Transcript Fermentation and Respiration

Fermentation and Respiration
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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
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 lacticacid 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