MCAT Biochemistry Review

Chapter 9: Carbohydrate Metabolism I: Glycolysis, Glycogen, Gluconeogenesis, and the Pentose Phosphate Pathway

Conclusion

This chapter is critically important in your studying for the MCAT. The processes of carbohydrate metabolism that do not require oxygen are heavily tested, as is their integration. The body has evolved in such a way that we can use, store, or create fuel 24 hours a day, depending on the demands of the internal and external environment. We can turn on pathways when we need them and turn them off when we don't. And the regulation of these pathways makes sense: for example, acetyl-CoA—a downstream product of glycolysis—can turn off the process of glycolysis and allow us to either store extra sugar as other biomolecules or generate sugar anew if we need it. Return to this chapter repeatedly during your studies to maximize points on metabolism on Test Day. In the next chapter, we'll turn our attention to the oxygen-requiring carbohydrate metabolism processes, including the citric acid cycle, the electron transport chain (ETC), and oxidative phosphorylation.

Concept Summary

Glucose Transport

·        GLUT 2 is found in the liver (for glucose storage) and pancreatic β-islet cells (as part of the glucose sensor). It has a high Km.

·        GLUT 4 is found in adipose tissue and muscle and is stimulated by insulin. It has a low Km.

Glycolysis

·        Glycolysis occurs in the cytoplasm of all cells, and does not require oxygen. It yields 2 ATP per molecule of glucose.

·        Important glycolytic enzymes include:

o   Glucokinase, which converts glucose to glucose 6-phosphate. It is present in the pancreatic β-islet cells as part of the glucose sensor and is responsive to insulin in the liver.

o   Hexokinase, which converts glucose to glucose 6-phosphate in peripheral tissues.

o   Phosphofructokinase-1 (PFK-1), which phosphorylates fructose 6-phosphate to fructose 1,6-bisphosphate in the rate-limiting step of glycolysis. PFK-1 is activated by AMP and fructose 2,6-bisphosphate (F2,6-BP) and is inhibited by ATP and citrate.

o   Phosphofructokinase-2 (PFK-2), which produces the F2,6-BP that activates PFK-1. It is activated by insulin and inhibited by glucagon.

o   Glyceraldehyde-3-phosphate dehydrogenase produces NADH, which can feed into the electron transport chain.

o   3-phosphoglycerate kinase and pyruvate kinase each perform substrate-level phosphorylation, placing an inorganic phosphate (Pi) onto ADP to form ATP.

·        The enzymes that catalyze irreversible reactions are glucokinase/hexokinase, PFK-1, and pyruvate kinase.

·        The NADH produced in glycolysis is oxidized by the mitochondrial electron transport chain when oxygen is present.

·        If oxygen or mitochondria are absent, the NADH produced in glycolysis is oxidized by cytoplasmic lactate dehydrogenase. Examples include red blood cells, skeletal muscle (during short, intense bursts of exercise), and any cell deprived of oxygen.

Other Monosaccharides

·        Galactose comes from lactose in milk. It is trapped in the cell by galactokinase, and converted to glucose 1-phosphate via galactose-1-phosphate uridyltransferase and an epimerase.

·        Fructose comes from honey, fruit, and sucrose (common table sugar). It is trapped in the cell by fructokinase, and then cleaved by aldolase B to form glyceraldehyde and DHAP.

Pyruvate Dehydrogenase

·        Pyruvate dehydrogenase refers to a complex of enzymes that converts pyruvate to acetyl-CoA.

·        It is stimulated by insulin and inhibited by acetyl-CoA.

Glycogenesis and Glycogenolysis

·        Glycogenesis (glycogen synthesis) is the production of glycogen using two main enzymes:

o   Glycogen synthase, which creates α-1,4 glycosidic links between glucose molecules. It is activated by insulin in liver and muscle.

o   Branching enzyme, which moves a block of oligoglucose from one chain and adds it to the growing glycogen as a new branch using an α-1,6 glycosidic link.

·        Glycogenolysis is the breakdown of glycogen using two main enzymes:

o   Glycogen phosphorylase, which removes single glucose 1-phosphate molecules by breaking α-1,4 glycosidic links. In the liver, it is activated by glucagon to prevent low blood sugar; in exercising skeletal muscle, it is activated by epinephrine and AMP to provide glucose for the muscle itself.

o   Debranching enzyme, which moves a block of oligoglucose from one branch and connects it to the chain using an α-1,4 glycosidic link. It also removes the branchpoint, which is connected via an α-1,6 glycosidic link, releasing a free glucose molecule.

Gluconeogenesis

·        Gluconeogenesis occurs in both the cytoplasm and mitochondria, predominantly in the liver. There is a small contribution from the kidneys.

·        Most of gluconeogenesis is simply the reverse of glycolysis, using the same enzymes. The three irreversible steps of glycolysis must be bypassed by different enzymes:

o   Pyruvate carboxylase converts pyruvate into oxaloacetate, which is converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (PEPCK). Together, these two enzymes bypass pyruvate kinase. Pyruvate carboxylase is activated by acetyl-CoA from β-oxidation; PEPCK is activated by glucagon and cortisol.

o   Fructose-1,6-bisphosphatase converts fructose 1,6-bisphosphate to fructose 6-phosphate, bypassing phosphofructokinase-1. This is the rate-limiting step of gluconeogenesis. It is activated by ATP directly and glucagon indirectly (via decreased levels of fructose 2,6-bisphosphate). It is inhibited by AMP directly and insulin indirectly (via increased levels of fructose 2,6-bisphosphate).

o   Glucose-6-phosphatase converts glucose 6-phosphate to free glucose, bypassing glucokinase. It is found only in the endoplasmic reticulum of the liver.

The Pentose Phosphate Pathway

·        The pentose phosphate pathway (PPP), also known as the hexose monophosphate (HMPshunt, occurs in the cytoplasm of most cells, generating NADPH and sugars for biosynthesis (derived from ribulose 5-phosphate).

·        The rate-limiting enzyme is glucose-6-phosphate dehydrogenase, which is activated by NADP+ and inhibited by NADPH and insulin.

Answers to Concept Checks

·        9.1

1.     

 

GLUT 2

GLUT 4

Important tissues

Liver, pancreas

Adipose tissue, muscle

Km

High (~15 mM)

Low (~5 mM)

Saturated at normal glucose levels?

No—cannot be saturated under normal physiological conditions

Yes—saturated when glucose levels are only slightly above 5 mM

Responsive to insulin?

No (but serves as glucose sensor to cause release of insulin in pancreatic β-cells)

Yes

2.    GLUT 4 is saturated when glucose levels are only slightly above 5 mM, so glucose entry can only be increased by increasing the number of transporters. Insulin promotes the fusion of vesicles containing preformed GLUT 4 with the cell membrane.

·        9.2

1.     

·        Hexokinase phosphorylates glucose to form glucose 6-phosphate, “trapping” glucose in the cell. It is inhibited by glucose 6-phosphate. It is irreversible.

·        Glucokinase also phosphorylates and “traps” glucose in liver and pancreas cells, and works with GLUT 2 as part of the glucose sensor in β-islet cells. In liver cells, it is induced by insulin. It is irreversible.

·        PFK-1 catalyzes the rate-limiting step of glycolysis, phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate using ATP. It is inhibited by ATP, citrate, and glucagon. It is activated by AMP, fructose 2,6-bisphosphate, and insulin. It is irreversible.

·        Glyceraldehyde-3-phosphate dehydrogenase generates NADH while phosphorylating glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. It is reversible.

·        3-phosphoglycerate kinase performs a substrate-level phosphorylation, transferring a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. It is reversible.

·        Pyruvate kinase performs another substrate-level phosphorylation, transferring a phosphate from phosphoenolpyruvate (PEP) to ADP, forming ATP and pyruvate. It is activated by fructose 1,6-bisphosphate. It is irreversible.

2.    Fermentation must occur to regenerate NAD+, which is in limited supply in cells. Fermentation generates no ATP or energy carriers; it merely regenerates the coenzymes needed in glycolysis.

3.    The binding of 2,3-BPG decreases hemoglobin's affinity for oxygen. Fetal hemoglobin must be able to “steal” oxygen from maternal hemoglobin at the placental interface; therefore, it would be disadvantageous to lower its affinity for oxygen.

·        9.3

1.    Galactose is phosphorylated by galactokinase, trapping it in the cell. Galactose-1-phosphate uridyltransferase produces glucose 1-phosphate, a glycolytic intermediate, thus linking the pathways.

2.    Fructose is phosphorylated by fructokinase, trapping it in the cell (with a small contribution from hexokinase). Aldolase B produces dihydroxyacetone phosphate (DHAP) and glyceraldehyde (which can be phosphorylated to form glyceraldehyde 3-phosphate), which are glycolytic intermediates, thus linking the pathways.

·        9.4

1.    Pyruvate, NAD+, and CoA are the reactants of the PDH complex. Acetyl-CoA, NADH, and CO2 are the products.

2.    Acetyl-CoA inhibits the PDH complex. As a product of the enzyme complex, a buildup of acetyl-CoA from either the citric acid cycle or fatty acid oxidation signals that the cell is energetically satisfied and that the production of acetyl-CoA should be slowed or stopped. Pyruvate can then be used to form other products, such as oxaloacetate for use in gluconeogenesis.

·        9.5

1.    Glycogen is made up of a core protein of glycogenin with linear chains of glucose emanating out from the center, connected by α-1,4 glycosidic links. Some of these chains are branched, which requires α-1,6 glycosidic links.

2.    Glycogen synthase attaches the glucose molecule from UDP-glucose to the growing glycogen chain, forming an α-1,4 link in the process. Branching enzyme creates a branch by breaking an α-1,4 link in the growing chain and moving a block of oligoglucose to another location in the glycogen granule. The oligoglucose is then attached with an α-1,6 link.

3.    Glycogen phosphorylase removes a glucose molecule from glycogen using a phosphate, breaking the α-1,4 link and creating glucose 1-phosphate. Debranching enzyme moves all of the glucose from a branch to a longer glycogen chain by breaking an α-1,4 link and forming a new α-1,4 link to the longer chain. The branchpoint is left behind; this is removed by breaking the α-1,6 link to form a free molecule of glucose.

·        9.6

1.    Gluconeogenesis occurs when an individual has been fasting for >12 hours. To carry out gluconeogenesis, hepatic (and renal) cells must have enough energy to drive the process of glucose creation, which requires sufficient fat stores to undergo β-oxidation.

2.     

Gluconeogenic Enzyme

Replaces

Pyruvate carboxylase

Pyruvate kinase

Phosphoenolpyruvate carboxykinase (PEPCK)

Pyruvate kinase

Fructose-2,6-bisphosphatase

Phosphofructokinase-1

Glucose-6-phosphatase

Glucokinase

3.    Acetyl-CoA inhibits pyruvate dehydrogenase complex while activating pyruvate carboxylase. The net effect is to shift from burning pyruvate in the citric acid cycle to creating new glucose molecules for the rest of the body. The acetyl-CoA for this regulation comes predominantly from β-oxidation, not glycolysis.

·        9.7

1.    The two major metabolic products of the pentose phosphate pathway are ribulose 5-phosphate and NADPH.

2.    NADPH is involved in lipid biosynthesis, bactericidal bleach formation in certain white blood cells, and maintenance of glutathione stores to protect against reactive oxygen species.

Shared Concepts

·        Biochemistry Chapter 4

o   Carbohydrates

·        Biochemistry Chapter 10

o   Carbohydrate Metabolism II

·        Biochemistry Chapter 12

o   Bioenergetics and Regulation of Metabolism

·        Biology Chapter 7

o   The Cardiovascular System

·        General Chemistry Chapter 5

o   Chemical Kinetics

·        General Chemistry Chapter 11

o   Oxidation–Reduction Reactions