All metabolic pathways are regulated. This means that the flow of metabolites through the pathway can either be increased or decreased in response to the metabolic needs of the organism. The steps that are the best candidates for regulation are those that have large negative free energy changes.

Recall that equilibrium occurs when the free energy change of a reaction, symbolized by delta G, is zero. When a reaction is near equilibrium, it is as likely to go forward as it is to go in the reverse direction, and there isn’t much to control. Reactions with large negative free energy changes are far from equilibrium. Such reactions are essentially irreversible. They are the major control points of glycolysis.

Think about marbles on a level surface. Like reactions close to equilibrium, there is no need to control their movement, because the marbles aren’t really going anywhere. But what happens when the marbles are on a steep incline? Without a barrier to control their movement, all the marbles quickly roll downhill.

It’s much the same way for metabolic reactions that are far from equilibrium. They are the only ones that are affected by regulation. In glycolysis, reactions 1, 3, and 10 all have large free energy changes. So these are the regulated reactions. The activity of the enzymes that catalyze these reactions is either increased or decreased under specific conditions.

Let’s start with reaction 1. Reaction 1 is catalyzed by the enzyme hexokinase. The activity of hexokinase is inhibited by glucose-6-phosphate. Since glucose-6-phosphate is also a product of the reaction, this is an example of feedback inhibition. Feedback inhibition is the inhibition of an enzyme by a reaction product. As more glucose-6-phosphate is produced, the reaction rate slows down. Hexokinase governs the rate-limiting step of glycolysis in the brain and in red blood cells.

In most cells, glycolysis is regulated during reaction 3. This is important whenever the rate-limiting hexokinase reaction is bypassed, such as in the break down of glycogen, which leads to the formation of glucose-6-phosphate rather than glucose. Like reaction 1, reaction 3 results in a large negative free energy change, and ATP is consumed. Reaction 3 commits the cell to completing the glycolytic pathway. It acts as the valve that controls the flow of substrate through the remaining steps.

Reaction 3 is catalyzed by the enzyme phosphofructokinase or PFK. Both activators and inhibitors regulate PFK. Activators increase the enzyme’s activity, whereas inhibitors decrease its activity. In glycolysis, ATP inhibits PFK activity. This makes sense since the purpose of metabolizing glucose is to generate ATP. If enough ATP is already present, there is no need to make more. The product of reaction 9, PEP, also inhibits PFK activity. Even though these molecules aren’t the direct products of reaction 3, they are both examples of feedback inhibition. Activators of PFK include ADP and AMP. Their levels increase when ATP levels are low, thus reversing the inhibitory effects of ATP. Citrate also affects PFK activity. Citrate is part of the TCA cycle.

Predict how citrate affects PFK activity. Is citrate an inhibitor or activator of PFK? Enter your answer and click Submit.

Yes, that’s correct.

No, that’s incorrect.

Citrate inhibits PFK activity. This is another example of feedback inhibition since citrate is part of the TCA cycle. High concentrations of citrate indicate that enough metabolites, such as pyruvate, are already feeding into the TCA cycle, and there is no need to make more. The regulation of PFK activity by citrate links glycolysis to the TCA cycle.

The enzyme pyruvate kinase or PK catalyzes reaction 10 of glycolysis, the step that forms pyruvate and generates ATP. The action of this enzyme results in a large negative free energy change, so it’s activity may be regulated. Reaction 10 commits the cells to either anaerobic fermentation or the TCA cycle. Like the enzyme of reaction 3, many different effectors regulate PK’s activity.

The steps that regulate glycolysis reactions affect the concentration of glucose in the blood. Blood glucose levels decrease as the production of ATP increases. When ATP production slows down, less glucose is consumed. In addition, insulin and other hormones and regulatory systems tightly control glucose levels. If glucose levels are too high, excess glucose is polymerized and stored as glycogen in animals or starch in plants. If levels are too low, glucose is released from these molecules and made available for use by cells. The regulation of glycolysis illustrates the general principles that apply to the regulation of all biochemical pathways.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education