Fermentation Primer: Lesson III

In last month’s lesson we discussed how inefficient alcohol fermentationis at producing energy from sugar, versus consuming it for cell growth and reproduction. We also touched on why yeast may have evolved this ability, as a survival strategy and also explained how glucose is converted into pyruvate.

Q: So, what happens to pyruvate within the yeast?

Pyruvate can be processed by the yeast in one of two ways:

  1. In the presence of Oxygen (under aerobic conditions) it enters the Citric Acid/Krebs Cycle or
  2. In the absence of Oxygen (under anaerobic conditions) it enters the Alcohol Fermentation Pathway

This lesson focuses on the aerobic option and lesson IV will explore the anaerobic one.

In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes. The conversion is a three-step process.

  1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium.
  1. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme (pyruvate dehydrogenase). This is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice (Remember: there are two pyruvate molecules produced at the end of glycolysis) for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps.
  2. The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD+, forming NADH. The high- energy electrons from NADH will be used later to generate ATP.
  3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.

In the presence of oxygen, acetyl CoA delivers its acetyl group to a four- carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule. This single pathway is called by different names: the citric acid cycle (for the first intermediate formed—citric acid, or citrate—when acetate joins to the oxaloacetate), the TCA cycle (since citric acid or citrate and isocitrate are tricarboxylic acids), and the Krebs cycle, after Hans Krebs, who first identified the steps in the pathway in the 1930s in pigeon flight muscles.

The Citric Acid Cycle / Krebs Cycle

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria. Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion.

Unlike glycolysis, the citric acid cycle is a closed loop:the last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of redox, dehydration, hydration and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP and reduced forms of NADH and FADH2 (see diagramon the right). This is considered an aerobic pathway because the NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur.

Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.

Join us again next month, as we continue our deep-dive into this fascinating world!

Source: Creative Commons (creativecommons.org) and Khan Academy (khanacademy.org)