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Link Reaction

Glycolysis can be fun…?

Glycolysis

Hey everyone, it’s been 5 weeks since I discussed Cells with you. Now I am here to make Glycolysis your new best friend! Yes, you know I am kidding… Or am I… *evil laugh* lol. Besides, if there is one thing Richie likes to do it is to make you understand and I assure you, you won’t be clicking that little white x in that red box at the top of your screen any time soon!

glycolysis factory

 

Steps in Glycolysis

Steps in Glycolysis

 

Glycolysis is a group of 10 intracellular (cytoplasmic) chemical reactions that makes up the most ancient metabolic pathway to synthesize pyruvate and other chemicals that include the energy currency, ATP, for the cell’s existence… How interesting!  Glycolysis literally means the splitting of 6 carbon glucose into two 3 carbon pyruvates. It is important to understand that it occurs in the cytoplasm and NOT THE MITOCHONDRIA. Yes, I know a lot of people who didn’t grasp this concept from A-levels. 😛 Furthermore, very primitive bacteria that lived in the geological time period when the planet was an anaerobic environment employed this pathway. How cool is that! The first five of the 10 metabolic reactions (in sequence) can be described as vitally investing energy in the form of ATP. The other five reactions produce a surplus, in other words a profit is achieved which ensures that the process of glycolysis doesn’t defeat the purpose of respiration which is to supply an organism with energy as there is a net gain in ATP.

Unfortunately, this is the end of the unpretentious, bigger picture of the process 😦

Worried man

However I will try my best to make you grasp the concepts of each reaction! 🙂

relieved

I assure you, the names of the involved enzymes are the toughest aspect of glycolysis, and learning the stages will be much more fun if you keep that into consideration. Here is a little tip: try learning it without the intimidating names of the enzyme catalysts first! So…. HERE WE GO!

The first reaction is a phosphorylation reaction that involves adding a phosphate (from an ATP) to the 6th carbon atom of glucose to create the structures, glucose-6-phosphate and ADP. This irreversible stage is called the first priming reaction and the enzyme that catalyzes it is called Hexokinase. The biological significance is to trap glucose within the cell since there are no channel proteins capable of transporting this modified form of glucose across the plasma membrane.

'Glucose' trapped in a 'Cell' lol

‘Glucose’ trapped in a ‘Cell’ lol

In the second reaction (a reversible reaction), the enzyme Phosphohexose isomerase converts the aldose sugar glucose-6-phosphate into its ketose isomer, fructose-6-phosphate.

The third reaction is in fact the irreversible second priming reaction, where fructose-6-phosphate is converted into fructose-1,6-bisphosphate (Regina Bailey. 2014), when a phosphate from another molecule of ATP is added to its first carbon via the enzyme Phospho-fructokinease-1 (PFK 1). As seen in the product’s name (fructose-1,6-bisphosphate), ‘bis’ implies that both phosphates are attached to different carbon atoms. Indeed, as we have seen, in the first priming reaction, the phosphate was added to carbon 6, while in this reaction the second phosphate was added to carbon 1. (Don’t say diphosphate!!!)

In the fourth reaction (reversible), the enzyme Aldoses catalyzes a lysis reaction, where the fructose-1,6-bisphosphate is split into glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate (DHAP) (Regina Bailey. 2014). The products that are synthesized are isomers of each other. They are both Triose sugars!

"Glucose" starring Van Damme doing the splits

“Glucose” starring Van Damme doing the splits

halfway there

Halfway through the process!

The fifth reaction, rapidly catalyzed by the enzyme Triose phosphate isomerase brings about the conversion of dihydroxyacetone-phosphate (DHAP) into a glyceraldehyde-3-phosphate. This reaction occurs very quickly because at the instant when the enzyme binds with the substrate, the intermediate formed is so highly unstable, that it is quickly converted into its product. Consequently, Triose phosphate isomerase is considered a kinetically perfect enzyme. You can now see that at the end of the ‘energy investing phase’ that 2 ATPs were used in phosphorylation reactions, while two glyceraldehyde-3-phosphate molecules were synthesized.

Fun fact!: All enzymes of glycolysis has induced fit structures!!!

redox is coming

The sixth step (or the first in the ATP generating phase) involves converting the two glyceraldehyde-3-phosphates into two 1,3-Bisphosphoglycerates. To achieve this, the enzyme Glyceraldehyde 3-phosphate dehydrogenase oxidizes the glyceraldehyde-3-phosphates (removes a hydrogen from each) and the co enzyme NAD+ collects the hydrogen and becomes reduced (NADH). The formation of NADH is essential for glycolysis to continue. The oxidation of the glyceraldehyde-3-phosphate fulfills the energy requirements for the same enzyme, Glyceraldehyde 3-phosphate dehydrogenase, to catalyze a phosphorylation reaction where an inorganic phosphate is added to the oxidized glyceraldehyde-3-phosphate to create the final product 1,3-Bisphosphoglycerate. Remember that in this stage, two 1,3-Bisphosphoglycerates are produced! In other words, a hydrogen is removed from each glyceraldehyde-3-phosphate and it forms NADH. A phosphate is then added to each modified glyceraldehyde-3-phosphate, to form the product 1,3-Bisphosphoglycerate.

In the seventh reaction, two molecules of ATP are being synthesized in a process known as Substrate Level Phosphorylation. The enzyme Phosphoglycerate Kinase removes a phosphate from the first carbon of each of the two 1,3-Bisphosphoglycerates and then these high energy phosphate groups are combined with ADP molecules to create two ATP. Once the phosphates have been removed, the molecules are now called 3-Phosphoglycerates.

The 3-Phosphoglycerates then undergo ‘change in structure’ when the enzyme Phosphoglycerate mutase converts them firstly to 2,3-bisphosphoglycerates (intermediates). Furthermore, the phosphate on carbon 3 is then removed hence two 2-Phosphoglycerates are formed.

changing shape

A 3-Phosphoglycerate undergoes a change in structure

In the ninth reaction, the two 2-Phosphoglycerates are converted into two Phosphoenolpyruvates (PEP) due to the loss of water (dehydration reaction) catalyzed by the enzyme, Enolase.

Finally, the last reaction!!!

yes-finally-pl-ffffff

You guessed it, here the two PEPs are converted into two pyruvates and two more ATP are being synthesized from ADPs. Pyruvate Kinase is the enzyme catalyzing this reaction. It removes the phosphates from the PEPs, and adds it to the ADPs.  This reaction releases a lot of energy in addition to what is stored in the ATPs. This energy is released as heat. It partly contributes to making you perspire during exercise. The two ATPs produced in this final step can be referred to as the energy/ATP profit gained from glycolysis.

Yay! Now we have two pyruvates. That’s pretty cool, but wouldn’t it be cooler if we knew what happens to each pyruvate after synthesis? It will be my pleasure to briefly discus three of these ‘fates’ with you.  🙂

Let us first observe the metabolic pathway in aerobic conditions. Once oxygen is present, the pyruvate is converted to Acetyl CoA. This pathway is called the Link Reaction since it connects Glycolysis to the Krebs cycle. Converting pyruvate into Acetyl CoA is a decarboxylation reaction. The removal of carbon dioxide would lead to a two carbon structure; simultaneously an NAD+ is reduced and the rest of the pyruvate is combined with Coenzyme-A to form Acetyl CoA.

Lactate Fermentation results  from  high physical activity

Lactate Fermentation results from high physical activity

In anaerobic conditions, Lactate fermentation (in animals) or ethanol production (in yeast and plants), are possible scenarios (Myda Ramersar 2011). Lactate fermentation mostly occurs in erythrocytes (red blood cells) and skeletal muscle. Red blood cells lack mitochondria therefore the Krebs cycle and Oxidative phosphorylation cannot occur (rendering the Link reaction useless). Consequently, glycolysis is the sole means for the cells to acquire their energy. Skeletal muscle cells, conversely, respire anaerobically only when oxygen availability is minimal. Since each cell contains a fixed amount of oxidized NAD+, once all of it is reduced by glycolysis, the process will come to a grinding halt. Imagine how bad that would be for your poor cells. 😦 Luckily the process of lactate fermentation (catalyzed by the lactate dehydrogenase enzyme) reconverts NADH into NAD+ by the following generalized equation:

pyruvate + NADH –> lactate + NAD+

Ethanol fermentation in a breadshell

Ethanol fermentation in a breadshell

You can see that the product of glycolysis (pyruvate) is involved in securing its continued synthesis!!!

In the production of ethanol for commercial wine/beer brewing, a fermentation pathway that involves the use of yeast’s ability to respire anaerobically, is utilized. This pathway incorporates the pyruvate first being converted into acetaldehyde by the enzyme pyruvate decarboxylase, and then to ethanol by the enzyme ethanol dehydrogenase. In the latter stage, NADH is oxidized hence glycolysis can continue! Oh and I am sure you guessed it, bread making also incorporates fermentation by yeast. 🙂 Did you realize the amount of science that is involved in making the dough rise? Mind boggling right!

I hope you follow my tip when studying glycolysis. It helps to get the general idea and then work on the details of the process. It helped me 🙂 I would like to take this time to thank you for reading our posts, attempting the MCQ and word search puzzle. Feel free to leave comments and for any post you wish, especially for the MCQ 🙂 The Biochemistry3RST team would love to know if we helped boost your understanding of any topic we blogged on! This might be my last reflection, but I am not abandoning you in your Biochem Jungle 😛 there is more to come. 🙂 Until next time guys! Time for me to rock and roll outa here! Richie, over and out!!!

my-job-here-is-done

References:

· Bailey, Regina. 2014. “10 Steps of Glycolysis”. Accessed March 9th, 2014.                                                                              http://biology.about.com/od/cellularprocesses/a/aa082704a.html

Ramersar, Myda, Mary Jones, Geoff Jones. 2011. Biology for CAPE Unit 1. Cambridge: Cambridge University Press.

Picture References:

Glycolysis Cycle- http://cnx.org/content/m44442/latest/Figure_07_07_02.jpg

Bad time-http://biochemicalminds.files.wordpress.com/2014/03/438e532c9e230ecc312a6c59d925469876c4d3b7b625269fe843155b755b3c55.jpg

Glycolysis Factory- http://dynamic.pixton.com/comic/t/a/6/q/ta6qxsk7epuocem8_v2_.png

Worried man- http://thumbs.dreamstime.com/z/worried-business-man-1741735.jpg

relieved- http://i.imgur.com/RkFZOm0.jpg

prison- http://img.naij.com/n/0b/3/prison-cube_1_.jpg

epic split- Van Damme http://static.squarespace.com/static/51b3dc8ee4b051b96ceb10de/t/52857605e4b0975b1b06bd27/1384478226902/jean-claude-van-dammes-epic-truck-splits-stunt.jpg

redox- http://cdn.memegenerator.net/instances/400x/37209536.jpg

halfway there- https://i.chzbgr.com/maxW500/7762225664/hB8C603B1/
changing shape- http://www.capstoneclassroom.com/product/covers_lg/9781432932763.jpg

FINALLY- http://www.chesnok.com/daily/wp-content/uploads/2013/10/yes-finally-pl-ffffff.jpeg

Link reaction- http://www.revisionworld.com/sites/revisionworld.com/files/rw_files/LinkReaction.JPG

running- http://www.dailyhiit.com/hiit-blog/wp-content/uploads/2013/10/running-wallpaper-41.jpg

breadshell- http://media.tumblr.com/bfa7ee6595e07c7553f6e3372f7ee1d8/tumblr_inline_mpd9c5Kekm1qz4rgp.jpg

My job is done- http://www.troll.me/images/the-chuck-norris/my-job-here-is-done.jpg