6.2 Carbohydrate Metabolism

Monosaccharide Metabolism

Galactose and fructose metabolism is a logical place to begin looking at carbohydrate metabolism, before shifting focus to the cell’s preferred monosaccharide, glucose. Once absorbed in the small intestine (Chapter 4), these monosaccharides are transported to the liver via the hepatic portal system. The figure below shows that galactose and fructose are phosphorylated (have a phosphate added to them) in the liver (a hepatocyte is a liver cell).

Figure 6.211 Uptake of monosaccharides into the hepatocyte

Galactose

As shown above, galactose is phosphorylated in the cells of the liver, resulting in a molecule called galactose-1-phosphate. Galactose -1-phosphate is converted to glucose-1-phosphate, before finally being converted to glucose-6-phosphate1. As shown below, glucose 6-phosphate can then be used in either glycolysis (the breakdown of glucose for energy) or glycogenesis (the production of glycogen for storage), depending on the person’s current energy state.

Figure 6.212 Conversion of galactose-1-phosphate to glucose-6-phosphate

Fructose

Unlike galactose, fructose cannot be used to form glucose 6-phosphate. Instead, fructose-1- phosphate is cleaved in the liver to form glyceraldehyde 3-phosphate, an intermediate in the process of glycolysis (see Section 6.23 below).

The Importance of Glucose-6-Phosphate

Within hepatocytes or myocytes (muscle cells), glucose-6-phosphate can be used either for glycogenesis (glycogen synthesis) or glycolysis (breakdown of glucose for energy production). If the person is in an anabolic state (e.g. after a meal), they will use glucose-6-phosphate for storage. If they are in a catabolic state (e.g. fasted), they will use it for energy production.

Figure 6.214 The “fork in the road” for glucose-6-phosphate

References & Links

1. Gropper SS, Smith JL, Groff JL. (2008) Advanced Nutrition and Human Metabolism. Belmont, CA: Wadsworth Publishing.

Glycogenesis & Glycogenolysis

As discussed earlier, glycogen is the stored form of glucose in humans. If a person is in an anabolic state, such as after consuming a meal, most glucose-6-phosphate within the myocytes (muscle cells) or hepatocytes (liver cells) is going to be stored as glycogen.

Glycogen is mainly stored in the liver and the muscle. It makes up ~6% of the weight of the liver, but only ~1% of muscle weight. However, since we have far more muscle mass in our body, there is 3-4 times more glycogen stored in muscle than in the liver2. This is of great practical importance since glycogen is an importance source of energy for muscle contraction. We have limited glycogen storage capacity in the liver. Thus, after a high-carbohydrate meal, our glycogen stores will reach capacity fairly quickly. After glycogen stores are filled, glucose will have to be metabolized in different ways for it to be stored in a different form, often as fat.

Glycogenesis

The synthesis of glycogen from glucose is a process known as glycogenesis. You will remember that glucose can be converted to glucose-6-phosphate (see Figure 6.211). If glucose storage (as glycogen) is required at any given time, the glucose-6-phosphate is converted to glucose-1- phosphate and then converted to glycogen (Figure 6.222).

Figure 6.222 Glycogenesis

Glycogenolysis

The process of liberating glucose from glycogen is known as glycogenolysis. This process is essentially the opposite of glycogenesis. Glycogen is hydrolyzed and the individual glucose molecules are phosphorylated (converted into glucose-6-phosphate) through the action of an

enzyme called glycogen phosphorylase as shown below3.

Figure 6.223 Glycogenolysis

References & Links

• http://en.wikipedia.org/wiki/File:Glycogen.png
• Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern Nutrition in Health and Disease. Baltimore, MD: Lippincott Williams & Wilkins.
• Gropper SS, Smith JL, Groff JL. (2008) Advanced Nutrition and Human Metabolism. Belmont, CA: Wadsworth Publishing.

Glycolysis

If a person is in a catabolic state (in need of energy) such as during fasting, most glucose-6- phosphate will be used for glycolysis.

Figure 6.231 The “fork in the road” for glucose-6-phosphate

Glycolysis is the breaking down of one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons). During the process, a net of two ATPs and two NADHs are also produced. The Figure 6.232 below shows the steps of glycolysis. Do not get overwhelmed, you will not have to learn every step. We will break it down into smaller sections and highlight the important intermediates, but I do want you to see how glucoses progresses through the various intermediate molecules before becoming pyruvate.

Figure 6.232 Glycolysis1

The following animation, using ball-and-stick models, allows you to control the 3 steps of glycolysis.

Required Web LinksGlycolysis Animation

3 steps of Glycolysis

• Energy investment step – 2 ATP are added to the 6-carbon glucose molecule resulting in one 6-carbon molecule of fructose 1,6-bisphosphate.

Figure 6.233 Glycolysis step 1, energy investment1

• Glucose Split – The 6-carbon fructose 1,6-bisphosphate molecule is split into two 3-carbon molecules of glyceraldehyde 3-phosphate.

Figure 6.234 Glycolysis step 2, glucose split1

• Energy harvesting step – The two molecules of glyceraldehyde 3-phosphate are eventually converted to two 3-carbon molecules of pyruvate resulting in a total “harvest” of 2 NADH and 4 ATPs (1 NADH and 2 ATPs are produced from each glyceraldehyde 3-phosphate.)

Figure 6.235 Glycolysis step 3, energy harvesting1

Thus, from a molecule of glucose, the harvesting step produces a total of four ATPs and two NADHs. Remember that in Step 1 we had to “invest” two molecules of ATP to get the process started. Therefore, the net output from one molecule of glucose is two ATPs and two NADHs. You will remember that NADH is a molecule that is used to manage energized electrons. In this case, the splitting of the glucose molecule releases two energized electrons, which are then managed by two NADH molecules. These energized electrons will ultimately be processed by the Electron Transport Chain to generate ATP in the process of Cellular Respiration.

The figure below shows the stages of glycolysis, as well as the transition reaction, citric acid cycle, and electron transport chain that are utilized by cells to produce energy. They are also the focus of the next 3 sections. Again, you’re not going to have to memorize each step. This is just to give you an overview of the entire process.

Figure 6.236 Glycolysis, transition reaction, citric acid cycle, and the electron transport chain2

References & Links

• http://en.wikipedia.org/wiki/File:Glycolysis.svg
• http://en.wikipedia.org/wiki/File:CellRespiration.svg

Links

Glycolysis Animation – http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html

Transition Reaction

If a person is in a catabolic state, or needs energy, how the pyruvate molecules produced in glycolysis will be used depends on whether adequate oxygen levels are present. If oxygen levels are adequate (aerobic conditions), pyruvate moves from the cytoplasm, into the mitochondria, and then undergoes the transition reaction. If oxygen levels are not adequate (anaerobic conditions), pyruvate will remain in the cytoplasm to be used to produce lactate. We are going to focus on the aerobic pathway for now. We will address what happens under anaerobic conditions in the anaerobic respiration section.

Figure 6.241 Pyruvate fork in the road. What happens depends on whether it is aerobic or anaerobic respiration1

The transition reaction (sometimes called the transition step) is the transition between glycolysis and the citric acid cycle. It also represents a transition in location from the cytoplasm to the mitochondrion. The transition reaction converts pyruvate molecule (3 carbons) into acetyl CoA molecules (2 carbons), producing carbon dioxide (CO₂) and NADH as shown below. The figure below shows the transition reaction with CoA and NAD entering, and acetyl-CoA, CO₂, and NADH being produced.

Figure 6.242 Transition reaction2

The acetyl is combined with coenzyme A (CoA) to form acetyl-CoA. The structure of CoA is shown below. You can think of coenzyme A as an acetyl manager…a molecule that will deliver the 2-carbon acetyl group into the citric acid cycle (see Section 6.25).

In summary, the transition reaction converts each 3-carbon pyruvate into a 2-carbon acetyl group, which is then managed by coenzyme A. The transition reaction also generates CO₂ (a waste product), and NADH (a reduced molecule, contains energized electrons that will be processed by electron transport chain to make ATP).

References & Links

• https://simple.wikipedia.org/wiki/Mitochondria#/media/File:Animal_mitochondrion_diagram_en_(edit).svg
• http://en.wikipedia.org/wiki/Image:Citric_acid_cycle_with_aconitate_2.svg
• http://en.wikipedia.org/wiki/Image:Coenzym_A.svg

The Citric Acid Cycle

Acetyl-CoA is a central point in metabolism, meaning there are a number of ways that it can be used. We’re going to continue to consider its use in an aerobic, catabolic state (need energy). Under these conditions, acetyl-CoA will enter the citric acid cycle (a.k.a. Krebs Cycle, TCA Cycle). The following figure shows the citric acid cycle. In the top left you will notice the acetyl-CoA we just produced.

Figure 6.251 The citric acid cycle1

The citric acid cycle begins by acetyl-CoA (2 carbons) combining with oxaloacetate (4 carbons) to form citrate (a.k.a. citric acid, 6 carbons). Coenzyme A is removed as part of this reaction leaving a single acetyl group to continue through the cycle. A series of transformations occur as the acetyl group is processed, creating a series of intermediates known as keto acids, until oxaloacetate if eventually reformed. During these intermediate steps, the acetyl group that was created during the formation of citrate is broken down and NADH, FADH₂, CO₂, and ATP are produced.

In summary, the Citric Acid Cycle processes each 2-carbon acetyl group from the transition reaction. The acetyl group is delivered by coenzyme A, and is progressively broken down, resulting in the production of carbon dioxide (a waste product), ATP, and NADH and FADH₂ (reduced molecules that contain energized electrons that will be processed by the Electron Transport Chain to make ATP).

The first video and the animation do a good job of explaining and illustrating how the cycle works. The second video is an entertaining rap about the cycle.

Required Web LinksVideo: Citric acid cycle (0:44) Citric acid cycle animationVideo: TCA (Kreb’s) Cycle Rap (3:01)

Through glycolysis, the transition reaction, and the citric acid cycle, multiple NADH and FADH₂ molecules are produced. Under aerobic conditions, these molecules will enter the electron transport chain to be used to generate energy through oxidative phosphorylation as described in the next section.

References & Links

• http://en.wikipedia.org/wiki/Image:Citric_acid_cycle_with_aconitate_2.svg
• http://en.wikipedia.org/wiki/File:CellRespiration.svg

Link

Citric Acid Cycle Animation – http://www.wiley.com/college/boyer/0470003790/animations/tca/tca.htm

Video

Citric acid cycle – http://www.youtube.com/watch?v=hw5nWB0xN0Y

TCA (Kreb’s) Cycle Rap – http://www.youtube.com/watch?v=aMBIs_Iw0kE

Electron Transport Chain

The electron transport chain is located on the inner membrane of the mitochondria, as shown below.

Figure 6.261 The pathways involved in aerobic respiration1

The electron transport chain contains a number of electron carriers. These carriers take the electrons from NADH and FADH₂, pass them down the chain of complexes and electron carriers, and ultimately produce ATP. More specifically, the electron transport chain takes the energy from the electrons on NADH and FADH2 to pump protons (H+) into the intermembrane space.

This creates a proton gradient between the intermembrane space (high) and the matrix (low) of the mitochondria. The protons will then move back out through the enzyme ATP synthase from high to low concentration. This is similar to how a person rides up a motorized ski-lift (the proton pump) only to use gravity (high to low concentration) to come back down the hill. ATP synthase uses the energy of the moving protons to synthesize ATP (think of a hydroelectric dam using moving water to generate electricity.) Oxygen is required for this process because it serves as the final electron acceptor, forming water. Collectively this process is known as

oxidative phosphorylation. The following figure and animation do a nice job of illustrating how the electron transport chain functions.

Figure 6.262 Location of the electron transport chain in the mitochondria2

Required Web LinkETC Animation

The electron transport chain generates 3 ATP for each NADH processed and 2 ATP for each FADH₂ processed. We can assess each of the catabolic steps of aerobic cellular respiration (steps that actually deconstruct the molecule of glucose) in terms of the number of NADH and FADH₂ molecules produced. For one molecule of glucose, the preceding pathways produce:

Glycolysis:2 NADH Transition Reaction: 2 NADH

Citric Acid Cycle:6 NADH, 2 FADH₂ Total10 NADH, 2 FADH₂

Note: some textbooks will use 2.5/1.5 ATP for NADH/FADH₂ instead of the 3/2 we are using here. This is due to the fact that the actual total varies from organism to organism, and even

from one round to the next within the same organism. For simplicity’s sake, we will stick with 3/2 here.

In the following section (Section 6.27), we will compute exactly how many ATP can be generated from the aerobic breakdown of a single molecule of glucose.

The first video does a nice job of illustrating and reviewing the electron transport chain. The second video is a great rap video explaining the steps of glucose oxidation.

Required Web LinksVideo: Electron Transport (1:43)Video: Oxidate it or Love it/Electron to the Next One (3:23)

References & Links

• http://en.wikipedia.org/wiki/File:CellRespiration.svg
• http://en.wikipedia.org/wiki/File:Mitochondrial_electron_transport_chain%E2%80%94Etc4.svg

Link

ETC Animation – http://www.science.smith.edu/departments/Biology/Bio231/etc.html

Videos

Electron Transport Chain – http://www.youtube.com/watch?v=1engJR_XWVU&feature=related Oxidate it or Love it/Electron to the Next One – http://www.youtube.com/watch?v=VCpNk92uswY&feature=response_watch

Aerobic Glucose Metabolism Totals

The table below shows the ATP generated from one molecule of glucose in the different metabolic pathways. As you look at Table 6.271 below, be sure to recognize that the ATP produced through Electron Transport is generated through the processing of the NADH and FADH₂ summarized in the previous section.

Notice that the vast majority of ATP is generated by the electron transport chain. Remember that this is an aerobic process and oxygen is the final electron acceptor. Oxygen is the key to the rich energy return of 38 ATP per molecule of glucose. If there were no oxygen, there would be no final electron acceptor. If there were no final electron acceptor, there would be no electron transport chain. If there were no electron transport chain, it would not be possible to process NADH and FADH₂. In the next section, we will see what happens if there is a limited supply of oxygen in our cells.

Table. 6.271 ATP generated from one molecule of glucose.

No References

• Anaerobic Respiration

Conditions without oxygen are referred to as anaerobic. In this case, the pyruvate will be converted to lactate in the cytoplasm of the cell as shown below.

Figure 6.281 Pyruvate fork in the road, what happens depends on whether it is aerobic or anaerobic respiration1

What happens if oxygen isn’t available to serve as the final electron acceptor? As shown in the following video, the ETC becomes backed up with electrons and can’t accept any more from NADH and FADH₂.

Web LinkVideo: What happens when you run out of oxygen? (0:37)

This leads to a problem in glycolysis because NAD are limited and it is needed to accept electrons, as shown below. Without the electron transport chain functioning, once all NAD molecules have been reduced to NADH, glycolysis cannot continue to produce ATP from glucose.

Figure 6.282 Why NAD needs to be regenerated under anaerobic conditions2

Thus, there is a workaround to regenerate NAD by converting pyruvate (pyruvic acid) to lactate (lactic acid) as shown below.

Figure 6.283. The conversion of pyruvic acid to lactic acid regenerates NAD3,4

However, anaerobic respiration only produces 2 ATP from one molecule of glucose, compared to the 38 ATP from one molecule of glucose we saw with aerobic respiration. The biggest producers of lactate are muscle cells under oxygen stress (lacking adequate oxygen). During periods of intense activity, we might not be able to supply our muscle cells with sufficient oxygen to support the aerobic breakdown of glucose. At that point, our muscle cells are forced

to breakdown glucose in the absence of oxygen (which is essentially a process of glycolysis), which results in a limited amount of ATP and lactate (lactic acid). The lactate is generated because the conversion of pyruvate to lactate allows us to recycle NAD. The lactate produced, while technically a waste product, is still a metabolically valuable commodity. Through what is known as the Cori cycle, lactate produced in the muscle can be sent to the liver. In the liver, through a process known as gluconeogenesis, glucose can be regenerated and sent back to the muscle to be used again for anaerobic respiration forming a cycle as shown below.

Figure 6.284 The Cori cycle5

It is worth noting that the Cori cycle also functions during times of limited glucose (like fasting) to spare glucose by not completely oxidizing it.

References & Links

• https://simple.wikipedia.org/wiki/Mitochondria#/media/File:Animal_mitochondrion_diagram

_en_(edit).svg

• http://en.wikipedia.org/wiki/File:CellRespiration.svg
• https://en.wikipedia.org/wiki/Pyruvic_acid#/media/File:Pyruvic-acid-2D-skeletal.png
• https://en.wikipedia.org/wiki/Lactic_acid#/media/File:Lactic-acid-skeletal.svg
• https://commons.wikimedia.org/wiki/File:CoriCycle-noLang.svg#/media/File:CoriCycle- eng.svg

Video

What happens when your run out of oxygen? – http://www.youtube.com/watch?v=StXlo1W3Gvg