GLYCOLYSIS

Glycolysis

Glycolysis

Introduction

Biology, physiology, and allied health biochemistry textbooks cover metabolic pathways such as glycolysis; however, most do not include much discussion of how these pathways are regulated within the cell. Because the details of these complex regulatory processes can be difficult for students to learn, we have developed a robust teaching analogy that compares the glycolytic synthesis of ATP to the generation of electrical power. Analogies comparing glucose catabolism to coupled industrial engines (1) and to family finances (2) have been published, but these do not include any notion of regulation. A detailed analogy comparing an enzyme to a butcher (5) deals with enzyme structure, mechanism, and regulation, but only for a single enzyme. Thus we feel that our glycolysis/power generation analogy, which envisions the regulation of an entire metabolic pathway, is uniquely useful.

Discussion

In our scheme, we identify three key regulatory enzymes in glycolysis: hexokinase, phosphofructokinase, and pyruvate kinase, along with some of their activators and inhibitors. Our analogy compares three key parts of the electric power generation system with these three major regulatory enzymes in the glycolytic pathway. The target audience for this analogy is lower-division undergraduates studying metabolism in courses such as introductory biology, physiology (especially human physiology), introductory biochemistry, and nursing/allied health courses.

Glycolysis

The 10 reactions in the glycolytic pathway, catalyzed sequentially by 10 different enzymes, oxidize glucose to pyruvate where NAD+ is nicotinamide adenine dinucleotide (oxidized) and NADH is nicotinamide adenine dinucleotide (reduced). Even though glycolysis is a spontaneous process, the glycolytic "pump" must first be "primed" before it can work; ATP is initially consumed in steps 1 and 3 (see Fig. 1A) to eventually produce even more ATP in steps 7 and 10. Similarly, in a power plant, energy must be expended initially to either ignite fuel in the combustion chamber or to reposition dam gates for hydroelectricity (Fig. 1B). In either case, energy must first be spent before it can be created. However, power companies don't have to generate electricity all the time. Instead, they match energy production to energy need (Silverstein 1995). Cells also perform the same function.

Glycolysis - the Lactic Acid System.

Net reaction:

Glucose + 2 ADP + 2 Pi + 2 NAD+--->2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Detail:

Glycolysis, also called the Embden-Myerhoff pathway, is the sequence of reactions, which converts a glucose molecule into two pyruvate molecules with the production of NADH and ATP. Specific enzymes control each of the different reactions, as shown in figure 5. Glucose comes either directly from digestion, or from short-term storage in the muscles or long-term storage in the liver. There is a net gain of two (2) ATP at the end of glycolysis (reaction shown above), using glucose as the source and three (3) from glycogen. This is because glucose needs to be converted to glucose 6-phosphate to enter the glycolytic pathway that requires the use of one ATP (Nelson 2005).

All the reactions of glycolysis take place entirely in the cytosol due to the abundance of ...