Question

1. A decrease in oxidative phosphorylation also leads to a decrease in reactive oxygen species (ROS). Why might this be an advantage for cancer cells?
2. What are the three major metabolic control points in glycolysis? What generic reaction do these enzymes perform? Why are these enzymes the ones most regulated?
3. Hexokinase II (the isoform usually discussed in biochemistry classes as part of glycolysis) catalyzes the first step in glycolysis. The isoform of hexokinase that is expressed in most tumor cells is bound to the mitochondrial outer membrane facing the cytosol. Propose two reasons why this would be beneficial to the tumor cell.
4. Another highly regulated enzyme in glycolysis is phosphofructokinase (PFK1). There is evidence from some tumors that PFK1 undergoes posttranslational proteolysis that yields a truncated functional enzyme (47 kD vs 85 kD) that is insensitive to citrate and ATP. How might this work and why would it be beneficial for a tumor cell?

Answer 1

Decrease in oxidative phosphorylation also leads to decrease in Reactive Oxygen Species (ROS) level within the cancer cells. Increased ROS level has a damaging effect on the cells (whether it is normal or abnormal), it is involved in DNA damage, oxidation of lipids and protein, inactivation of enzymes either by oxidative damage to its protein part or oxidising the cofactor it requires for its activity (for example elevated ROS level can inhibit the glycolytic enzyme within the cancer cells). So the decreased level of ROS might be beneficial for cancer cells as it will protect the cancer cells from various kinds of damaging effect of ROS.

The three major metabolic control points in glycolysis are Hexokinase2, Phosphofructokinase1 and Pyruvate kinase. Hexokinase phosphorylates glucose forming glucose 6-phosphate, Phosphofructokinase1 convert fructose 6-phosphate to fructose 1,6-bisphosphate, and Pyruvate kinase catalyse conversion of phosphoenolpyruvate to pyruvate. These enzymes are most regulated because they catalyse the irreversible steps of the glycolysis all other steps are reversible so the activity of these enzymes control the fate of glycolysis.

Hexokinase II binding to mitochondrial membrane inhibit the release of intermembrane space proteins involving in apoptosis, thus inhibiting apoptosis. Hexokinase II upregulation also help upregulate glycolysis which help to meet the energy demand of cancer cells.

Phosphofructokinase (PFK1) that is truncated might have change in the site where these inhibitors bind to and inactivate the enzyme. Phosphofructokinase (PFK1) that is insensitive to citrate and ATP will be beneficial for a tumor cells as it will not be inhibited by ATP produced during glycolysis and citrate the product of TCA cycle. Thus the glycolysis rate will not be affected by the level of ATP and Citrate within the cancer cells.