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Chapter 5: Problem 17

Describe the metabolic pathway by which glucose can be converted into fat. How can end-product inhibition by ATP favor this pathway?

Short Answer

Expert verified
The metabolic pathway for converting glucose into fat, known as lipogenesis, primarily occurs in the liver and adipose tissue. This process starts with glycolysis, where glucose is converted to pyruvate, then transported to the mitochondria for conversion into Acetyl-CoA. Acetyl-CoA is carboxylated into Malonyl-CoA, entering fatty acid synthesis where it's transformed into fatty acids using NADPH. These fatty acids are esterified with glycerol-3-phosphate to produce triglycerides, which are stored in adipocytes. End-product inhibition by ATP can favor lipogenesis since high ATP levels can inhibit glycolysis at PFK-1, promoting glucose-6-phosphate conversion into NADPH through the pentose phosphate pathway, ultimately supporting fatty acid synthesis.

Step by step solution

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1. Overview of glucose conversion to fat

The metabolic pathway that converts glucose into fat is called lipogenesis, which has several steps that lead to the production of fatty acids. These fatty acids can then be stored as triglycerides in adipose tissue. Lipogenesis primarily occurs in the liver and adipose tissue.
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2. Conversion of glucose to pyruvate

The first step in this process is glycolysis, which converts one glucose molecule into two pyruvate molecules, generating ATP and reduced NADH in the process. This occurs in the cytosol of the cell.
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3. Conversion of pyruvate to Acetyl-CoA

The pyruvate molecules are then transported into the mitochondria, where they are converted into Acetyl-CoA by the enzyme pyruvate dehydrogenase. This conversion results in the release of CO鈧� and the production of NADH.
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4. Carboxylation of Acetyl-CoA to Malonyl-CoA

The next step occurs in the cytosol, where Acetyl-CoA is carboxylated to Malonyl-CoA by the enzyme Acetyl-CoA carboxylase. This is the rate-limiting and irreversible step in the lipogenesis process and requires ATP and bicarbonate as substrates.
05

5. Fatty acid synthesis

The Malonyl-CoA then enters the fatty acid synthesis pathway, where it is converted into fatty acids through a series of reactions involving the condensation, reduction, dehydration, and further reduction. This process requires NADPH, which is commonly produced from the pentose phosphate pathway or via malic enzyme.
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6. Formation of triglycerides

The synthesized fatty acids are then transported to the endoplasmic reticulum, where they are esterified with glycerol-3-phosphate to produce triglycerides. Finally, triglycerides are assembled into lipid droplets and stored in fat cells (adipocytes).
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7. End-product inhibition by ATP

End-product inhibition occurs when the final product of a metabolic pathway acts as an inhibitor for an enzyme involved in the same pathway. In the case of lipogenesis, high levels of ATP can inhibit glycolysis by allosterically inhibiting phosphofructokinase-1 (PFK-1). This decreases the flow of glucose through glycolysis, leading to an accumulation of glucose-6-phosphate. The excess glucose-6-phosphate can then be channeled into alternative pathways, such as the pentose phosphate pathway, which produces NADPH. As mentioned earlier, NADPH is an essential cofactor in fatty acid synthesis. Therefore, high levels of ATP can favor lipogenesis by promoting glucose conversion into fatty acids.

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Most popular questions from this chapter

For many years, the total number of molecules of ATP produced for each molecule of glucose in aerobic respiration was given as 38 . Later, it was estimated to be closer to 36 , and now it is believed to be closer to 30 . What factors must be considered in estimating the yield of ATP molecules? Why are the recent numbers considered to be approximate values?

A friend, wanting to lose weight, eliminates all fat from her diet. How would this help her to lose weight? Could she possibly gain weight on this diet? How? Discuss the health consequences of such a diet.

What purpose is served by the formation of lactic acid during anaerobic metabolism? How is this accomplished during aerobic respiration?

The conversion of glucose 6-phosphate to free glucose, which can be secreted into the blood, occurs in a. the liver. b. skeletal muscles. c. both \(a\) and \(b\).

The net gain of ATP per glucose molecule in lactic acid fermentation is _____; the net gain in aerobic respiration is generally _____. a. \(2 ; 4\) b. \(2 ; 30\) c. \(30 ; 2\) d. \(24 ; 38\)
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