Reading Guide
Chapter 6: Metabolism
To prepare for class you should read sections 6.1-6.5. You should have a basic idea of the
big picture of metabolism, what pathways are operating within a type of metabolism
(fermentation, aerobic respiration, or anaerobic respiration) and the final products from
each of them.
This chapter covers the processes involved in bacterial metabolism, both catabolic and
anabolic reactions. We will focus on catabolic reactions, the processes that bacteria use to
generate ATP and discuss the three different types of bacterial metabolism; fermentation,
aerobic respiration, and anaerobic respiration. The key pathways operating in these three
types of bacterial metabolism are glycolysis, the TCA cycle, and the electron transport
chain.
Before we begin our discussion of bacterial metabolism and metabolic pathways it is
important to understand some of the basic concepts that form the foundation. The
pathways that we will talk about are catabolic pathways.
Catabolic pathways will generate ATP in the process of breaking bonds or oxidizing
organic compounds. ATP can be made by bacterial cells by one of three ways:
1. substrate level phosphorylation
2.oxidative phosphorylation,
3.photophosphorylation.
Which of these three methods is how cells make ATP within a metabolic pathway such as
glycolysis by the transfer of a phosphate group from an organic compound to ADP?
Which process is the generation of ATP through oxidation/reduction reactions in the
electron transport chain?
Another key concept for understanding bacterial metabolism is that enzymes function in
metabolic pathways. Enzymes are proteins that lower the activation energy of a
reaction. Without enzymes, these metabolic reactions would not occur. Enzymes have a
specific site, called the active site where the substrate binds. Once the substrateenzyme binds and forms a complex, the substrate can then be changed/modified and
becomes the product. Some enzymes require a non-protein component called a cofactor
to function. Cofactors often improve the fit of the substrate with the active site of the
enzyme (see Fig. 6.12). Some examples of cofactors are magnesium and zinc.
Coenzymes are a special kind of organic cofactor that loosely bind molecules or carry
electrons to help the reaction. Examples of coenzymes are NAD+ and FAD.
There many things influence the activity of enzymes and how fast the enzyme can
interact with the substrate to generate product. Some factors which influence enzyme
activity are environmental factors, such as temperature and pH. (see fig 6.13) In
addition, enzyme inhibitors can also influence enzyme activity. There are two major
classes of enzyme inhibitors, competitive and non-competitive inhibitors.
Competitive inhibitors bind to the active site of the enzyme, actually “competing” for
the same site as the substrate. If the active site is occupied, then the substrate can not be
turned into product….and enzyme activity is decreased. A good example of a competitive
inhibitor is the drug sulfanilamide. This drug is chemically similar enough to the
compound PABA. PABA is a precursor used to generate folic acid, an essential precursor
to nucleic acid synthesis. (see fig 6.15)
Non-competitive inhibitors bind to the enzyme at a place “other” than the active site.
This “other” site is called an allosteric site. By binding to this allosteric site on the
enzyme the actual shape or conformation of the active site is altered. This change in the
shape of the active site means the substrate no longer fits, essentially inhibiting the
process of the enzyme interacting with the substrate and generating product. (see fig
6.14) Allosteric can also mean the inhibitor is in the way, or blocking the active site so
that the substrate cannot fit.
Now you are ready to begin looking closer at the central metabolic pathways of bacteria.
The metabolic processes found in bacteria are fermentation, aerobic respiration, or
anaerobic respiration. (see Table 6.3) Each of these metabolic processes consists of
metabolic pathways. For example, in fermentation the ONLY metabolic pathway that is
operating is glycolysis whereas in aerobic respiration the pathways operating are
glycolysis AND the tricarboxylic acid cycle AND the electron transport chain. Which
type of metabolic process will generate the most ATP? Why? Continue to read the rest of
the chapter (through section 6.5) and we will discuss the metabolic processes in greater
detail in class.