Lecture 28 Summary

Tips for Exam 3, scheduled for Monday.

The regulation of the glycolytic pathway also has a logic, one that is context-dependent. We finish our study of glycolysis with a look at some control issues. In order to fully understand the regulation of glycolysis, we must know a little bit about its alter-ego pathway, gluconeogenesis.

Control of glycolysis and allosteric regulation of PFK

Control of glycolysis illustrates a key principle: the flux through a pathway is regulated at steps that are associated with a significant drop in free energy, i.e. at exergonic steps. The first significantly exergonic step that is unique to a given pathway is referred as the committed step of that pathway, and the committed step would seem to be a logical choice for regulation of the flux through that pathway. The most important point of control of glycolysis is held to be at the reaction catalyzed by phosphofructokinase [PFK, EC 2.7.1.1]. The reaction catalyzed by PFK is the committed step of glycolysis, and indeed PFK displays allosteric regulation. In general, PFK activity is regulated by energy charge: As a indicator of relatively high energy status, ATP acts as an inhibitor (as well as a substrate!) of PFK, while AMP (signaling low energy status), activates the enzyme. Furthermore, fructose 2,6-bisphosphate relieves the inhibitory effect of ATP, a regulatory mechanism important in liver. Levels of fructose 2,6-bisphosphate are controlled by covalent modification (phosphorylation) of a bifunctional enzyme responsible for its production as well as its breakdown.

Other regulatory features: Hexokinase and pyruvate kinase: Feed-forward activation by FBP; product inhibition by ATP. cellular glucose uptake by transporters. Glycolysis vs. gluconeogenesis; Substrate cycles.

Substrate cycles

If the rates of two directly opposing reactions, the phosphofructokinase reaction of glycolysis and that of the fructose 1,6-bisphosphatase reaction of gluconeogenesis are measured (with the help, say, of isotope-labeled substrates) it is found that both reactions can and do take place at the same time. Since the net result of this substrate cycle is simply the hydrolysis of ATP, it was believed at one time that this was a wasteful "futile cycle". Subsequently, it was discovered that substrate cycles can provide an effective means to create large changes in metabolic flux.

Learning objectives

• Describe the gluconeogenic pathway: Inputs and outputs, net reaction, names of intermediates, unique steps
• Explain the meaning and metabolic logic of reciprocal regulation of glycolysis and gluconeogenesis.
• Define what is meant by the term committed step, and explain why it is typically subject to regulation
• Describe the regulation of phosphofructokinase, hexokinase, and pyruvate kinase.
• Explain how the roles of skeletal muscle, heart muscle, and liver relate to the regulatory properties of the corresponding isozymes of phosphofructokinase, hexokinase, and pyruvate kinase.
• Define the term metabolic flux.
• Describe how regulation of substrate cycles contributes to control of metabolic flux
• Describe the biological role of substrate cycles in thermogenesis.

Page updated 11-29-09

References

1. Berg, Tymoczko, and Stryer. Biochemistry (BTS): 6th edition (2007, Freeman) pp.383-388