Carbohydrate Creation and Utilization

This is part two of a series covering yeast's metabolism of sugars. The first part (renamed for more catchy-ness) can be found here.

In this section we will discuss the synthesis and degradation of storage carbohydrates within a cell. For medically minded people, these processes are used to balance blood glucose levels within humans (and most mammals). First we need to familiarize ourselves with the nomenclature.

Maltose and trehalose, both made
 from two glucose molecules:
maltose has an α(1→4) bond,
trehalose has an α(1→1)α bond.

Monosaccharides are the simplest carbohydrates. What's a carbohydrate? Carbo- containing carbon; hydrate - a compound in which water (H₂O) is bound to other elements; hence C_X(H₂O)_X, where X > 3 (typically), is the chemical formula for most carbohydrates. Examples include glucose, fructose, galactose, etc..

Disaccharides are two monosaccharides linked together, remember your greek δίς - two. Disaccharides form via dehydration (the removing of a water molecule), and can be linked on any hydroxyl group (-OH). This results in several types of compounds being possible from the same two monosaccharides. 

Polysaccharides would obviously contain many individual monosaccharides linked together.

To form very complex carbohydrates it is necessary to put branches in this ever-growing chain of monosaccharides. 

Ok, now on to the show.

Glyconogenesis/glycogenolysis

When yeast are first introduced into a new environment, they break up internal glycogen (a very large chain of glucose molecules) reserves to produce their glucose without transporting external glucose across the cell membrane in order to reach a stable, internal osmotic pressure before the onset of full fermentation.

In order to make the glycogen reserves that yeast need, glucose-6-phosphate (the product of the first step of glycolysis) is acted on by the enzyme phosphoglucomutase to form glucose-1-phosphate.

Then, the enzyme uridyl transferase forms UDP-glucose and pyrophosphate (which is broken up into two phosphate groups by the enzyme pyrophosphatase; these P_i groups are used in many metabolic processes).

Glycogen synthase then assembles smaller glycogen molecules using UDP-glucose and preexisting glycogen molecules or glycoginin (a "seed" protein). Larger glycogen molecules are created by adding branches of smaller glycogen molecules utilizing the enzyme transglycosylase (α(1,4 →1,6))

To utilize glycogen as an energy source, large glycogen molecules are broken by a different transglycosylase (α(1→ 4)) enzyme into smaller glycogen molecules. The enzyme glycogen phosphorylase then forms glucose-1-phosphate from the terminal glucose molecules of a chain with the addition of a phosphate group. This process will work down the chain until 4 glucose molecules are left before the branch point, at which point it is joined to the terminal position of another chain. Phosphoglucomutase then transforms these glucose-1-phosphate molecules into glucose-6-phosphate (the opposite reaction of the first step of glycogenesis) that can be used directly in glycolysis.

Trehalose synthesis
D-glucose-6-phosphate can also be directed down a different pathway to produce trehalose. UDP-glucose and D-glucose-6-phosphate are the substrates that trehalose-6-phosphate synthase acts on to produce UDP and trehalose-6-phosphate. Trehalose-6-phosphate phosphatase then cleaves the phosphate group off of T6P  producing a free phosphate group (that can be used in further metabolic processes) and trehalose.

This pathway has several benefits: it consumes ATP which creates a loss of energy, in turn driving glycolysis; it stabilizes cell membrane structures which helps protect the cell from temperature swings; it helps prevent damage to membranes by its ability to prevent phase transition events in lipid bi-layers; it prevents glycolysis from progressing too rapidly by diverting phosphorylated sugars used in glycolysis; it also exerts a restrictive control on the influx of sugar by resticting hexokinase activity. These last two act as flow-valves by limiting the amount of glucose that can enter glycolysis to moderate (healthy) levels preventing stalled metabolism that can occur when glycolysis progresses too quickly.

Takeaways

• Only small portions of glucose-6-phosphate are diverted to either of these paths
• The yeast will utilize glycogen during lag phase for its sole carbon source while it trys to reach equilibrium with the external osmotic pressure
• Trehalose is vital for yeast health, especially when put under external temperature stress or changes in osmotic pressure

The importance of a functional trehalose biosynthetic pathway for the life of yeasts and fungi. 2003

Reserve carbohydrates metabolism in the yeast Saccharmyces Cerevisiae. 2000

The role of trehalose synthesis for the acquisition of thermotolerance in yeast. 1994

(P.S. sorry for this crappy bibliography, but i didn't take great notes on sources, and really don't have the time. Also, sorry about only one picture, it takes time to make them; I promise the next post will be more . . . colorful)