Download Metabolism 4 - DR CLEM KUEK

Catabolism in heterotrophs
Overview of energy flow through the cell
Overview of carbon flow through the cell
• Captured
as ATP and NADH is cytoplasmic membranes (or mitochondria) from
• Enters the cell
in heterotrophs as organic compounds
o catabolism of organic compounds
o lithotrophy of inorganic compounds
or ATP and NADPH in thylakoids (chloroplasts) from light
e.g. glucose, amino acids, fatty acids, glycerol.
•
in autotrophs as CO2
• Converted to other compounds
organic compounds: via catabolism (glycolysis; TCA cycle) to small molecular weight
intermediate metabolites
CO2: via fixation pathways (Calvin and Hatch-Slack cycles) to glucose
cell
Used to biosynthesise cell components, storage compounds, excreted as CO2 , wastes
and other compounds e.g. antibiotics
ZIP/Lect+Prac/MPAG/Metabolism4.doc
ATP, ADP
other phosphorylated compounds
NADH; NADPH
organic compounds
• Used for biosynthesis (cell growth and reproduction), motility, nutritent transport into the
• Fate of these compounds
Dr. Clem Kuek
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Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism4.doc
Energy from organic compounds
Glycolysis
Pathways for the oxidation of organic compounds to
produce energy fall into 2 groups
Oxidation of glucose to pyruvate
• Reduces carbon number from 6 to 3
(different in phosphorylation)
Fermentation
2
• Releases energy for anabolism and fermentation
Respiration
• Several amphibolic pathways (catabolic and anabolic) pathways
Embden-Meyerhof in eukaryotes and bacteria
Others found in bacteria
• Glucose derived from polysaccharides and other sugars
• Carbon compounds derived from amino acids and lipids may enter
glycolytic pathways at various stages
No externally supplied electron acceptor required as
oxidation is coupled with reduction of organic compound
Dr. Clem Kuek
External electron acceptor is required
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Dr. Clem Kuek
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The Embden-Meyerhof Pathway 2
The Embden-Meyerhof Pathway
2. Oxidative reactions of 3C compounds
Two stages
2 NAD
2 NADH
1. Preparatory reactions of 6C compounds
Glucose
ATP
ADP
P Glucose 6-phophate
P Fructose 6-phosphate
ATP
ADP
P Fructose 1,6-diphosphate
Dihydroxyacetone
phosphate P
• Phosphorylation and oxidation
(at 1,3-diphosphoglycerate)
P 1,3-diphosphoglyceric acid
P
Features
Features
2P
• Substrate phosphorylation of ADP
• Dehydration upgrades phosphate bond to
2 NAD
2 NADH
• Phosphorylation with 2 P
P 3-phosphogylceric acid
To energize 3C intermediate
product
• Isomerization
Glucose to fructose
• Cleavage
6C to 3C compounds
• Net loss of 2 ATP
high-energy bond
• Net gain of 2 ATP
P
2
2-phosphoglyceric acid
2H2O
P
Phosphoenolpyruvic acid
2 ADP
2 ATP
2
P Glyceraldehyde 3-phosphate
Pyruvate
Overall reaction in glycolysis
+
+
Glucose + 2ADP + 2Pi + 2NAD 2 pyruvate + 2ATP + 2NADH + 2H
Dr. Clem Kuek
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Dr. Clem Kuek
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The Pentose Phosphate Pathway 2
Alternatives to glycolysis
3NADP +
The Pentose Phosphate Pathway
3NADPH + 3 H+
3 glucose-6-P
a.k.a the Hexose Monophosphate pathway
3NADP +
3NADPH + 3 H+
3 6-phosphogluconate
3CO 2
3 ribulose-5-P
3H2O
• A sole pathway, or in conjunction with the Embden-Meyerhof or
xylulose-5-P
ribose-5-P
Entner-Doudoroff pathways
Important in anabolism as well as catabolism
glyceraldehyde-3-P
• Significance
Fructose-6-P
sedoheptulose-7-P
erythrose-4-P
Source of energy though of greater importance in biosynthesis
1 ATP (net) per glucose molecule via glycolysis (pyruvate from glyceraldehyde-3-phosphate)
NADPH produced is a source of electrons for reduction in biosynthesis;
Fructose-6-P
12 NADPH per glucose molecule
Synthesizes 4 and 5 C sugars various purposes
glyceraldehyde-3-P
Fructose-6-P
e.g. ribose-5-phosphate for nucleic acids; ribulose-1,5-biphosphate is the primary CO2 acceptor
in photosynthesis
Pyruvate
3 Glucose-6-P + 6NADP + + 3H2O 2 Fructose-6-P + Glyceraldehyde-3-P + 3CO2 + 6NADPH + 6H+
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Dr. Clem Kuek
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The Entner-Doudoroff pathway 2
The Entner-Doudoroff pathway
Glucose
ATP
• Alternative to the Embden-Meyerhof pathway for glucose catalysis
Found in some G(-) bacteria e.g. Pseudomonas, Rhizobium.
Generally not found in G(+) bacteria
ADP
Glucose-6-phosphate
NAD
• Generates
• Three stages
Oxidation of glucose-6-P to
6-phosphogluconate
Dehydration to 2-keto-3-deoxy6-phosphogluconate
cleavage to pyruvate and
glyceraldehyde-3-P
NADPH +
6-phosphogluconic acid
2 pyruvate per glucose molecule
1 ATP (net), 1 NADPH, 1 NADH per glucose
H2O
Pyruvic acid
2-keto-3-deoxy-6phosphogluconic acid
Glyceraldehyde-3-phosphate
Glycolysis
2ATP
Pyruvate
Dr. Clem Kuek
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Dr. Clem Kuek
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The Phosphoketolase pathway 2
The Phosphoketolase pathway
Glucose
AT
AD
• An alternative to the Embden-Meyerhof pathway
Glucose-6-P
NA
may be linked to reductive/fermentative pathway from pyruvate
present in heterofermentative lactic acid bacteria
NADH +
6-phosphogluconic acid
NA
• Generates
oxidation of glucose to xylulose-5-P
cleavage to acetyl-P and
glyceraldehyde-3-P
oxidation of glyceraldehyde-3-P to
pyruvate
CO2
NADH +
Ribulose-5-P
pyruvate and acetyl-P during oxidation
o which ferment (reduce) to lactic acid and ethanol
1 ATP and 3 NADH per glucose molecule
Xylulose-5-P
Glyceraldehyde-3-P
Pyruvate
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Dr. Clem Kuek
• Three stages
Acetyl-P
Acetyl-CoA
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The Krebs/TCA/Citric Acid cycle
• Oxidation of acetyl groups to CO2 with production of NADH,
FADH2 and some ATP
4 NADH+H, 1 FADH+H and 1GTP per pyruvate molecule
• 4, 5 and 6C compounds available for biosynthesis
Carbon skeletons supplied for amino acid biosynthesis by α-ketoglutarate,
succinate and oxaloacetate
Glucose biosynthesis (gluconeogenesis) from malate/oxaloacetate
• Present in organisms with aerobic respiration
The cycle is not fully developed in anaerobes (they lack the α-ketoglutarate
dehydrogenase complex)
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