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Fermentation and Respiration. Embden-Meyerhof (glycolysis) Fermentation products Respiration and electron transport Electron-transport phosphorylation Citric acid cycle Energy production from fermentation and respiration Alternate modes of energy generation. Overview of fermentation.
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Fermentation and Respiration • Embden-Meyerhof (glycolysis) • Fermentation products • Respiration and electron transport • Electron-transport phosphorylation • Citric acid cycle • Energy production from fermentation and respiration • Alternate modes of energy generation
Preparatory reactions for glycolysis • Refer to figure 4.32 (p. 119) • Preparatory reactions add phosphates (from ATP) • Fructose(1,6)diphosphate is the common intermediate for hexose fermentation • Two phosphates needed so that after splitting, each 3-carbon unit is ionized
Oxidation Reactions • After splitting 6-carbon fructose into two 3-carbon glyceraldehyde-3-phosphate • Key reaction: oxidation (NAD) plus phosphorylation yields 1,3-diphosphoglycerate • Phosphoryl transfer to ADP, making ATP • Isomerization yields phosphoenol pyruvate • Phosphoryl transfer (ATP) from phosphoenol pyruvate yields pyruvate
Fermentation Reactions • Electrons from NADH must be passed on • Homolactic fermentation (muscles; most lactic-acid bacteria) • Simplest fermentation • pyruvate is reduced to lactate • Alcoholic (yeasts; uncommon in bacteria) • Pyruvate + NADH -> *acetaldehyde + CO2 + NADH -> ethanol + CO2 • In any fermentation, products must balance reactants (C, H, O); C6H12O6 -> 2 x C3H6O3
Phosphoroclastic Reaction • This reaction is not a balanced fermentation • Pyruvate + NADH -> acetyl-CoA + HCOOH + NADH -> acetate + CO2 + H2 + NADH (phosphoroclastic reaction) • Must be coupled with another, NADH-consuming, reaction • Acetyl-CoA is transferred to phosphate (acetyl phosphate), and the phosphate is then transferred to ADP, making ATP and acetate • This reaction provides an extra ATP, but doesn’t dispose of extra electrons (NADH) from early oxidation reactions
Reduction of Pyruvate • Reduction reactions must balance phosphoroclastic reactions • Ethanol production (4-electron reduction [or 2NADH] of acetate) • Pyruvate + 2NADH -> acetate + HCOOH + 2NADH -> ethanol + HCOOH + NAD+ • 2 pyruvate + 2NADH -> acetate + ethanol + 2HCOOH + 2NAD+ + ATP • This is a dominant reaction of the mixed acid fermentation (enteric bacteria such as Escherichia coli)
Gas Production in the Mixed Acid Fermentation • Gas formation • At low pH (< 6), many enteric bacteria produce formate-hydrogen lyase • HCOOH -> H2 + CO2 • H2 is major gas detected by Durham tubes; CO2 is also detected when produced rapidly; when produced slowly it can dissolve (e.g., in the gut)
Other Reductions in Mixed Acid Fermentation • Reductive carboxylation to succinate • Acetoin (butane diol) fermentation • Overview: • Lactate (minor product, as in homolactic) • Formate or H2 + CO2 (major product) • Acetate (important product, but must be balanced by equimolar amounts of ethanol, succinate, or butane diol) • Yields an extra ATP for each acetate formed
Acetoin Fermentation • Part of mixed acid fermentation; dominant fermentation by some (e.g., Enterobacter) • Also called butylene glycol or butane diol fermentation • Pyruvate + NADH -> *aldehyde + HCOOH + NAD+ • *aldehyde + pyruvate -> acetolactate -> CO2 + acetoin (2-keto-3-hydroxy-butane) • Acetoin + NADH -> 2,3-butanediol + NAD+
Other Fermentation Products • Reductive pathways to balance phosphoroclastic reaction • Propionate fermentation • Butyrate fermentation
