Pièce de Résistance

Do you like to eat? Because I do. I also like to run, which requires quite a bit of anaerobic metabolism. The combination of the need to break down the glucose in the copious amounts of mashed potatoes I consumed during Sunday dinner and the need to generate large amounts of ATP without the luxury of oxygen makes me a big fan of glycolysis. Going hand in hand with this is my passion for phosphofructokinase-1.

PFK-1 is the most important enzyme in glycolysis. But wait, you say, in order for glycolysis to occur, one needs all of its associated enzymes; it’s impossible for one to be more important than the other. PFK-1, however, catalyzes the committed step of glycolysis, the point of no return. Phosphohexose isomerase does its job, and the rest of the process may or may not occur. PFK-1 shows up to work, though, and you’re guaranteed molecules of that glorious ATP. 

Some of you get more pumped about Sunday’s cheesecake than the mashed potatoes (even with the roasted garlic infusion). You are more concerned about fat metabolism than glycolysis. Well, have no fear, young grasshoppers, because PFK-1 plays a role in that as well. As it turns out, PFK-1 regulates triacylglycerol synthesis; mice (and presumably people, though experimentally removing vital enzymes in humans is generally frowned upon) deficient in PFK-1 have decreased fat stores. Wait, you argue, in a country where two thirds of the population is obese, isn’t that a good thing? First of all, I’m guessing that you will change your mind the next time you are starving in the Sahara, far from Aunt Aljean’s green bean casserole. In all seriousness, though, that’s part of what makes PFK-1 so fascinating. Understanding the interplay between PFK-1 and fats has the potential to lead to medical treatments of lipid-related diseases, such as obesity and diabetes. 

There are people out there who don’t share my enthusiasm for running, or anaerobic activity in general. A few of you might not even be all that jazzed about eating. How about breathing? Or even just life on the whole? Because even for those of who for whom glycolysis seems a minor annoyance, existing solely to cause lengthy memorization quizzes in Chem 324, PFK-1 is critical. Shutting it down decreases the concentration of later glycolytic intermediates, like 2,3-bisphosphoglycerate. 2,3-BPG increases heme affinity for oxygen, enhancing the body’s oxygen supply. Deficient PFK-1 therefore means difficultly breathing.

Similarly, faulty PFK-1 causes an accumulation of earlier glycolytic intermediates, specifically glucose and glucose-6-phosphate. This results in amplified glycogen synthesis. At first glance, a lot of glycogen may seem pretty inoffensive. Unfortunately, however, it has all kinds of nasty side effects. For example, the increased solute concentration in red blood cells causes water to rush in and the erythrocytes to burst. That’s not good. Also, because the spleen is responsible for recycling red blood cells, it enlarges. That’s no good either. Increased glycogen concentration causes the heart to swell too. The result? Death. That’s really not good. 

As you can see, PFK-1 is essential for life. Stuffing fans, apple pie aficionados, runners, couch potatoes, even those of you who just want a refrigerator magnet of a two-subunit protein conveniently colored in your favorite team hues (any Wolverines fans out there?), we all need PFK-1. 

Jenkins, Christopher M., Jingyue Yang, Harold F. Sims, and Richard W. Gross. "Reversible High Affinity Inhibition of Phosphofructokinase-1 by Acyl-CoA A MECHANISM INTEGRATING GLYCOLYTIC FLUX WITH LIPID METABOLISM." The Journal of Biological Chemistry 286.1423 Jan. (2011): 11937-50. PubMed. Web. 20 Apr. 2011. <http://www.ncbi.nlm.nih.gov/pubmed/21258134>.

Getty-Kaushik, Lisa, Jason C. Viereck, Jessie M. Goodman, Zifang Guo, and Nathan K. LeBrasseur. "Mice Deficient in Phosphofructokinase-M Have Greatly Decreased Fat Stores." Obesity 18.3 Mar. (2010): 434-40. PubMed. Web. 20 Apr. 2011. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2871150/>.

Garcia, M, A Pujol, A Ruzo, E Riu, and J Ruberte. "Phosphofructo-1-kinase deficiency leads to a severe cardiac and hematological disorder in addition to skeletal muscle glycogenosis." PLoS Genetics 5.8 Aug. (2009): 1000615. PubMed. Web. 20 Apr. 2011. <http://www.ncbi.nlm.nih.gov/pubmed/19696889>.

Interesting Research Regarding Phosphofructokinase-1

Phosphofructokinase-1 plays a number of vital roles in the body, as shown by the following articles.

http://www.ncbi.nlm.nih.gov/pubmed/21258134
Regulation by Fatty Acids

Phosphofructokinase-1 catalyzes the first committed step of glycolysis and is therefore tightly regulated. One such mechanism of regulation is by acylation by long-chain acyl-CoA molecules such as palmitoyl-CoA. Interestingly, PFK-1 is not regulated by short-chain acyl-CoA molecules. Long-chain acyl-CoA molecules inhibit PFK-1 through acylation, which causes increased membrane association. This acylation can then be reversed by cellular thioesterases.

A variety of allosteric regulators play a role in the regulation of PFK-1 by acyl-CoA molecules, making the mechanism quite complex. For example, AMP and ADP protect PFK-1 from inhibition by acyl-CoA, while ATP does not. As a consequence, glycolysis can continue to occur even when fatty acid oxidation is increased, such as under hypoxic conditions. Another complexity of the acyl-CoA regulation system involves the calmodulin bonding site of PFK-1. Acylation changes the conformation of this bonding site, allowing calmodulin association. Calmodulin previously had been identified as a regulator of PFK-1; this new information gives insight into the regulation mechanism of calmodulin.

Knowledge of the regulation by acyl-CoA molecules, the product of the branch point between lipid anabolism and catabolism in lipid metabolism, of glycolytic pathways has vast medical implications. As the prevalence of lipid-related diseases, such as diabetes, increases, understanding of this interplay is vital for medicine and pharmacology.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2871150/
Decreased Fat Stores

Triacylglycerol synthesis require glucose metabolism because the glycerol component is synthesized from the glycerol phosphate intermediate of glycolysis. Previously, the rate limiting step in this process was thought to be uptake of glucose. Recently, however, it has been shown that phosphofructokinase-1 also plays a large role in the regulation of triacylglycerol synthesis. In fact, mice that are deficient in PFK-1 have significantly decreased fat stores. They do not have a decreased number of adipocytes; rather, the number of triacylglycerides contained by the adipocytes is decreased. Multiple forms (isozymes) of PFK-1 exist in the body. This decrease in fat storage was seen only in response to a deficiency in the muscle isozyme of PFK-1, indicating something special about PFK-M.

The mechanism by which PFK-M regulates adipocyte metabolism remains under discussion. The current thought is based on the fact that PFK-M is more strongly activated by its product, fructose 1,6-bisphosphate, than other PFK isozymes. This leads to the generation of glycolytic oscillations, which produce pulses of glycerol phosphate, the precursor for triacylglycerol. When PFK-M is lacking, these pulses do not occur. Subsequently, triacylglycerol synthesis is decreased.

http://www.ncbi.nlm.nih.gov/pubmed/19696889
Glycogen Storage Disease

Phosphofructokinase-1 catalyzes the commited step of glycolysis (Fructose 6-phosphate +ATP à 1,6-Fructose bisphosphate). A deficiency in this enzyme results the blockage of glycolysis and an accumulation of the first substrates of glycolysis, specifically glucose and glucose 6-phosphate. This has a variety of medical applications. First, the lack of ability to metabolize glucose 6-phosphate, the product of glycogen breakdown, leads to an accumulation of glycogen, known as glycogen storage disease. In both mice and humans, this glycogen accumulation results in an enlarged heart and early mortality.

Second, a deficiency in PFK-1 causes decreases exercise tolerance, as ATP cannot be generated by glucose metabolism. The body compensates for this by up-regulating the enzymes of the citric acid cycle, which can produce energy from acetyl-CoA produced by fatty acid oxidation, and the enzymes of the pentose phosphate pathway, which siphons off glucose 6-phosphate for nucleotide generation. This compensation, however, is not enough to overcome the blockage of glycolysis. This chronic lack of ATP results in degradation of skeletal muscle fibers.

A third medical implication of PFK-1 deficiency involves erythrocytes. The accumulation of glucose and glucose 6-phosphate in the red blood cells causes water to enter the cell via osmosis, leading to hemolysis (rupturing of red blood cells). Consequently, the concentration of reticulocytes, immature red blood cells, increases. Because the spleen removes old erythrocytes, the spleen also enlarges. Finally, when PFK-1 activity in erythrocytes is blocked, the concentration of 2,3-bisphosphoglycerate, a later intermediate in glycolysis, decreases. 2,3-BPG increases heme affinity for oxygen, meaning that the amount of available oxygen decreases. This may be one reason why up-regulation of oxidative energy producing pathways is not enough to compensate for a lack of glycolysis.

As you can see, defective PFK-1 can have some pretty nasty implications. Don't worry though--it's only important in people who eat.

Visual Representations of PFK-1

Surface representation on black background colored by spectrum b factors.

Maize and blue surface representation of chains on white background.

Cartoon on black background with organic molecules highlighted and colored by element.

Ribbons on a black background colored by chainbows.

Cartoon on light gray background colored by chain.

Spheres on black background colored by element.

Cartoon on black background colored by secondary structure.