BC 351 Module 9

Discuss the outlook for targeting metabolic changes in cancer cells for treating cancer patients.

Targeting of metabolic changes in cancer cells with metabolic enzyme inhibitors, etc. like those described in the table above is promising for anticancer therapies. They may be particularly useful in combination with drugs exploiting other changes found in cancer cells. However, just like other changes in cancer cells, metabolic changes are often specific to cancer types (lung, skin, pancreatic, colon, etc.) and specific cancers among these types. Therefore, it will be important to develop means to determine the metabolic profiles typical of cancer types and for specific cancers to effectively target them through their metabolic changes.

Does the overall process have a positive or negative change in free energy?

The overall process to go must and does have an over all negative free energy change. The negative free energy change of two Na+ ions flowing across the plasma membrane down the electrochemical gradient is more negative than the positive free energy change of transporting one glucose molecule into the cell against the concentration gradient.

Compare symport and antiport cotransport systems.

A symport system transports two solutes in the same direction while an antiport system transports two solutes in the opposite direction; i.e., it acts as an exchange system.

Compare passive (channel and transporter), primary active, and secondary active transport.

Passive transport proceeds down a concentration gradient with the release of free energy. Active transport proceeds against a concentration gradient and requires the input of energy. Primary active transport uses an energy source, like ATP or light, directly. It often functions to create an electrochemical gradient. Secondary active transport uses an electrochemical gradient created by a primary active transporter.

Do you think a glucose gradient could be used to drive Na+ pumping with this enzyme and why or why not?

Probably not, since the conformational changes that occur do not include changes in the affinity of the transporter for Na+. Otherwise, if they did, any enzyme should theoretically be able to catalyze the forward or reverse reaction and which reaction catalyzes is only dictated by the overall free energy change.

Diagram or describe the mechanism or primary active transport (antiport) by the Na+/K+ ATPase (Na+ pump).

The E1 form of the Na+ pump has the Na+ and K+ binding sites facing the cytoplasm and has a high affinity for Na+ and a low affinity for K+. When E1 is bound by Na+ in the cytoplasm, this triggers its self-phosphorylation using ATP. The phosphorylated E1 changes conformation (phosphorylation is an allosteric regulator) to become the E2 form. The E2 form has the binding sites facing the exterior of the cell and has a low affinity for Na+ so these ions are released outside the cell and has a high affinity for K+ so it binds K+ ions. K+ binding triggers dephosphorylation, which leads to an allosteric change back to the E1 form. The E1 form has the binding sites facing the cytoplasm and has a low affinity for K+ so these ions are released and a high affinity for Na+ starting the cycle over.

Explain how a symport secondary active transport system can use the energy of a Na+ gradient to accomplish the active transport of glucose or an amino acid.

The Na+/K+ ATPase maintains a large electrochemical gradient of Na+ ions across the plasma membrane. Thus, a symport system that transports Na+ ions and a second solute (in the same direction) will derive energy from the movement of Na+ ions down an electrochemical gradient. This system can be used to accomplish the active accumulation of the second solute against a concentration gradient. The negative ΔG of the flow of Na+ ions down the gradient must be more negative than the positive ΔG of the pumping of the second solute, for example glucose, against the concentration gradient is positive. In other words, the net ΔG of the two coupled reactions must be negative.

Otto Warburg and his coworkers first described "aerobic glycolysis", which has come to be known as the Warburg effect. Describe aerobic glycolysis, in terms of the basic changes that occur in cancer cell metabolism relative to normal somatic cells.

The Warburg Effect or aerobic glycolysis refers to cells exhibiting increased glucose uptake and lactate production in cancer cells. This has been determined to be due to a high rate of glycolysis, converting glucose to pyruvate. However, pyruvate conversion to acetyl CoA is inhibited and its conversion to lactate by lactate dehydrogenase is up regulated. Since Warburg's group first described this phenomenon cancer cells have also been shown to be highly dependent on glutamine as a precursor for nitrogen containing compounds and for TCA cycle anaplerotic (filling up) functions. This is the basis for PET scans with radiolabeled glucose for detecting tumors in patients. It has only recently begun to be exploited as an "Achilles heel" for anticancer therapies.

How is the negative free energy change (-ΔG) of Na+ passage into the cell tied (physically) to transport of glucose against a concentration gradient?

The binding of Na+ ions allosterically converts the transporter to a form with a high affinity for glucose so that it binds glucose even at the lower concentration found outside the cell. Glucose binding causes a second conformational change so that the binding sites are now facing the cytoplasm and the lower Na+ concentration, leading to its release and a second conformation change to a form with a very low affinity for glucose. This allows glucose release at the relatively high concentration in the cytoplasm. Thus, Na+ ions flowing down their concentration gradient can lead to and power conformational changes that result in pumping of glucose against a concentration gradient.

Describe the three main types of metabolic pathways, indicating those that involve oxidation and those that involve reduction of metabolites. How are they connected and interrelated?

The three main types of metabolic pathways are catabolic (oxidation), anabolic (reduction) and amphibolic. Catabolic pathways couple the oxidation of nutrients to the production of ATP. Anabolic pathways couple to hydrolysis of ATP to the biosynthesis of macromolecules. Amphibolic pathways can do both. The catabolic pathways are connected with the anabolic pathways by way of ATP and NADH, which catabolic pathways produce and regenerate, for the anabolic pathways to use.

Describe the two examples of normal cells exhibiting aerobic glycolysis discussed in Lecture 31.

The two examples of "normal" aerobic glycolysis are cells in early mammalian embryos up to the 16-cell stage and in brain cells. Early embryonic cells are dividing rapidly, suggesting that aerobic glycolysis may be an attribute of rapidly dividing cells and perhaps providing an important function useful to cancer cells. In the brain, astrocytes and neuron cells form a commensal metabolic relationship. Astrocytes carry out aerobic glycolysis producing lactate. Nerve cells take up the lactate, convert it back to pyruvate and oxidize it through the TCA cycle and produce ATP by oxidative phosphorylation. Astrocytes also function to take up glutamate and convert it back to glutamine, which nerve cells release as a neurotransmitter. Glutamate is toxic to nerve cells at high levels and glutamine is an important nutrient.