Fermentations

NADH must be oxidized to NAD+ in order to oxidize glyceraldehyde-3-P

In the absence of an electron transport chain pyruvate or a derivative serves as the electron acceptor for NADH

Can lead to the production of some ATP

Commonalities of fermentations

NADH is oxidized to NAD+

The electron acceptor is often pyruvate or a pyruvate derivative

The substrate is partially oxidized

Commonalities of fermentations

ATP is produced by substrate-level phosphorylation

Oxygen is not needed

Fermentations

Different fermentations are often characteristic of particular microbial groups

Alcoholic fermentations

Many fungi and some bacteria, algae and protozoa ferment sugars to ethanol and CO2

Pyruvate is decarboxylated to form acetaldehyde

Acetaldehyde reduced to form ethanol

Lactic acid fermentation

Carried out by bacteria, fungi, algae, protozoa and animal muscle cells

Pyruvate is reduced to lactate

Homolactic fermenters

Reduce almost all their pyruvate to lactate using lactate dehydrogenase

Heterolactic fermenters

Form substantial amounts of products other than lactate

Products include lactate, ethanol and CO2

Formic acid fermentation

Pyruvate converted to formic acid and other products

2 types of formic acid fermentations:

1. Mixed acid fermentation

2. Butanediol fermentation

Mixed acid fermentation

Results in the production of ethanol and a mixture of acids including acetic, lactic, succinic and formic acids

Formic hydrogenlyase will degrade formic acid to H2 and CO2

Occurs in Escherichia, Salmonella, Proteus and other genera

Butanediol fermentation

The second type of formic acid fermentation

Pyruvate converted to acetoin

NADH reduces acetoin to 2,3-butanediol

Large amounts of ethanol and small amount of mixed acid fermentation acids also produced

Butanediol fermentation

Characteristic of Enterobacter, Serratia, Erwinia and some species of Bacillus

Voges-Proskauer test

Differentiates between mixed acid fermenters and butanediol fermenters

Detects acetoin

Positive for butanediol fermenters and negative for mixed acid fermenters

Methyl red test

Mixed acid fermenters acidify media to a greater extent than butanediol fermenters

Change in color from red to yellow indicates pH has dropped below 4.4 and is read as positive

Mixed acid fermenters are positive in the methyl red test

Stickland Reaction

Some bacteria obtain energy from the fermentation of amino acids

One amino acid is oxidized and another is reduced to regenerate NAD+

Acetate, CO2, NH3 and ATP are generated

Stickland Reaction

Many amino acids can be fermented by this reaction

Amino acids can be fermented by other mechanisms besides the Stickland reaction

Fermentations

Many commercial products are the result of fermentation reactions

Alcoholic beverages and bread (alcoholic fermentation)

Yogurt, Sauerkraut, Pickles (lactic acid fermentation)

Fermentations

The tricarboxylic acid cycle

Represents stage 3 of catabolism

Most of the energy from the complete oxidation of glucose is released in the TCA cycle

Also known as citric acid cycle or Kreb’s cycle

The tricarboxylic acid cycle

Pyruvate is first oxidized, decarboxylated and joined to CoA to form acetyl-CoA

Acetyl-CoA combines with oxaloacetate to form citrate

The tricarboxylic acid cycle

Cycle broken down into three stages:

6 carbon stage

5 carbon stage

4 carbon stage

The 6 carbon stage

6 carbon citrate decarboxylated and oxidized to form -ketoglutarate

The 5 carbon stage

5 carbon -ketoglutarate is decarboxylated, oxidized and joined to CoA to form succinyl-CoA

The 4 carbon stage

4 carbon succinyl-CoA produces GTP by substrate-level phosphorylation and forms succinate

Succinate oxidized by FAD to form fumarate

The 4 carbon stage

Fumarate malate oxidized by NAD+ to form oxaloacetate

Oxaloacetate starts cycle again

The tricarboxylic acid cycle

Catabolism of carbohydrates, lipids and amino acids results in the production of acetyl-CoA which can be oxidized in the TCA cycle

One molecule of acetyl-CoA yields 3 NADH, 1 FADH and 1 GTP

The three stages of catabolism (organic molecules)

Little ATP synthesized

Oxidation of glucose to 6 CO2 4 ATP

Most ATP comes from oxidation of NADH and FADH2 in the electron transport chain

Fermentation, aerobic and anaerobic respiration

Differ regarding the final electron acceptors

Electron transport chains

Eukaryotic and prokaryotic electron transport chains differ regarding their electron carriers and the details of construction

Both operate according to the same basic principles

Electron transport chains

Electrons move from a carrier with a lower standard reduction potentials (EO) to a carrier with a higher EO

Electron transport chains

Electrons move from a carrier with a lower standard reduction potentials (EO) to a carrier with a higher EO

Mitochondrial electron transport chain

Electrons pass from NADH to FMN in complex I

Electrons from succinate can be passed to FAD in complex II

Both complexes pass electrons to Coenzyme Q (ubiquinone)

Mitochondrial electron transport chain

Coenzyme Q passes electrons to complex III

Electrons passed to cytochrome c then to complex IV

Mitochondrial electron transport chain

Electrons eventually combine with 1/2 O2 and 2 H+ to form H2O

Protons pumped across the membrane at various points during electron transport