MICROBIOLOGY (BIOL& 260)
CHAPTER 5
MICROBIAL METABOLISM
METABOLISM
Anabolic reactions
1. synthesis reactions
2. require energy
3. A + B + energy --> AB
Catabolic reactions
1. breakdown reactions
2. release energy
3. AB --> A + B + energy
ENZYMES
Biological catalysts
Reduce activation energy
Enzymes are not consumed in the reaction
Enzyme names end in -ase
COMPONENTS OF ENZYMES
Apoenzyme--protein portion produced by the cell
Cofactor--inorganic “partner” (mineral)
Coenzyme--organic “partner” (vitamin)
Holoenzyme--whole, functioning enzyme
Includes apoenzyme with the cofactor or coenzyme
ENZYMATIC ACTIVITY
Substrate is the substance which binds to and is changed by the enzyme
The substrate binds to the active site to create the enzyme-substrate complex
Products are released from the active site
ENZYME ACTIVITY FACTORS
1. Temperature--optimum temp.
a. too high--denaturation of enzyme
b. too low--decrease in molecular kinetics
2. pH--optimum pH
a. too acidic (low pH)--denaturation
b. too alkaline (high pH)--denaturation
3. Substrate Conc.--increase in enzymatic activity until saturation point
4. Inhibitors
Competitive inhibitors--bind to the active site; chemical structure is similar to the
substrate
Noncompetitive inhibitors--bind to the allosteric site; cause the active site to change
its shape
FEEDBACK INHIBITION
Provides control over enzymatic activity
End-products of an enzyme series inhibit (as allosteric inhibition) previous enzymes of
the series
Prevents overproduction of end-products
OXIDATION AND REDUCTION
Energy production often occurs via the loss (oxidation) or gain (reduction) of electrons
Electron transport chain is an example
RIBOZYMES
Catalysts
RNA molecules
Substrate molecules aere RNA strands
PHOSPHORYLATION
1. Substrate level phosphorylation--a phosphorylated compound transfers a phosphate
to ADP to form ATP; glycolysis
2. Oxidative phosphorylation--final electron acceptor is oxygen (or another inorganic
compound); electron transport chain
3. Photophosphorylation--conversion of light energy to chemical energy (ATP and
NADPH); photosynthesis
CARBOHYDRATE CATABOLISM
1. Glycolysis
2. Kreb’s cycle (tricarboxylic acid cycle)
3. Electron transport chain
GLYCOLYSIS
Also called the Embden-Meyerhof pathway
Glucose enters
Anaerobic
Requires addition of 2ATP molecules
Produces a net of 2 ATP and 2 NADH
End products are 2 pyruvic acid molecules
Occurs in the cytoplasm
KREBS CYCLE
Produces carbon dioxide
Produces 2ATP molecules (one per cycle)
Produces NADH
Produces FADH2
Occurs in the mitochondria
ELECTRON TRANSPORT CHAIN
Converts NADH and FADH2 into ATP
Series of oxidation and reduction reactions in proteins embedded in the inner
mitochondrial membrane
One NADH yields approx. 3 ATP molecules
One FADH2 yields approx. 2 ATP molecules
CHEMIOSMOSIS
Protons are pumped out of the cell by means of the proteins of the electron transport
chain
A proton-motive force is created
Protons are only allowed to diffuse across the membrane through certain channels (ATP
synthase) which drives the production of ATP
ALTERNATE METABOLIC PATHWAYS
1. Pentose Phosphate Pathway
2. Enter-Doudoroff Pathway
3. Anaerobic respiration
4. Fermentation
PENTOSE PHOSPHATE PATHWAY
Cyclic means of breaking down glucose to yield precursors to other compounds:
amino acids
glucose from carbon dioxide
nucleic acids
NADPH (12 from one glucose molecule)
Can produce only one glucose molecule
ENTER-DOUDOROFF PATHWAY
Glucose to pyruvic acid
Produces 2 NADPH molecules
Produces 1 ATP molecule
Typically not found in gram-positive bacteria
ANAEROBIC RESPIRATION
Use of another inorganic molecule as an electron acceptor besides oxygen:
1. nitrate ion
2. nitrite ion
3. nitrous oxide
4. nitrogen gas
5. sulfate
6. carbonate
FERMENTATION
Releases energy from various organic compounds
Doesn’t require oxygen
Doesn’t require Kreb’s cycle or electron transport chain
Uses organic molecule as electron acceptor
Produces modest amounts of ATP
LIPID CATABOLISM
Lipids are broken down by lipases
Products enter glycolysis or the Kreb’s cycle
PROTEIN CATABOLISM
Extracellular proteases break down protein
Amino acids are deaminated (this releases ammonium ions-NH4+)
Some amino acids have their -COOH group removed (decarboxylation)
Resultant products enter the Kreb’s cycle
PHOTOSYNTHESIS
Overall equation is the reverse of cellular respiration
Light reactions (photophosphorylation)
Light independent reactions
LIGHT REACTIONS
1. Cyclic photophosphorylation
2. Noncyclic photophosphorylation
CYCLIC PHOTOPHOSPHORYLATION
Photon of light energizes the electron
Electron is passed through electron carriers
Protons are pumped across the membrane
Chemiosmosis produces ATP from ADP
Excited electron ultimately returns to where it began (chlorophyll)
NONCYCLIC PHOTOPHOSPHORYLATION
Photon-excited electrons pass through electron carriers to produce ATP from proces of
chemiosmosis
Electrons become incorporated into NADPH
Electrons from water replace these electrons
Oxygen is released
CALVIN-BENSON CYCLE (LIGHT INDEPENDENT)
Similar to the Kreb’s cycle except carbon dioxide goes in and glyceraldehyde 3phosphate is produced (two such molecules can be converted into glucose)
To make one glucose the cycle must turn 6 times
Requires 6 carbon dioxide, 18 ATP, and 12 NADPH
NUTRITIONAL TYPES
Photoautotrophs (plants)--obtain energy from the sun, obtain carbon from carbon
dioxide
Photoheterotrophs (some species of nonsulfur bacteria)--obtain energy from light,
obtain carbon from organic compounds
Chemoautotrophs (hydrogen, sulfur, iron and nitrifying bacteria)--obtain energy from
inorganic compounds, obtain carbon from carbon dioxide
Chemoheterotrophs (ourselves)--obtain energy from organic compounds, carbon from
organic compounds
LEARNING OBJECTIVES FOR CHAPTER 5
Following successful study of this chapter, students should be able to:
1. Define or describe each of the following terms: metabolism, catabolism, anabolism,
metabolic pathways, enzymes, apoenzyme, cofactor, coenzyme, holoenzyme, activation
energy, enzyme-substrate complex, substrate, active site, allosteric inhibition, feedback
inhibition, ribozymes, oxidation, reduction, redox reactions, aerobe, anaerobe,
photoautotrophs, photoheterotrophs, chemoautotrophs, chemoheterotrophs
2. List and describe the factors which affect enzymatic activity (eg. temperature, pH,
substrate concentration, competitive inhibitors, noncompetitive inhibitors)
3. List and describe the various types of phosphorylation: substrate-level
phosphorylation, oxidative phosphorylation, photophosphorylation.
4. Describe the general characteristics for each of the following metabolic pathways:
glycolysis, pentose phosphate pathway, Entner-Doudoroff pathway, Krebs cycle, electron
transport chain, chemiosmosis, anaerobic respiration, fermentation, cyclic
photophosphorylation, noncyclic photophosphorylation, Calvin-Benson cycle,