BACKGROUND KNOWLEDGE

Glycolysis

Introduction

The function of glycolysis is quite simple and straight forward. Attention should be focused on the regulation of the pathway. Remember that all the steps in glycolysis are fully reversible. The pathway of glucose to pyruvate involves the break down of glucose, so this pathway is called glycolysis. The reverse direction, from pyruvate to glucose is called gluconeogenesis.

There are 4 key enzymes which are involved in the regulation of glycolysis. We will go into more detail of all of them and point out interesting effects that affect the human body. This will hopefully help you to remember some key molecules which you have encountered in the "essential knowledge" section.

Look at diagram 1-02 to see the key molecules with their key enzymes. Compare this diagram with figure 1-01 to see the more important molecules, which you should not need for your examinations. Remember where the enzymes of glycolysis are found within the cell!

Hexokinase

This enzyme is involved in the conversion of glucose into glucose-6-phosphate G6P. This helps the cell to maintain a diffusion gradient into the cell at high blood glucose concentrations and prevents the diffusion out of the cell at low blood glucose concentrations.

There are 3 different types of this enzyme. Hexokinase I-III are found in most tissues of the body, while hexokine IV (called glucokinase) is found within the liver. The difference between glucokinase and hexokinase I-III is that glucokinase only works at high glucose concentrations.

When blood glucose levels are high, most tissues including brain and muscles will have enough glucose available for uptake. As the blood glucose levels have to return to normal again (remember homeostasis), the liver takes up the excess glucose by using glucokinase.

When blood glucose levels are low, most tissues, especially the brain, require glucose for respiration, may it be anaerobic or aerobic. There is a lot of glucose stored in the liver, therefore the liver does not need to take up any glucose during starvation. Instead, the liver glucokinase is inhibited (as it only works in high concentrations) and glucose is left for the brain and other tissues to allow them to continue with their work.

During starvation, the liver also releases glucose by gluconeogenesis (the reverse of glycolysis). The glucose can then be released without being taken up again by the glucokinase enzyme.

Type I-III hexokinase are regulated by allosteric inhibition. If there is an accumulation of G6P, the enzyme is inhibited. G6P accumulates if there is too much ATP available and no more ATP is needed by the cell. If the cell does not need ATP then it also does not need to take up more glucose. Therefore, by inhibiting hexokinase I-III, glucose stays in the blood and can be transported to the liver, where glucokinase is activated because of high glucose levels. Glucose is then stored in the liver. As you might have noticed, glucokinase is active at high glucose concentrations, so this particular enzyme is not affected by allosteric inhibition.

Phosphofructokinase PFK

This enzyme is also a key regulator of glycolysis. The enzyme promotes glucose production, therefore gluconeogenesis, at low blood glucose levels as glucagon binds to the cell. Remember that the hormone glucagon is released into the blood if glucose levels are low. Insulin is released when glucose levels are high. It directs the enzyme towards glycolysis, therefore making pyruvate.

Phosphoglycerate Kinase

1,3-BPG is directly converted to 3-Phosphoglycerate.

1,3-BPG is also converted to 2,3-BPG by the enzyme BPG mutase. 2,3-BPG is then converted to 3-Phosphpglycerate by 2,3-BPG phosphatase. This is an important reaction in association with red blood cells. Red blood cells do not have any cell organelles and therefore cannot respire aerobically, even though they have got loads of oxygen bound to them.

2,3-BPG is a negative regulator for haemoglobin affinity for oxygen. The more 2,3-BPG is present in the cytoplasm of a red blood cell, the lower is the afinity for oxygen. We will come back to this feature below, when looking at pyruvate kinase.

Pyruvate Kinase (PK)

Converts phosphoenolpyruvate into pyruvate. There are 2 types of PK, an adult type and a fetal type.

The fetal type of PK has a greater activity than the adult type of PK. Glucose is quickly converted to pyruvate and there are hardly any intermediates accumulating during glycolysis. Therefore, there is less 1,3-BPG present, as it it quickly converted into its next intermediate. Consequently, less 2,3-BPG is formed and the affinity for oxygen is higher. Fetal red blood cells have a higher affinity for oxygen than maternal red blood cells. The advantage of this is that oxygen can be transported across the placenta in a diffusion gradient.

The adult type has a lower activity of PK. Intermediates of glycolysis are more likely to develop and stay in the cell for a longer period of time. 1,3-BPG is also converted to 2,3-BPG which accumulates and causes the affinity for oxygen to decrease. Adult red blood cells have a low oxygen affinity.

Anaerobic Respiration

Provides a faster formation of ATP than aerobic respiration. Red blood cells depend on thsi pathway as there no organelles present in these cells.

Pyruvate is converted to lactic acid by the enzyme LHD. This produces 2NAD+ molecules which are required for the continuation of the glycolytic pathway for the formation of ATP. This is clearly shown in diagram 1-01.

Lactic acid is transported out of the cell and taken up by aerobic tissues again, so that it can be converted to pyruvate again, also by the enzyme LHD. Therefore, the reaction is reversible. The more ATP is formed, the higher will be the release of lactic acid. Lactic acid, as the name suggests, is acidic and lowers the blood pH. An accumulation of excess lactic acid can therefore have severe consequences on the body systems.

Figure 1-02: Key enzymes in the glycolytic pathway.