Question

"In many different species, lactate dehydrogenase (LDH) constitues a major checkpoint of anaerobic glycolysis, by catalyzing the reduction of pyruvate into lactate. This enzyme has recently received a great deal of attention since it may constitute a valid therapeutic target for diseases so different as malaria and cancer. In fact, the isoform expressed by Plasmodium falciparum (pfLDH) is a key enzyme for energy generation of malarial parasites. These species mostly depend on anaerobic glycolysis for energy production, since they lack a citric acid cycle for ATP formation. Therefore, several small organic molecules have been recently designed and developed with the aim of blocking this new potential antimalarial chemotherapeutic target. Moreover, most invasive tumor phenotypes show a metabolic switch (Warburg effect) from oxidative phosphorylation to an increased anaerobic glycolysis, by promoting an upregulation of the human isoform-5 of lactate dehydrogenase (hLDH-5 or LDH-A), which is normally present in muscles and in the liver. Hence, inhibition of hLDH-5 may constitute an efficient way to interfere with tumor growth and invasiveness."

a. Why is conversion of pyruvate to lactate necessary for glycolysis under anaerobic conditions? Why is there no conversion of pyruvate to lactate under aerobic conditions? Explain your answer.

b. Which glycolytic step would be affected by lactate dehydrogenase inhibitors ? What glycolytic intermediate(s) would accumulate in the presence of lactate dehydrogenase inhibitors? Explain your answer.

c. Explain why the presence of lactate dehydrogenase inhibitors may cause mortality of P. falciparum and interfere with tumor growth and invasiveness?

Answer 1

a) Under anaerobic conditions, when sufficient oxygen is not present, pyruvate and NADH produced in glycolysis are further converted to NAD+ from NADH by reduction of pyruvate to lactate. Lactate is converted back to pyruvate by the enzyme lactate dehydrogenase. This pyruvate can then undergo anaerobic respiration to give ethanol and CO2.

In aerobic conditions, since sufficient oxygen is available, pyruvate formed is directly used to produce ATP by citric acid cycle. Pyruvate is converted to Acetyl-CoA, which enters the TCA. Since this can directly happen, there is no lactic acid formation. Besides, the aerobic mode of respiration is far more efficient than the anaerobic mode deneration 38 ATP as opposed to 2 ATP from the anaerobic mode.

b) Glycolytic step number 4 i.e. conversion of Glyceraldehyde -3- Phosphate and Dihydroxyacetone phosphate will be affected and these glycolytic intermediates will accumulate along with pyruvate. This is because enzyme LDH converts pyruvate and NADH to Lactate and NAD+. In presense of its inhibitors, this step will not take place leading to accumulation of pyruvate. Further, since NAD+, which is produced in the LDH-catalyzed reaction, is required by glycolytic enzyme glyceraldehyde- 3-phosphate dehydrogenase (which promotes the conversion of glyceraldehyde-3-phosphate to 1,3-biphosphoglycerate), and is necessary for the regular continuation of glycolysis, LDH inhibitors will lead to an accumulation of NADH, not allowing its convertion to NAD+ by LDH. Thus the glycolytic intermediates, Glyceraldehyde -3- Phosphate and Pyruvate will accumulate.

c) ATP is the energy currency of the cell. Since the organism P. falciparum solely depends on anaerobic mode of respiration which depends on lactate fermentation, an inhibition of LDH will not allow any energy production. The glycolytic intermediates will accumulating and the pathway will not proceed ahead. A deficit in energy production and excessive accumulation of glycolutic intermediates will inhibit its proliferation and disable various cellular functions, ultimately causing its death.