Part 1 – Alcoholic Fermentation
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Glycolysis or Embden–Meyerhof–Parnas
(EMP pathway)
In alcoholic fermentation, the 6-carbon sugar molecule (glucose) is first converted into two 3-carbon molecules
of different structures, which are finally converted to two molecules of
ethanol. It happens in 12 steps, and each step is catalysed by a different
enzyme. The transformation from glucose to pyruvate is called
glycolysis
(glyco- comes from glucose, and –lysis means decomposition).
It is also called
Embden–Meyerhof–Parnas (EMP pathway), after their discoverers.
Step 1: Glucose phosphorylation, C6 has its –OH
group replaced by a phosphate group from ATP, which has become ADP. ATP (adenosine triphosphate) is a biochemical
energy source which contains 3 phosphate groups and ADP (adenosine diphosphate)
contains 2 phosphate groups. (*
For enzyme
that catalyses the step, see image above)
ATP ADP
Step 2:
C2 linked up with C1, the 6-angled glucose-6-p is transformed into a 5-angled fructose-6-p.
Step 3:
Phosphorylation C1 (the same as step 1).
Step 4:
1x fructose-1,6-diphosphate has been split into 1x dihydroxyacetone phosphate (DHAP)
and 1x glyceraldehyde-3-phosphate (GAP).
Step 5:
DHAP can be transformed into GAP, or vice versa.
Step 6:
NAD (nicotinamide adenine dinucleotide) is used as
oxidizing agent to remove one H-atom from C1 and one H-atom from C2. This is called oxidation in chemistry. A
Pi is attached to C1.
Pi is an inorganic phosphate group which
is present in cytoplasm of the cell. Note that it has an H-atom more than the
phosphate group from the ATP.
Step 7:
Dephosphorylation C1, the Pi is replaced by a
OH-group from ADP which has become ATP. The leftover H-atom is returned to C2.
Step 8:
C2 –OH group and C3 phosphate groups have their position exchanged
Step 9: H2O has split
off.
Step 10: Dephosphorylation
C2, ADP has become ATP. The O stays at C2 and H moves to C3.
Step 11: Decarboxylation C1, the leftover H-atom has joined C2.
Step 12:
NADH+H gave off H+H and has become NAD again. This is called
reduction in chemistry. The regained NAD will then be reused for step 6. As a matter of fact, for the yeast, the main purpose of this
step is to regenerate NAD, not to please the wine drinkers.
* NOTE: Step 12 will only happen in anaerobic conditions (=in the absence of
oxygen).
* In aerobic conditions (=in the presence of oxygen), acetaldehyde will be
attached to Coenzyme A and transported from cytoplasm into mitochondria to
generate ATP (energy), which is needed for the yeast multiplication. This will happen
in two steps: the
Tricarboxyylic Acid Cycle
(TCA cycle ) or
Citric Acid Cycle (see Part 2) and the
Electron Transport Chain (see Part 3).
Purpose
alcoholic fermentation
* For a winemaker, alcohol fermentation is aimed at producing wine.
* For yeast, alcoholic fermentation is a mean to produce energy for growth under
anaerobic condition.
Step 1-5 is the investing stage in which 2 ATPs are used for
phosphorylation.
Step 6-12 is the producing stage in which 2 ATPs are produced.
Since a glucose yields two ethanols, there is a net gain of 2 ATPs.
Yeasts
In spontaneous alcoholic fermentation different yeast
species may participate. In the early stages of alcoholic fermentation usually
predominate Candida,
Hansenniaspora
and Kloeckera.
Later, in the middle stages prevail Pichia and Metschnikowia. Finally, in the latter stages
of fermentation Saccharomyces
cerevisiae predominates because of its greater resistance to higher
concentrations of ethanol and sulphur dioxide. Some other yeasts, like Brettanomyces,
Kluyveromyces,
Schizosacchaomyces,
Zygosaccharomyces,
and Torulaspora may also be present, which may cause some
organoleptic defects. To prevent undesirable yeast developing, wineries add
sulphur dioxide to the must. Nowadays, most wineries inoculate selected dry
yeast (e.g. Saccharomyces cerevisiae) in order to guarantee alcoholic
fermentation without any deviation. There are many yeast cultures and each with
their own specific characteristics. By adding a particular type of yeast to the
must, the winemaker can affect the odour, taste or texture to a certain extent.
However, other wineries, especially traditional wine cellars, continue to use
spontaneous alcoholic fermentation because they believe it gives their wines
greater complexity.
Sugars
* In chemistry, sugars get the suffix –ose. Sugars of 3,4,5,6,
and 7 carbons are called resp. triose, tetrose,
pentose, hexose and heptose.
* In grapes, 98% of the sugars consist of glucose (48.5%) and fructose (49.5%) .
They both have 6 carbons, so they are hexose. They both have the same molecular
formula C6H12O6 , but they have a different structure.
* In open chain form they both have a D-form (Dextra = right) and an L-form (Levo = left), depending on the OH-group of C5 on the right or left.
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An
important basic rule is that C-atom always has 4 bonds, H-atom only 1 bond and O-atom always 2 bonds.
The –OH is actually –O–H, but the bond between O and H is often
omitted to show that it is a characteristic group.
In the molecules left, C1 and C6 have only 3 bonds. They have to
donate or to receive an H in order to create 4 bonds.
A single bond (–) is saturated,
it can no longer bind any atom.
A double bond (=) is unsaturated, it can bind another
atom.
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* In ring form, both the D-form and the L-form have a α-form or a
β-form, depending on the OH-group of the first angle stands up (=β ) or down(=α).
In grape juice the glucose and fructose are α- D-glucose and α- D-fructose.
Alcohols
In chemistry alcohols get the suffix
–ol. Alcohols of 1,2,3 or 4 carbons e.g. are called resp. methanol,
ethanol, propanol and butanol.
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P.S. - What do the yeast do in the
presence of oxygen? Part 2 – TCA-Cycle
will come next month.
