Microbial Metabolism
Chapter 5
Metabolism
Metabolism - all of the chemical
reactions within a living organism

1. Catabolism
–
–

breakdown of complex organic molecules into
simpler compounds
releases ENERGY
2. Anabolism
–
–
( Catabolic )
( Anabolic )
the building of complex organic molecules from
simpler ones
requires ENERGY
Enzymes - catalysts that speed up and
direct chemical reactions

A. Enzymes are substrate specific
–
–
–
–
–
Lipases
Sucrases
Ureases
Proteases
DNases
Lipids
Sucrose
Urea
Proteins
DNA
Enzyme Specificity can be explained
by the Lock and Key Theory
E + S -----> ES ------> E + P
Naming of Enzymes - most are named
by adding “ase” to the substrate






Sucrose
Sucrase
Lipids
Lipase
DNA
DNase
Proteins
Protease
removes a HydrogenDehydrogenase
removes a phosphate
phosphotase
Naming of Enzymes




Grouped based on type of reaction they
catalyze
1. Oxidoreductases
oxidation & reduction
2. Hydrolases
hydrolysis
3. Ligases
synthesis
More about Enzymes

Sometimes an enzyme needs help
–
–
Protein alone = apoenzyme
Helper molecule: cofactor


–
–
Could be inorganic like a metal ion (Fe+2)
Could be organic coenzyme (like CoA, NAD)
Apoenzyme + cofactor = holoenzyme.
Cofactors have an effect on nutrition


Bacteria have certain mineral requirements.
Vitamins are cofactors that are needed in the “diet”.
Enzyme Components
2 Parts
1. Apoenzyme - protein portion
2. Coenzyme (cofactor) - nonprotein
Holoenzyme - whole enzyme
Coenzymes

Many are derived from vitamins

1. Niacin
–

2. Riboflavin
–

NAD (Nicotinamide adenine dinucleotide)
FAD (Flavin adenine dinucleotide)
3. Pantothenic Acid
–
CoEnzyme A
Factors that Influence Enzymatic
Activity
Denaturation of an Active Protein
Enzymes can be stopped

Conditions that disrupt the 3D shape
–
–

Acidic, alkaline, high salt, high temperature, etc.
These conditions thus affect growth of cell also.
Inhibitory molecules affect enzymes
–
Competitive inhibitors

–
Non-competitive inhibitors

–
Fit in active site but are not changed; prevent normal substrate from
binding, prevent reaction.
Bind permanently to active site or other site which changes molecular
shape; prevents reaction.
Allosteric inhibitor: temporary binding, regulates.
Competitive Inhibition
.

Both the substrate and the
inhibitor fit into the active
site, but the inhibitor isn’t
altered by the enzyme. As
long as the inhibitor is in the
active site, the substrate
cannot enter the active site
and react. The more inhibitor
molecules that are present,
the more often one of them
occupies the active site
ghs.gresham.k12.or.us/.../ competitiveinhib.htm
Allosteric sites
In allosteric site, inhibitor is not reacted, but causes a shape
change in the protein. The substrate no longer fits in the active
site, so it is not chemically changed either.
ghs.gresham.k12.or.us/.../ noncompetitive.htm
Competitive Inhibitors -compete
for the active site

1. Penicillin
–

competes for the active site on the enzyme
involved in the synthesis of the pentaglycine
crossbridge
2. Sulfanilamide (Sulfa Drugs)
–
competes for the active site on the enzyme that
converts PABA into Folic Acid

Folic Acid - required for the synthesis of DNA and RNA
Selective Toxicity
Non-competitive Inhibitors - attach to
an allosteric site

Feedback Inhibitionstops the cell from
wasting chemical
resources by making
more of a substance
than it needs.
Energy Production

1. Oxidation
–

refers to the loss of Hydrogens and or electrons
2. Reduction
–
the gain of Hydrogens and or electrons
NAD Cycle
Carbohydrate Catabolism



Microorganisms oxidize carbohydrates as
their primary source of energy
Glucose - most common energy source
Energy obtained from Glucose by:
–
–
Respiration
Fermentation
Aerobic Cellular Respiration

Electrons released by oxidation are passed
down an Electron Transport System with
oxygen being the Final Electron Acceptor

General Equation:

Glucose + oxygen----> Carbon dioxide + water


ATP
Chemical Equation

C6H12O6 + 6 O2 -------> 6 CO2 + 6 H2O


38 ADP + 38 P
38 ATP
Aerobic Cellular Respiration

4 subpathways

1. Glycolysis
2. Transition Reaction
3. Kreb’s Cycle
4. Electron Transport System



1. Glycolysis
(splitting of sugar)

Oxidation of Glucose into 2 molecules of
Pyruvic acid
Embden-Meyerhof Pathway

End Products of Glycolysis:

–
–
–
2 Pyruvic acid
2 NADH2
2 ATP
2. Transition Reaction

Connects Glycolysis to Krebs Cycle

End Products:
–
–
–
2 Acetyl CoEnzyme A
2 CO2
2 NADH2
3. Krebs Cycle
(Citric Acid Cycle)

Series of chemical reactions that begin and
end with citric acid

Products:
–
–
–
–
2
6
2
4
ATP
NADH2
FADH2
CO2
4. Electron Transport System

Occurs within the cell membrane of Bacteria

Chemiosomotic Model of Mitchell
–
34 ATP
How 34 ATP from E.T.S. ?
3 ATP for each NADH2
2 ATP for each FADH2
NADH2

2
2
6


Glycolysis
T. R.
Krebs Cycle

Total
10

10 x 3 = 30 ATP



FADH2

Glycolysis
T.R.
Krebs Cycle
0
0
2

Total
2

2 x 2 = 4 ATP

Total ATP production for the
complete oxidation of 1 molecule
of glucose in Aerobic Respiration


Glycolysis
Transition Reaction
Krebs Cycle
E.T.S.

Total



ATP
2
0
2
34
38 ATP
Overview of aerobic metabolism


Energy is in the C-H bonds of glucose.
Oxidation of glucose (stripping of H from C atoms) produces
CO2 and reduced NAD (NADH)
–

Electrons (H atoms) given up by NADH at the membrane,
energy released slowly during e- transport and used to
establish a proton (H+) gradient across the membrane
–
–

Energy now in the form of NADH (“poker chips”)
Energy now in the form of a proton gradient which can do work.
Electrons combine with oxygen to produce water, take e- away.
Proton gradient used to make ATP
–
Energy now in the form of ATP. Task is completed!
Definitions

Substrate level phosphorylation
–

Oxidative (respiratory) phosphorylation
–

Chemical reaction coupled to ATP synthesis
Pumping of protons powered by electron transport
with oxygen as terminal electron acceptor yields
ATP
Photophosphorylation
–
Pumping of protons powered by absorption of
light.
Central Metabolism:
Funneling all nutrients into central
pathways
•Many other
molecules
besides glucose
can serve as a
source of carbon.
Central Metabolism:
A source of building blocks for
biosynthesis
BUT, these
molecules can’t be
broken down to CO2
for energy AND
used for biosynthesis
Other ways to make ATP




Photosynthesis: light driven ATP synthesis.
Anaerobic respiration: organic compounds
oxidized, electrons passed down e- transport
chain to some molecule other than oxygen
(e.g. NO3, SO4).
Inorganic molecules can be oxidized with
ATP synthesis by e- transport and
chemiosmosis.
Fermentation: common anaerobic pathway
used by many medically important bacteria.
Anaerobic Respiration

Electrons released by oxidation are passed
down an E.T.S., but oxygen is not the final
electron acceptor

Nitrate (NO3-)
----> Nitrite (NO2-)
Sulfate (SO24-)
----> Hydrogen Sulfide (H2S)
Carbonate (CO24-) -----> Methane (CH4)


Fermentation

Anaerobic process that does not use the
E.T.S. Usually involves the incomplete
oxidation of a carbohydrate which then
becomes the final electron acceptor.

Glycolysis - plus an additional step
Fermentation may result in numerous
end products
1. Type of organism
2. Original substrate
3. Enzymes that are present and active
1. Lactic Acid Fermentation




Only 2 ATP
End Product - Lactic Acid
Food Spoilage
Food Production
–
–
–

Yogurt
- Milk
Pickles
- Cucumbers
Sauerkraut - Cabbage
2 Genera:
–
–
Streptococcus
Lactobacillus
2. Alcohol Fermentation


Only 2 ATP
End products:
–
–
alcohol
CO2

Alcoholic Beverages
Bread dough to rise

Saccharomyces cerevisiae

(Yeast)
3. Mixed - Acid Fermentation

Only 2 ATP
End products - “FALSE”

Escherichia coli and other enterics

Propionic Acid Fermentation


Only 2 ATP
End Products:
–
–

Propionic acid
CO2
Propionibacterium sp.
Fermentation
Figure 5.18b
Lipid Catabolism
Protein Catabolism
Biochemical tests

Used to identify
bacteria.
Figure 10.8
Photosynthesis - conversion of light
energy from the sun into chemical energy



Chemical energy is used to reduce CO2 to
sugar (CH2O)
Carbon Fixation - recycling of carbon in the
environment (Life as we known is dependant on this)
Photosynthesis
–
–
–
Green Plants
Algae
Cyanobacteria
Chemical Equation
6 CO2 + 6 H2O + sunlight -----> C6H12O6 + 6 O2


2 Parts:
–
–
1. Light Reaction
2. Dark Reaction
Light Reaction

Non-Cyclic Photophosphorylation
–
–
–

O2
ATP
NADPH2
Light Reaction (simplified)
2. Dark Reaction
Macronutrients











Carbon (CO2 or organic compounds)
Hydrogen (H2O or organic compounds)
Oxygen (H2O or organic compounds)
Nitrogen (NH3, NO3-, organic N-compounds)
Phosphorus (PO43-)
Sulfur (H2S, SO42-, organic compounds)
Potassium (K+)
Magnesium (Mg2+, salts)
Sodium (Na+)
Calcium (Ca2+, salts)
Iron (Fe3+, Fe2+, or salts)
Iron as a nutrient


Needed for aerobic metabolism
(cytochromes, iron-sulfur proteins)
Insoluble under aerobic conditions
–
–
Fe(OH)3, FeOOH
Solubilized by siderophores
Micronutrients and growth factors

Micronutrients: Metals and metalloids
–
–

Generally not necessary to add to medium
Deficiencies can arise when medium constituents
are very pure
Growth factors: organic requirements
–
Vitamins, amino acids, purines, pyrimidines,
acetate
Culture media


Defined: all chemicals are ostensibly known
Complex (undefined): contains substances
with unknown chemistries, such as peptones,
yeast extract, lake water, soil extract, etc.