Reading Guide
Chapter 6: Metabolism
To prepare for class you should read sections 6.1-6.6. 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, meaning that the cell will
generate ATP in the process of breaking bonds or oxidizing organic compounds. ATP can
be made by bacterial cells in one of three ways, substrate level phosphorylation, oxidative
phosophorylation, or 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 concept key to the topic of bacterial metabolism is enzymes and their
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 is
now known as a 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. 6.10). Some examples of cofactors are magnesium and zinc.
Coenzymes are a special kind of organic cofactor that act as loosely bound carriers of
molecules or carry electrons. Examples of coenzymes are NAD+ and FAD.
There are many factors which can influence the activity of an enzymes, or the rate
by which the enzyme can interact with a substrate generating product. Some factors
which influence enzyme activity are environmental factors, such as temperature and pH.
(see fig 6.11) In addition, enzyme inhibitors can also influence the activity of an enzyme.
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 6.13) 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 6.12)
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 6.6) and we will discuss the metabolic
processes in greater detail in class.