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Chapter 6: Carbohydrate Metabolism

1. PURPOSE

  1. Glycolysis and the TCA cycle are the major pathways of energy production in the body.

  2. The body can't just rely on glucose—other carbohydrates, proteins, and lipids are also important sources of energy

  3. The next 4 chapters will cover these additional pathways involved in energy production and storage

  4. Diseases resulting from dysfunction of these side pathways are a big part of how the Step 1 likes to test metabolism

2. STRATEGY

  1. Each side pathway has a function: What does the body need it for?

  2. Symptoms often relate to limiting the body's ability to perform this function

  3. Don't memorize every enzyme/step of the pathways—focus on the 1–2 enzymes per pathway that are tested

  4. Use mnemonics for groups of diseases like glycogen storage, and make your own if you need to

3. OVERVIEW OF METABOLIC PATHWAYS

  1. Chapter 5 focused on the main path from glucose to ATP production (glycolysis, TCA cycle, and oxidative phosphorylation) as well as glucose synthesis by gluconeogenesis

  2. Chapter 6 focuses on other carbohydrates

    1. Fructose and galactose are common dietary carbohydrates that enter into the glycolysis pathway for energy production

    2. Glycogen is a polysaccharide, consisting of multiple glucose subunits, for energy storage

Table 6-1.Summary of pathways.
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Table 6-1. Summary of pathways.
Pathway Location Major diseases
Carbohydrates (Chapter 6)
Gluconeogenesis Liver (mitochondria → cytosol → ER) Hypoglycemia
Non-glucose monosaccharides All (cytosol)

Fructose intolerance

Classic galactosemia
Glycogenesis/Lysis Liver, muscle (cytosol) Glycogen storage diseases (e.g., Von Gierke, Pompe)
Pentose phosphate All (red blood cells clinically relevant) Glucose-6-phosphate dehydrogenase (G6PD) deficiency
Amino acids (Chapter 7)
Amino acid catabolism All (cytosol → mitochondria)

Maple syrup urine disease

Homocystinuria
Catecholamine synthesis All (locally in brain) Phenylketonuria
Urea cycle Liver (mitochondria → cytosol) Ornithine transcarbamylase deficiency
Lipids (Chapter 8)
Lipogenesis/Lysis Cytosol, mitochondria

Carnitine deficiency

Medium-chain acyl-CoA dehydrogenase deficiency
Cholesterol synthesis Liver Hypercholesterolemia
Ketone bodies (Chapter 9)
Ketone synthesis Liver (mitochondria) Diabetic ketoacidosis
Ketone catabolism Muscle, brain Diabetic ketoacidosis
Figure 6-1.

Overview of metabolic pathways.

An illustration shows the multiple metabolic pathways that originate from glucose. Galactose is shown at the top, as the starting point of the pathway. Galactose is shown to undergo glycogen synthesis to form glycogen, which in turn forms glucose through a resonance arrow. Glucose is shown to undergo glycolysis to form pyruvate, which in turn forms glucose again through gluconeogenesis. Glucose also undergoes H M P shunt and then glycolysis to form pyruvate. In addition, fructose is also shown to undergo glycolysis to form pyruvate. The pyruvate is shown to form lactate through a double-sided resonance arrow. The pyruvate is also shown to form acetyl coenzyme A, which is shown to be involved in the T C A cycle, which is represented by a circular arrow that starts from acetyl coenzyme A and also ends with the arrow pointing toward the same substrate. Another curved arrow branches leftward from the T C A cycle, and this is labeled oxidative phosphorylation. Acetyl coenzyme A is shown to also form cholesterol and. using a resonance arrow, fatty acids. In addition amino acids are also shown to undergo T C A cycle. The caption reads, “overview of metabolic pathways.”
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Figure 6-2.

Links between glucose metabolism and other carbohydrates.

A flowchart shows the conversion of glucose to glycogen, galactose, and acetyl coenzyme A. Acetyl coenzyme A in turn undergoes the T C A cycle in the mitochondria. In the first step glucose converts to glucose-6-phosphate. This in turn is shown to be related by a resonance arrow with glucose-1-phosphate, which forms galactose and glycogen, both of which are connected to glucose-1-phosphate by a resonance arrow. The glucose-1-phosphate also forms fructose-6-phosphate through a resonance arrow. This in turn forms fructose-1, 6-bisphosphate, which forms glyceraldehyde 3 P, as represented by a resonance arrow. The glyceraldehyde 3 P is also shown to be formed from fructose. The flyceralddehyde-3-phosphate forms phosphoenolpyruvate, as indicated by another resonance arrow. This phosphoenolpyruvate is shown to form pyruvate, which moves into the mitochondria to undergo the TCA cycle. The pyruvate is also shown to form lactate separately, and is indicated by a resonance arrow. In the flowchart all the single-direction reaction arrows are colored differently from the resonance arrows and glucose-6-phosphate, fructose, acetyl coenzyme A, and pyruvate are specifically highlighted. The caption reads, “links between glucose metabolism and other carbohydrates.”
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Figure 6-3.

Fructose metabolism.

A set of reactions indicate the conversion of fructose to fructose-1-phosphate and its further reactions until glycolysis. In the first step, fructose reacts in the presence of fructokinase to form fructose-1-phosphate. During this process, A T P converts to form A D P. This fructose-1-phosphate undergoes multiple reactions in the presence of aldolase B to eventually form glyceraldehyde-3-phosphate, which is labeled as glycolysis. The caption reads, “fructose metabolism.”
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4. FRUCTOSE METABOLISM

  1. Fructose is found in “naturally sweet” things—e.g., fruit, honey

  2. Both fructose and galactose (next section) follow very similar process that you can think of in 2 main steps

    1. First, like glucose they have to be phosphorylated and trapped in the cell: Fructokinase does this

    2. Then, it is converted by aldolase B (and others) through intermediates into a compound used in glycolysis (glyceraldehyde-3-P)

  3. The actual process has more steps in it, but it's highly ...

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