113 BIOLOGY - MICROBIOLOGY
Lecture 6: Microbial Metabolism - Activities and regulation of enzymes
The term metabolism refers to the sum of all chemical reactions within a living organism.
Metabolic activity can be dividied into two classes of reactions:
- Catabolic reactions release energy and are employed for breaking down nutrients
= Many catabolic reactions are hydrolysis reactions
= An example of catabolism is hydrolysis of proteins to individual amino acids
- Anabolic reactions require energy and are used in synthesizing molecules needed by the cells
= Many anabolic reactions are dehydration synthesis reactions
= An example of anabolism is synthesis of proteins from amino acids
- Adenosine triphosphate (ATP) serves as a link between catabolism and anabolism (Tortora et al.,
Figure 5.1)
= Catabolic reactions may be linked to dehydration synthesis of ATP from ADP and free
phosphate; the energy released in the catabolic reaction is "stored" in the ADP-P covalent
bond
= The energy for anabolic reactions may be obtained by hydrolysis of ATP to ADP +
phosphate.
The metabolic reactions of living systems are catalyzed by enzymes
- By a catalyst, we we mean something that increases the frequency of productive collisions in a
chemical reaction
- Enzymes, most of which are large globular protein molecules, are highly specific, affecting only
certain substrates and certain chemical reactions
- Many enzymes consist of both a protein portion, the apoenzyme, and a nonprotein cofactor;
together, these make up a holoenzyme (Tortora et al., Figure 5.2)
= If the cofactor is an organic molecule, it is called a coenzyme; most of the compounds that we
refer to as "vitamins" act as coenzymes or components of coenzymes (Tortora et al., Table
5.2)
= Many coenzymes act by serving as temporary acceptors or donors of chemical groups
Nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide
phosphate (NADP+) serve in the transfer of electrons in association with dehydrogenases
Coenzyme A (CoA) serves as a carrier of acetyl groups
= When a coenzyme is tightly bonded to its apoenzyme, it is called a prosthetic group; a good
example is the iron-containg heme group found in cytochromes
= Ions of metals, including magnesium, manganese, zinc and cobalt, are also employed as
cofactors of enzymes
- Enzymes are thought to work through association of reactants (substrates) with the holoenzyme
(Tortora et al., 5.3)
= Figure The substrate(s) bind(s) to a specific region on the surface of the enzyme, the enzyme's active
site, forming the enzyme-substrate complex
= The configuration of the enzyme-substrate complex favors the catalyzed reaction, yielding the
reaction products, which are released from the enzyme
= The enzyme can now go on to bind more substrate
- Enzymes are classified according to the nature of the reaction that that they catalyze (Tortora et
al., Table 5.1); note that the names of enzymes usually end in "-ase"
The activity of an enzyme (the rate of the reaction catalyzed by the enzyme) is influenced by a
variety of factors
- Every enzyme has an optimum temperature at which its activity is highest (Tortora et al., Figure
5.4a)
= As with noncatalyzed reactions, reaction rate tends to increase with increasing temperature
= At some point, reaction rate drops drastically with increasing temperature, as the enzyme
becomes denatured (Tortora et al., Figure 5.5)
- The pH of the environment in which an enzyme operates also influences activity Tortora et al.,
Figure 5.4b), mostly through ionization of carboxy and amino groups
- Enzyme activity varies with substrate concentration in a characteristic manner (Tortora et al.,
Figure 5.4c)
= At some level of substrate, the enzyme is said to be saturated, as all active sites are
"occupied"
= Under normal cellular conditions, reaction rate is more likely to be determined by substrate
concentration than by enzyme concentration
- Enzyme activity is also influenced by (and, in living systems, is often regulated by) inhibitors
(Tortora et al., Figure 5.6)
= Competitive inhibitors are molecules that compete with the normal substrate for the enzyme's
active site (Tortora et al., Figure 5.6b)
= Noncompetitive inhibitors bind to a part of the enzyme other than the active site (Tortora et
al., Figure 5.6c)
The binding site of a noncompetitive inhibitor is called an allosteric site
In some cases, allosteric binding of a compound can actually activate enzyme activity
= Feedback inhibition (Tortora et al., Figure 5.7) is an important biochemical control
mechanism involving allosteric noncompetitive inhibitors
In feedback inhibition, the end-product of a reaction sequence acts as an allosteric inhibitor
of an enzyme catalyzing a reaction early in the reaction sequence
Feedback inhibition is especially important in regulating levels of metabolic intermediatesa