Question

The concept of energy charge is a very useful way to think about how ATP levels control pathway activity. Discuss the role of ATP in the glycolytic pathway (between glucose and pyruvate and/or lactate) with a focus on how energy charge would alter the pathway and how the pathway activity would alter energy charge. Include both general aspects and specific ATP use and generation.

Answer 1

Glycolysis was one of the first metabolic pathways studied and is one of the best understood, in terms of the
enzymes involved, their mechanisms of action, and the regulation of the pathway to meet the needs of the
organism and the cell. The glycolytic pathway is extremely ancient in evolution, and is common to essentially all
living organisms. The earliest biochemical studies of glycolysis over 100 years ago used cell free extracts of yeast,
in which it was observed that glucose could be converted to carbon dioxide and ethanol in the same manner carried
out by intact yeast cells to make beer and bread. These experiments were the first to demonstrate that the
reactions of life were not inextricably tied to living cells but could occur in a cellfree
system, the foundation of
modern biochemistry.
In glycolysis, the sixcarbon
sugar glucose is oxidized and split in two halves, to create two molecules of pyruvate (3
carbons each) from each molecule of glucose. Along the way, the cell extracts a relatively small amount of energy
from glucose in the form of ATP, 2 ATP molecules collected for each glucose molecule that starts down the glycolytic
path. The pyruvate produced has one of three metabolic fates, to either become acetylCoA,
ethanol, or lactate.
When oxygen is available, the pyruvate can be converted to acetylCoA
and enter the Krebs Cycle, where the

acetylCoA
will be completely oxidized and generate ATP through oxidative phosphorylation. Fermentation is much
less efficient than oxidative phosphorylation in making ATP, creating only 2 ATP per glucose while oxidative
phosphorylation creates 36 ATP per glucose in mammalian cells. Oxidative phosphorylation does not work in the
absence of oxygen, however, and in the absence of oxygen glycolysis is forced to a halt due to a lack of NAD+,
unless NAD+ is regenerated through fermentation. In yeast, fermentation allows the yeast to continue producing
energy and survive in the absence of oxygen, producing ethanol and carbon dioxide from pyruvate. In mammalian
muscle, strenuous exertion can create conditions in which oxygen is consumed faster than blood can provide it,
forcing the muscle to use fermentation and create lactic acid in this case and make your muscles sore after a
workout.
There are ten enzymes that catalyze the steps in glycolysis that convert glucose into pyruvate, and the entire
pathway is located in the cytoplasm of eukaryotic cells. The activity of the pathway is regulated at key steps to
ensure that glucose consumption and energy production match the needs of the cell. The steps along the pathway
each involve a change in the free energy of the products and reactants, and as long as the overall change in free
energy is negative, the reaction continues forward, like water flowing down hill to its lowest energy point. The key
steps in the regulation of glycolysis, or any pathway, are those that catalyze the ratelimiting,
irreversible steps
along the pathway. In glycolysis in mammals, the key regulatory enzyme is phosphofructokinase, which catalyzes
the ratelimiting
committed step. Phosphofructokinase is activated by AMP and inhibited by ATP, among other
regulatory mechanisms. Thus, when ATP is low (and AMP is high), phosphofructokinase will be activated and
generate more ATP. Similarly, when ATP is abundant, phosphofructokinase will be inhibited to prevent wasting
glucose on making energy when it is not needed.
Although the glycolytic pathway was one of the first studied, it is still relevant to many issues faced in modern
biology. Failure to provide energy can have lethal consequences for cells the
absence of oxygen caused by a stroke
or a heart attack that prevents ATP generation can have lethal consequences for the cells involved. Cancer cells
often generate energy through glycolytic fermentation more than oxidative phosphorylation, suggesting that
manipulation of metabolism may provide a therapeutic strategy. Well known glycolytic enzymes such as
glyceraldehyde3phosphate
dehydrogenase may play roles in other cellular processes such as apoptosis. Future
studies may reveal additional functions of glycolytic enzymes.