Part 2 – TCA Cycle
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v Tricarboxylic Acid Cycle (TCA-cycle) is also called Citric Acid Cycle (CA-cycle) or Krebs Cycle, named after its
discoverer, Sir Hans Adolf Krebs. Please NOTE that TCA-cycle is taking place in mitochrondion, while Alcoholic Fermentation in cytoplasm. In TCA-cycle, each step is also catalysed
by a different enzyme, which is not mentioned in the image, but underneath for
the sake of easier reading and understanding.
v TCA-labyrinth
The TCA-cycle looks like a labyrinth, but “where there's
a will, there's a way”. If you take it under the magnifier step by step, you’ll
soon find the way out. Seeing is believing! First in cytoplasm, the 1st
(and the only) pyruvic acid is carboxylated to become oxaloacetic acid: CH3-CO-COOH + CO2 à COOH-CH2-CO-COOH
The 2nd pyruvic acid is decarboxylated, attached
to Coenzyme A and oxidized by NAD+ to become acetyl-CoA.
Both oxaloacetic acid and acetyl-CoA are transported into
mitochrondion where the TCA-cycle can begin.
v Total yield
EMP-pathway and TCA-cycle
Yeasts consume sugar (C6H12O6)
to obtain ATP for growth. In EMP-pathway, 1x glucose is transformed into 2x
pyrucic acid. That means 1x glucose needs 2x TCA-cycles to work it out. Via
decarboxylation, 1x TCA-cycle will remove 3x CO2 (at Acetyl-CoA formation and at step 5 and 6).
With 2x TCA-cycle, the total input of 1x glucose (C6 and O6)
are removed, along with another 6 oxygen, deriving from the metabolism. Via oxidation,
the captured hydrogen (H) , including H12 from the glucose, are
transported to the ETC (Electron Transport Chain) to produce 34 ATPs. Via Pi-phosphorylation,
2 ATPs in EMP-pathway and 2 ATPs in TCA-cycle are obtained. At best, the
metabolism of 1x glucose can lead to yield maximum 38 ATPs.
|
Glycolysis
|
step 1 and 3
step 7 and 10
step 6
|
2 x 1 ATP (used)
2 x 2 ATP
2 x 1 NADH (x3
in ETC)
|
2 ATP (-/-)
4
6 (1xNADH gives 3 ATP in the ETC)
|
|
Acetyl CoA
|
|
2 x 1 NADH (x3
in ETC)
|
6
|
|
TCA Cycle
|
step 4,6,10
step 7
stap 8
|
2 x 3 NADH (x3
in ETC)
2 x 1 ATP
2 x 1 FADH (x2
in ETC)
|
18
2
4
|
|
|
|
|
38 ATP in total
|
v Yeast growth in aerobic and anaerobic conditions
Under aerobic conditions yeasts produce 38 ATPs from each glucose and the
yeast growth is optimum. That's why at the beginning of the fermentation, the
must moves tumultuously and the temperature rises rapidly.
As the fermentation progresses, CO2 increases
and the oxygen diminishes. Under anaerobic conditions, yeasts yield only 2 ATPs
per glucose. The yeast growth is minimum, but the alcohol production is maximum.
v Yeast growth in 4 phases:
(a) Lag phase: During the first
hours the yeast population does not increase. The yeasts need to adapt to the
must conditions (high sugar level, low pH, temperature, and SO2 if
present). The normal initial population, if no yeasts are inoculated, is around
104 cells/ml. *(104 = 10 x 10 x
10 x 10 = 10000)
(b) Exponential growth
phase: Once the yeasts have adapted to the environmental conditions,
they multiply exponentially, increasing their population up to 107 –
108 cells/ml.This phase can
last from 2 to 6 days. The sugar concentration declines rapidly.
(c) Stationary phase: Oxygen is running
out and the yeast growth is practically halted. The yeast population remains
nearly stable. Practically all sugars are converted to ethanol now.
(d) Decline phase:
Sugar is running out. The increasing ethanol and other by-products are toxic to
them. The yeast population gradually decreases until it has almost completely
disappeared.
v Changes in
sugar concentration and alcohol content during fermentation
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P.S. I trust that you’ve got a good view of the alcoholic
fermentation already. Coming next month, Part 3 – ETC (Electron Transport
Chain), is the last piece of the fermentation puzzle. What is ETC? How ATPs are
synthesized, and why is oxygen needed? Don't miss it if you want to get the big
picture of the alcoholic fermentation. And as an added bonus, you'll acquaint
with two good friends in wine chemistry: OXIDATION and REDUCTION. Cheerio!
