CHAPTER
5
Microbial Metabolism
Chapter Outline
Basic Chemical Reactions Underlying Metabolism (pp. 125–132)
Lipid Catabolism
Protein Catabolism
Photosynthesis (pp. 144–149)
Chemicals and Structures
Light-Dependent Reactions
(pp. 125–132)
Catabolism and Anabolism
Metabolism is the sum of complex biochemical reactions within an organism. Catabolism
is the breakdown of nutrient molecules and the release of energy (are exergonic), which is then
stored in ATP (adenosine triphosphate) molecules. Anabolic reactions synthesize macromole-
charged). The electron donor loses an electron and is oxidized. If the electron is part of a hydro-
gen atom, the reaction is called a dehydrogenation reaction.
Three electron carrier molecules often required in metabolic pathways are nicotinamide ad-
enine dinucleotide (NAD+), nicotinamide adenine dinucleotide phosphate (NADP+), and
flavin adenine dinucleotide (FAD).
permanently changed in the process. Enzymes are often named for their substrates, which are
the chemicals they cause to react.
Naming and Classifying Enzymes
Enzymes are classified into six categories based on their mode of action: hydrolases add
hydrogen and hydroxide from the hydrolysis of water to split larger molecules into smaller ones;
RNA molecules functioning as catalysts are called ribozymes. Ribozymes process RNA
molecules in eukaryotes. Ribosomal enzymes catalyze the actual protein synthesis reactions of
ribosomes; thus, ribozymes make protein enzymes.
Enzyme Activity
Enzymes catalyze reactions by lowering the activation energy, which is the amount of
enzyme. Enzyme activity proceeds at a rate proportional to the concentration of substrate mole-
cules until all the active sites on the enzymes are filled to saturation.
Some enzymes can be activated by the binding of a cofactor to the allosteric site, a site away
from the active site of an enzyme. In allosteric activation, the binding of an activator to the allo-
steric site results in a conformational change in the active site, thereby activating the enzyme.
Glycolysis (the Embden-Meyerhof pathway) involves the splitting of a glucose molecule in
a series of 10 steps that ultimately results in the splitting of glucose into two molecules of
pyruvic acid, and a net gain of two ATP and two NADH molecules. The 10 steps of glycolysis
can be divided into three stages: energy-investment, lysis, and energy-conserving. Three of these
steps involve substrate-level phosphorylation in which a phosphate (and its energy) is trans-
Acetyl-coenzyme A (acetyl-CoA) is formed when two carbons from pyruvic acid join coenzyme
A. Two molecules of acetyl-CoA, two molecules of CO2, and two molecules of NADH are pro-
duced. The removal of the CO2 is decarboxylation.
The Krebs Cycle
Acetyl-CoA enters the Krebs cycle, a series of eight enzymatic steps that transfers energy and
energy from these electrons is used to pump protons (H+) across the membrane. The proton gra-
dient that is produced results in the synthesis of ATP by a process called chemiosmosis.
The four categories of carrier molecules in the electron transport system are flavoproteins,
ubiquinones, metal-containing proteins, and cytochromes.
Aerobes use oxygen atoms as final electron acceptors in the electron transport chain in a pro-
synthesize ATP by oxidative phosphorylation and photophosphorylation.
About 34 molecules of ATP are synthesized during chemiosmosis from the oxidation of 10
molecules of NADH and two molecules of FADH2. Thus, there is a theoretical net yield of 38
ATP molecules from the aerobic respiration of one molecule of glucose via glycolysis (four
molecules of ATP produced minus two molecules of ATP used), the Krebs cycle (two molecules
oids, and fatty acids.
Fermentation
Fermentation is the partial oxidation of sugar to release energy using an organic molecule with-
in the cell as the final electron acceptor. The essential function of fermentation is the
regeneration of NAD+ for use in glycolysis. Two common fermentation pathways reduce
FADH2.
Protein Catabolism
Protein catabolism by prokaryotes involves protease enzymes secreted to digest large
proteins outside their cell walls. The resulting amino acids move into the cell and are used in
anabolism. Others undergo deamination (removal of the amino group) to produce substrates for
chloroplast membrane is the stroma. There are two photosystems, photosystem I (PS I) and pho-
tosystem II (PS II), in order of their discovery.
Instructor’s Manual for Microbiology with Diseases by Body Systems, 5e
Light-Dependent Reactions
The light absorption and redox reactions of photosynthesis are classified as light-dependent re-
chemiosmosis.
Noncyclic Photophosphorylation
In noncyclic photophosphorylation, photosystem II donates electrons to photosystem I,
and the electrons are used to reduce NADP+ to NADPH in addition to ATP. Therefore, in
noncyclic photophosphorylation, a cell must constantly replenish electrons to PS II. In
Because anabolic reactions are synthesis reactions, they require energy and metabolites, both of
which are often the products of catabolic reactions. Amphibolic reactions are metabolic reac-
tions that can proceed toward catabolism or toward anabolism depending on the needs of the
cell. Examples are found in the biosynthesis of carbohydrates, lipids, amino acids, and nucleo-
tides.
and three molecules of fatty acid—a reverse of the catabolic reaction. Steroids result from com-
plex pathways involving polymerizations and isomerizations of sugar and amino acid metabo-
lites. Carotenoids are lipid pigments found in many bacteria and plants.
Amino Acid Biosynthesis
Precursor metabolites for amino acid synthesis are produced during glycolysis, the Krebs
Integration and Regulation of Metabolic Functions
(pp. 157–159)
Catabolic and anabolic pathways interact with each other in several ways. Energy released in
catabolic reactions is used to drive anabolic reactions. Catabolic pathways produce precursor
metabolites for use as substrates for anabolic reactions. Amphibolic reactions are anabolic or