Part 2 - TCA Cycle

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 1^st (and the only) pyruvic acid is carboxylated to become oxaloacetic acid:   CH₃-CO-COOH + CO₂ à COOH-CH₂-CO-COOH

The 2^nd 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!