C rise in temperature approximately doubles the speed of enzymatic reactions, until optimal activity is reached; this explains why bacteria tend to grow more rapidly at higher temperatures. If the temperature gets too high, however, proteins will become denatured and no longer function. Most enzymes function best at low salt concentrations and at pH values slightly above 7. Not surprisingly then, most bacteria that have been studied grow fastest under these same conditions. Some prokaryotes, however, particularly certain members of the Archaea, are found in environments where conditions are extreme. They may require high salt concentrations, grow under very acidic conditions, or be found where temperatures are near boiling. ■ pH, p. 23 ■ temperature and growth requirements, p. 86

Allosteric Regulation

Cells can rapidly fine-tune or regulate the activity of certain key enzymes using other molecules that reversibly bind to and distort them (figure 6.13). This has the effect of regulating the activity of metabolic pathways. These enzymes can be controlled because they are allosteric enzymes (allo means "other"), which have a binding site called an allosteric site that is separate from their active site. When a regulatory molecule, or effector, binds to the allosteric site, the shape of the enzyme changes. This distortion alters the relative affinity, or chemical attraction, of the enzyme for its substrate. In some cases the binding of the effector enhances the affinity for the substrate, but in other cases it decreases it.

Allosteric enzymes generally catalyze the step that either initiates or commits to a given pathway. Because their activity can be controlled, they provide the cell with a means to modulate the pace of metabolic processes, turning off some pathways and activating others. Cells can also control the amount of enzyme they synthesize; this control mechanism, which will be discussed in chapter 7, also involves allosteric proteins. ■ regulation, p. 183

The end product of a given biosynthetic pathway generally acts as an allosteric inhibitor of the first enzyme of that pathway—a mechanism called feedback inhibition (figure 6.13b). This mechanism allows the product of the pathway to modulate its own synthesis. For example, the first enzyme of the multistep pathway used to convert the amino acid threonine to isoleucine is an allosteric enzyme that is inhibited by the binding of isoleucine. This amino acid must be present at a relatively high concentration, however, to bind and inhibit the enzyme. Thus, the pathway will only be shut down when a cell has accumulated sufficient isoleucine to fill its immediate protein synthesis needs. Because the binding of the effector is reversible, the enzyme can again become active when isoleucine levels decrease.

Compounds that reflect a cell's relative energy stores often regulate allosteric enzymes of catabolic pathways, enabling cells to modulate the flow through these pathways in response to changing energy needs. High levels of ATP inhibit certain enzymes and, as a consequence, slow catabolic processes. In contrast, high levels of ADP warn that a cell's energy stores are low, and they function to stimulate the activity of some enzymes.

Enzyme Inhibition

Enzymes can be inhibited by a variety of compounds other than the effectors normally used by the cell for regulation (table 6.6). These compounds can be used to prevent microbial growth. The site on the enzyme to which the molecules bind determines whether they function as competitive or non-competitive inhibitors.

Non-Competitive Inhibition

Non-competitive inhibition occurs when the inhibitor and the substrate act at different sites on the enzyme. Allosteric inhibition, discussed previously, is an example of non-competitive reversible inhibition and is exploited by the cell to modulate its

Optimum temp. Temperature -

Figure 6.12 Environmental Factors that Influence Enzyme Activity (a) A rise in temperature increases the speed of enzymatic activity until the optimum temperature is reached. If the temperature gets too high the enzyme becomes denatured and no longer functions. (b) Most enzymes function best at pH values slightly above 7.

1 2 3 4 5 6 7 < I >9 10 11 12 13 Highacidity Qrfirmiriphl Lowacidity (low pH) -> (high pH)

1 2 3 4 5 6 7 < I >9 10 11 12 13 Highacidity Qrfirmiriphl Lowacidity (low pH) -> (high pH)

Figure 6.12 Environmental Factors that Influence Enzyme Activity (a) A rise in temperature increases the speed of enzymatic activity until the optimum temperature is reached. If the temperature gets too high the enzyme becomes denatured and no longer functions. (b) Most enzymes function best at pH values slightly above 7.

Active site

Allosteric inhibitor

Active site

Allosteric inhibitor

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