Tricarboxcylic acid cycle

• Anaerobic, cell membrane or mitochondria• Each pyruvate gives up its carbon as CO2

– 6 total• Oxaloacetate is regenerated with every turn

– Pick up molecule• 2 ATP are produced

– Substrate level phosphorylation

Net yield of product per glucose molecule:

•6 CO2 •2 ATP•8 NADH = 3 ATP•2 FADH2 = 2 ATP

TCA cycle occurs twice per glucose

ELECTRON TRANPORT

• Aerobic or anaerobic• Final electron acceptor:

– aerobic respiration - oxygen – anaerobic respiration - CO2, NO3

-, SO42

• Inner mitochondrial membrane or plasma membrane • Electrons move down chain and set up H+ gradient

– drives chemiosmosis

Electron transport systems consist of separate protein complexes

Oxidative Phosphorylation – series of redox reactions creating a stepwise release of energy

Proton Motive Force generated by chemical and electrical gradient

Proton flow across membrane is exerogonic

Using the PMF, ATP synthesis is catalyzed by ATP synthase (ATPase), through a process called chemiosmosis

Complete Aerobic Catabolism of Glucose

• C6H12O6 + 6O2 + 36ADP + 36P → 6CO2 + 6H2O + 36ATP– (eukaryote)

• C6H12O6 + 6O2 + 38ADP + 38P → 6CO2 + 6H2O + 38ATP– (prokaryote)

Typical net energy yield:36 ATP for eukaryotes 38 ATP for prokaryotes

By-products of aerobic respiration are H2O and CO2

Substrate-Level Phosphorylation– 2 ATP (net gain Glycolysis)– 2 ATP (TCA cycle)– 4 Total from substrate-level

phosphorylation

Oxidative Phosphorylation– 6 ATP (NADH Glycolysis)– 28 ATP (NADH/FADH2 TCA

cycle)– 34 total from oxidative

phosphorylation

Total ATP gain ~ 36 to 38

Many compounds can serve as terminal electron acceptors

Anaerobic Respiration

• E.coli – Nitrate reduction– N03

- + 2e- + 2H+ N02-+ H20

• Paracoccus, Bacillus and Pseudomonas– Denitrification– N03

- N02- NO N2O N2

Nitrate reduction and ammonification

Denitrification in Paracoccus

• Desulfovibrio– Reduce sulfate– acetate + SO4

-2 + 3H+ 2CO2 + H2S + 2H2O

• Archaea – Methanogens that reduce carbonate– HC03

- + 4H2 + H+ CH4 + 3H2O

• Common– Not associated with any one phylogenetic group – Except methanogenesis

• Involves:– membrane system– generation of ion gradient – formation of ATP via ATP synthase

• Less efficient than aerobic respiration– Electron acceptors have less positive reduction

potentials than oxygen– lower energy yield

Fermentation

• Used by organisms that can’t respire– lack of suitable inorganic electron acceptor or

lack of electron transport chain• Anaerobic; Occurs in the cytoplasm

– Partial oxidation of substrate• NADH oxidized back to NAD+• Uses organic compound as terminal electron

acceptor– Typically pyruvate or derivative

• NO oxidative phosphorylation so ATP yield is low

• Lactic acid fermentation– pyruvate reduced to lactate – pyruvate accepts electrons and protons from NADH

• Alcohol fermentation– pyruvate decarboxylated to form acetaldehyde– NADH transfers electrons and protons to acetaldehyde

reducing it to ethanol

Catabolism of Other Organic Compounds

• Carbohydrates are the main energy source– glucose

• Microbes may also utilize lipids and proteins – Both must be broken down into their individual

components– Each component is oxidized separately

Lipid Catabolism

• Lipases• Fatty acids and glycerol

– Fatty acid converted into acetyl CoA, enters TCA cycle– Glycerol converted into DHAP, enters glycolysis

Protein Catabolism

• Proteases • Amino acids

– can NOT be catabolized directly– transamination– decarboxylation– dehydrogenation