L18: Catabolism Flashcards
Photo autotrophic
Photosynthetic
Use sunlight to produce chemical energy (CO2 as C source)
Example: cyanobacteria, photosynthetic bacteria, microalgae
Chemolithoautotrophic organisms
Oxidise inorganic compounds -> produce chemical energy (CO2 as C source)
Some specialised bacteria
Heterotrophic organisms
Oxidise organic compounds (originally produced by autotrophic organisms) to harvest chemical energy (and C)
Fungi, protozoans and many bacteria (and all animals)
Complementary processes in heterotrophic organisms
Catabolism
Anabolism
Catabolism
Larger more complex molecules are broken down into smaller, simpler molecules -> release energy
Fuelling reactions supply ATP, reducing power and precursor metabolites
Anabolism
Synthesis of more complex molecules from simpler ones with input of energy
How is energy generated during catabolism?
Degradation of fuel molecules during catabolism -> releases energy ‘stored’ in chemical bonds
Cells harvest energy in form of e
Energy is harvested in redox reactions -> make ATP
Redox reactions
Involve pair of e carrier compounds (redox)
Donor: becomes oxidised
Acceptor: becomes reduced
Reduced acceptor can then act as donor and original donor can act as acceptor
Electron transport chains (ETCs)
E carrier compounds can be linked in cascade through which e is passed(ETC)
Important e carrier pairs involved in catabolism: NAD+ (nicotinamide adenine dinucleotide)/NADH, FAD (flavin adenine dinucleotide)/FADH2, also ubiquinone (coenzyme Q), flavoproteins, cytochromes
ATP
The universal energy currency
High energy molecule because of energy required to maintain bonds between bulky, -vely charged phosphate groups
Energy harvested from catabolic reactions: stored in ATP and released to drive energy-requiring cellular activities
Link between catabolism and anabolism
Exergonic
Energy yielding reactions
Endergonic
Energy requiring reactions
Catabolism involving aerobic respiration
Macromolecules are degraded to smaller, simpler molecules (glycolysis, TCA cycle)
Some ATP is produced during glycolysis and TCA cycle (substrate level phosphorylation)
More ATP is produced in ETC from reducing generated in glycolysis & TCA cycle (oxidative phosphorylation: harvest of e in redox reactions, ETC & proton motive force)
Glycolysis
Conversion of glucose to pyruvate
- Embden-Meyerhof Payhway: most common pathway. Generates pyruvate from glucose in multi-step process. Occurs in all major groups of microbes. Functions in presence or absence of O2. Generates energy (ATP, reducing power) & compounds for biosynthesis
- Pentose Phosphate Pathway: most microbes have this alternative/complementary pathway -> pyruvate
- Entner-Doudoroff Pathway: some G-ve bacteria use this pathway -> pyruvate. Soil bacteria e.g. Pseudomonas, Rhizobium. Not used in prokaryotes or most G+ves
TCA cycle: conversion of pyruvate to CO2
- Pyruvate is oxidised to acetyl-CoA (2-C)
- Acetyl-CoA is condensed with oxaloacetate to form citric acid (6-C)
- Citric acid is oxidised and CO2 is released
Generate energy (ATP and reducing power). Compounds for biosynthesis formed. Oxaloacetate is regenerated
Most microbes have TCA cycle: aerobic bacteria, most fungi and algae, free living protozoa
Some microbes do not: TCA cycle enzymes may be present and involved in making biosynthesis intermediates
ATP from substrate level phosphorylation
Phosphate group from phosphate-containing intermediate of glycolysis pathway or TCA cycle -> transferred directly to ADP to yield ATP (occurs only during some reaction steps in glycolysis and TCA cycle.e.g. In final step in glycolysis)
Harvesting electrons as reducing power
E are harvested by electron carriers (EC) during degradation of fuel molecules
E are packets of energy, and ECs use them to reduce other compounds in redox reactions: to harvest e NAD(and FAD) form redox pairs with fuel molecules during fuel molecule degradation, e transferred to NAD -> NADH, e transferred to FAD -> FADH2, ECs acquired reducing power from fuel molecules
Harvesting e as reducing power example: NAD acquires reducing power when it reacts with malate in TCA cycle
e is transferred from malate to NAD -> malate is oxidised to oxaloacetate
NADH is reduced to NADH (reducing power transferred from malate to NADH)
NADH has reducing power able to be transferred to an ETC
ATP from oxidative phosphorylation
Process by which ATP is synthesised as result of e transport driven by oxidation of chemical energy source
- e harvested by ECs during fuel molecule degradation enter an ETC
- Passage of e through ETC by series of interlinked EC pairs
- Generation of proton motive force from ETC
- Formation of ATP using proton motive force
Involves redox
Step 1 of ATP from oxidative phosphorylation
Harvest of e by ECs during fuel molecule degradation:
NAD & FAD -> redox pairs with fuel molecules during degradation
e are transferred to NAD and FAD (producing NADH and FADH)
NADH and FADH transfer e to ETC
Step 2 of ATP from oxidative phosphorylation
(Passage of e through ETC)
ECs transfer e to ETC
e flow to redox pairs (ECs) having progressively more +ve reduction potentials
e are eventually transferred to terminal e acceptor
Difference in reduction potential between EC redox pair and terminal e acceptor redox pair determines free energy released
Terminal e acceptor redox pair in aerobic respiration O2 is final acceptor and is reduced to H2O
Step 3 in ATP from Oxidative Phosphorylation
(Generation of proton motive force from ETC)
Energy released in ETC is used to pump protons (H+) to outside of membrane
Charge difference set up across membrane (i.e potential energy)
Potential energy from chemical and electrical potential differences: proton motive force
Step 4 in ATO oxidative phosphorylation
(Formation of ATP using proton motive force)
Electrochemical potential energy released during flow of protons back across membrane down charge and proton conc gradients
Energy used to form ATP, catalysed by ATP synthase embedded in membrane: a rotary motor
How much energy does aerobic respiration yield?
Glycolysis (substrate level phosphorylation): ATP yield = 2
TCA cycle (substrate level phosphorylation): ATP yield = 2
ETC (Oxidative phosphorylation): ATP yield=28
-> total ATP yield (in eukaryotes) during aerobic respiration: ATP yield=32
Catabolism by fermentation
Key requirements for maintenance of glycolysis pathway is presence of EC NAD
NAD accepts e during fuelling reactions of glycolysis -> formation of a phosphate transferring intermediate essential to formation of ATP by substrate level phosphorylation (SLP)
-> glycolysis will cease in absence of supply of NAD to accept e
Some heterotrophic microbes do not respire
Some lack on ETC
Sometimes those with ETC find themselves in environment where there are no suitable terminal e acceptors
In both cases has to be way to regenerate NAD
Overcome problem by fermentation (energy yielding process in which organic molecule is oxidised without exogenous e acceptor)
Pyruvate (end product of glycolysis) acts as e acceptor which: regenerates NAD, ensures ATP formation by SLP is maintained, produces variety of fermentation by-products
Common microbial fermentatikns
Depending on pathway used, range of alcohols and organic acids can be produced by particular microbes
Named on basis of major alcohol or acid formed
How much energy does fermentation yield?
Fermentation only yields ATP by substrate level phosphorylation
Fermentation produces 2 ATP molecules per molecule of glucose compared to approx. 32 ATP yielded by aerobic respiration
Difference implies that microbial growth bu fermentation will be much slower than by aerobic respiration (less ATP per fuel molecule degraded)
