Lesson 4 - Yeast Biochemistry
This lesson describes the biochemical pathways that explain the activities of yeast in brewing. It begins by looking at what metabolism is and how we control it. It then looks at how nutrients, including sugar, are taken up by brewing yeast. Next, it explores metabolism in more detail, focusing on sugar metabolism and looking at two particular modes of metabolism, respiratory and fermentative. Finally, it considers the need for redox control and takes a brief look at gluconeogenesis and the storage of carbohydrates in yeast.
Gluconeogenesis
We saw earlier in this lesson that most of the steps in glycolysis are reversible. This section explains why this is so.
A key pathway like glycolysis might look like it would only work in one direction, towards pyruvate. In brewing fermentations this is true; however, as we have seen, yeast cells have a very flexible metabolism and under some circumstances the flow of glycolysis is reversed.
This is called gluconeogenesis, literally ’the making of new glucose’. It does not happen in brewing because it needs oxygen and no glucose for it to occur. In other words, the cells need to be derepressed.
If we grew brewing yeast on a medium that had a lot of nutrients and oxygen but only a small amount of sugar, the sugar would quickly run out. Growth would stop, but only for a period during which the cell re-orders its genome to deal with the new situation. Perhaps surprisingly, the new phenotype enables the cell to grow on the alcohol that it made earlier using the sugar and there is a second burst of growth. This is called diauxy, meaning ‘two periods of growth’.
The cell uses the respiratory pathways, and a set of genes are turned on that produce enzymes which bypass the irreversible steps in glycolysis and so glucose is synthesised. The same thing happens with a lot of other substrates, such as glycerol, as the sole carbon source. These are called oxidative carbon sources. A relatively common mutation in some brewing yeast strains is that they lose some mitochondrial functions such as oxidative phosphorylation. These produce very small colonies called ‘petites’ on glucose media. We can test for these because they cannot grow on oxidative substrates such as glycerol.