Chemoorganoheterotroph
Metabolism Overview:
Reduction;
e- gain
from donor
Oxidation;
e- loss to
acceptor
Metabolic Pathways
• Although we can recognize a substrate and product of
individual enzymatic reactions; metabolic functions are
often performed by several enzymatic reactions in a
“pathway”.
• Pathways can be linear, branched, cyclic or even spiral.
• Pathway activity is controlled in three ways:
– Metabolites and enzymes may be localized in different parts of
the cell; called metabolic channeling.
– The total amount of enzymes in a pathway can vary (gene
expression).
– Pathway activity is controlled by critical regulated enzymes.
These “pacemaker enzymes” are often the rate-limiting step in
the pathway.
Metabolic
Pathways
Feedback Inhibition:
(“end-product
inhibition”)
• rate limiting enzyme
is first in pathway and
allosteric.
• end-product is a
negative effector
(inhibitor) of first
enzyme
Metabolic
Pathways
Feed Forward Activation:
• rate limiting enzyme of a
branch point is allosteric.
• earlier-substrate is a
positive effector (activator)
of forward reaction enzyme.
NOTE: the example also
illustrates feedback
inhibition.
+
Metabolic Pathways
• Amphibolic Pathways
– Catabolic direction
– Anabolic direction
• Separate regulatory enzymes
each way; function as “check
valves” for flow control.
• Other pathway enzymes are
reversible; ΔGo’≈0; their
equilibrium shifts based on
concentration of reactants &
products.
• Gycolysis is a good example.
Catabolism:
• Hydrolysis of complex (polymeric) organics
• Breakdown of Glucose (6C) to Pyruvate (3C)
– Glycolytic Pathway (Embden-Meyerhof)
– Pentose Phosphate Pathway (PPP)
– Entner-Doudoroff Pathway (E-DP)
• Fate of Pyruvate
– Fermentation
– Krebs Cycle (Tricarboxylic Acid, i.e. Citric Acid)
• Electron Transport Chain & Oxidative Phosphorylation
– Aerobic Respiration
– Anaerobic Respiration
Hydrolysis of Complex Organics
• Role of exo-enzymes and enzymes of cytoplasm or lysosome.
–
–
–
–
Glucosidase (polysaccharides → simple sugars)
Proteases (proteins → amino acids)
Nucleases (nucleic acids → nucleotides)
Lipases (lipids → glycerol & fatty acids)
• Hydrolysis reactions yield little energy & mostly lost as heat.
• Simple sugars (e.g. glucose) enter catabolic reactions early at
Glycolysis, PPP, E-DP, or used to biosynthesize other
polysaccharides.
• Others may be degraded to some intermediate of a major
catabolic pathways (e.g. fatty acids to acetyl-CoA) or used
directly in anabolic reactions (e.g. some amino acids).
Catabolism of Macromolecules
• Carbohydrates:
– Some monosaccharides may
require interconversion to
glucose or other sugar
intermediate.
– C5 & C4 sugars to PPP.
• Proteins:
– Amino acid deamination and
transamination.
– Mostly enter pathways as
carboxylic acid intermediates.
• Nucleic Acids
– Ribose sugar to PPP
– Purines and Pyrimidines to
component amino acids.
• Lipids
– Glycerol and fatty acids
– Fatty acids to acetyl-CoA
via β-oxidation.
Catabolism of
Glucose
Glycolosis: Both R & F;
ends in pyruvate; ATP via
substrate level
phosphorylation (SLP).
Respiration: pyruvate
oxidized by Krebs Cycle
(ATP by SLP); Electron
Transport Chain &
Chemiosmosis (ATP via
oxidative phosphorylation);
terminal electron acceptor
(O2 or other like NO3-).
Fermentation
only 2-4 ATP
Fermentation: Lack of
respiration; pyruvate is
reduced to another organic;
little ATP yield by SLP.
Aerobic Respiration 38 ATP
Glucose → Pyruvate via Glycolysis
Six Carbon (Investment)
Stage:
Hexose phosphorylation and
cleavage to two Glyceraldehyde-3Phosphates.
Five enzymatic reactions in this
stage; three are reversible.
Enzyme #1: Hexokinase (- G6P) &
Enzyme #3: Phosphofructokinase
(+ AMP; - ATP; - Citrate) are
regulated enzymes.
Requires an investment of 2 ATP to
“prime” the reaction.
Glucose → Pyruvate via Glycolysis
Three Carbon (Yield) Stage:
Two glyceraldehyde-3-phosphates
can pass per glucose.
Five more enzymatic reactions for a
total of ten in Glycolysis. Four at this
stage are reversible.
Enzyme #10: Pyruvate Kinase
(+ F1,6BP; - ATP) is regulated.
As the carbon becomes oxidized the
phosphate bonds elevate in their
energy potential.
Two steps involve SLP for ATP.
Total of 4 ATP yield here, minus 2
ATP invested earlier = 2 ATP for all
of Glycolysis, plus 2 NADH.
Glucose →
Pyruvate via E-DP
Two stage linear pathway like
Glycolysis; First stage unique;
second stage identical.
(KDPG)
KDPG the unique intermediate.
Yields 1 ATP, 1 NADH, 1NADPH.
Note one pyruvate is generated at
each stage of the pathway.
Found in some Gram negative
bacteria instead of Glycolysis
(Rhizobium, Agrobacterium,
Azotobacter, Pseudomonas)
Glucose →
Pyruvate via
PPP
Found in most bacteria.
Can produce pentoses (5C)
from hexoses (6C) via
oxidative decarboxylation,
which forms NADPH. Source
of ribose for nucleosides.
(GP)
Other unique sugars are
produced (4C, 7C); source of
erythrose for aromatic amino
acids (Phe, Tyr, Trp)
G3P enters Gycolysis to
produce ATP and pyruvate.
(GP)
Fermentation:
Lack of any respiration accumulates NADH.
Without NAD+, Glycolysis or E-DP won’t
proceed, i.e. no source of ATP!
Fermentation pathways couple NADH
oxidation and pyruvate reduction, or
reduction of another endogenous organic.
Permits some ATP production; slow growth.
Many species specific types.
Krebs (TCA) Cycle
Respiration allows pyruvate to be
completely oxidize to CO2.
First oxidative decarboxylation converts
pyruvate to Acetyl-CoA by the multienzyme Pyruvate Dehydrogenase
Complex – highly regulated
“pacemaker”.
Acetyl-CoA (2C) forms citric acid (6C) by
condensation with oxaloacetate (4C).
During the next 7 cyclic steps:
Two additional decarboxylations.
1 ATP by SLP.
3 NADH & 1 FADH2 produced.
Back to 1 oxaloacetate
Two cycles are required per glucose.
Β-Oxidation of Fatty Acids
The spiral pathway of fatty acid synthesis is nearly the reverse of this.