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Chapter 23: Problem 4

Proteins as Fuel during Fasting When muscle proteins are catabolized in skeletal muscle during a fast, what are the fates of the amino acids?

Short Answer

Expert verified
Amino acids from muscle protein can be converted into glucose for energy or used in the citric acid cycle; ammonia byproduct is excreted as urea.

Step by step solution

01

Amino Acid Release

During fasting, the body breaks down muscle proteins to release amino acids into the bloodstream. This occurs when your body requires an alternative energy source due to the depletion of glucose reserves.
02

Amino Acid Transport

The amino acids released from muscle protein degradation are transported through the bloodstream to the liver, where they undergo further processing.
03

Deamination Process

In the liver, amino acids undergo deamination, which removes the amino group (-NH2) from the amino acids. This process produces keto acids and ammonia.
04

Conversion to Glucose or Energy

The carbon skeletons (keto acids) left after deamination can enter the gluconeogenesis pathway to produce glucose, providing energy to the brain and other tissues. Alternatively, they can be used in the citric acid cycle to generate ATP for immediate energy needs.
05

Ammonia Conversion to Urea

Ammonia produced during deamination is toxic, so the liver converts it into urea through the urea cycle. Urea is then excreted from the body through urine, removing nitrogen waste.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Amino Acid Metabolism
Amino acid metabolism plays a crucial role during fasting. When fasting, your body lacks glucose, the primary energy source. So, muscle proteins are broken down to release amino acids. These amino acids serve dual purposes: they help produce energy and provide nitrogen.
Then, they travel through the bloodstream, reaching the liver for further processing.
Inside the liver, amino acids are transformed to fulfill the body's energy requirements and maintain essential functions. The liver is vital in maintaining balance and allocating resources where needed.
Gluconeogenesis
Gluconeogenesis is the process by which the liver creates glucose from non-carbohydrate sources. During fasting, it's especially important because it provides glucose when dietary sources are absent. Once deaminated, the remaining carbon skeletons of amino acids, known as keto acids, are used in gluconeogenesis.
This pathway helps preserve brain function by ensuring it receives enough glucose.
It is an essential survival mechanism that maintains normal blood glucose levels during prolonged periods without food. With gluconeogenesis, the body effectively transforms components of amino acids into a vital energy supply for the brain and other tissues.
Urea Cycle
The urea cycle is a crucial detoxification process in the liver, where toxic ammonia is converted into urea. Ammonia results from the deamination of amino acids.
Because ammonia can be harmful to cells and tissues, the liver must convert it into a safer substance for elimination.
Urea, being less toxic, is then transported through the bloodstream to the kidneys, where it is safely eliminated from the body in urine.
This conversion process ensures nitrogen is effectively and safely removed, protecting the body from the harmful effects of ammonia buildup.
Deamination
Deamination is the process of removing an amino group from an amino acid. This is a critical step in amino acid metabolism, especially during fasting.
It yields two products: ammonia and a carbon skeleton (or keto acid).
  • Ammonia: A potentially toxic byproduct that is converted to urea.
  • Carbon skeleton: Enter metabolic pathways like gluconeogenesis to produce glucose or contribute to energy production in the citric acid cycle.
This process not only helps manage waste but also reorients resources to provide energy during fasting periods.

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Most popular questions from this chapter

Sources of Glucose during Starvation The typical human adult uses about 160 g of glucose per day, 120 g of which is used by the brain. The available reserve of glucose \((\sim 20 \mathrm{g} \text { of circulating glucose and } \sim 190 \mathrm{g} \text { of glycogen })\) is adequate for about one day. After the reserve has been depleted during starvation, how would the body obtain more glucose?

Decisions on Drug Safety The drug Avandia (rosiglitazone) is effective in lowering blood glucose in patients with type 2 diabetes but also carries an increased risk of heart attack. If it were your responsibility to decide whether this drug should remain on the market (labeled with suitable warnings of its side effects or should be withdrawn, what factors would you weigh in making your decision?

Thermogenesis Caused by Thyroid Hormones Thyroid hormones are intimately involved in regulating the basal metabolic rate. Liver tissue of animals given excess thyroxine shows an increased rate of \(\mathrm{O}_{2}\) consumption and increased heat output (thermogenesis), but the ATP concentration in the tissue is normal. Different explanations have been offered for the thermogenic effect of thyroxine. One is that excess thyroxine causes uncoupling of oxidative phosphorylation in mitochondria. How could such an effect account for the observations? Another explanation suggests that the thermogenesis is due to an increased rate of ATP utilization by the thyroxine-stimulated tissue. Is this a reasonable explanation? Why?

Regulation of Hormone Levels in the Blood The halflife of most hormones in the blood is relatively short. For example, when radioactively labeled insulin is injected into an animal, half of the labeled hormone disappears from the blood within 30 min. (a) What is the importance of the relatively rapid inactivation of circulating hormones? (b) In view of this rapid inactivation, how is the level of circulating hormone kept constant under normal conditions? (c) In what ways can the organism make rapid changes in the level of a circulating hormone?

Metabolism of Glutamate in the Brain Brain tissue takes up glutamate from the blood, transforms it into glutamine, and releases the glutamine into the blood. What is accomplished by this metabolic conversion? How does it take place? The amount of glutamine produced in the brain can actually exceed the amount of glutamate entering from the blood. How does this extra glutamine arise? (Hint: You may want to review amino acid catabolism in Chapter \(18 ;\) recall that \(\mathrm{NH}_{4}^{+}\) is very toxic to the brain.)
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