Amino Acid Metabolism Quiz

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Questions and Answers

What is the primary pathway for the disposal of nitrogen in the body?

Conversion to fatty acids

Recycling in amino acid synthesis

Formation of urea (correct)

Elimination as NH4+

Which process is involved in the removal of the α-amino group from amino acids?

Glycolysis

Gluconeogenesis

Hydrolysis

Transamination (correct)

What occurs during the second phase of amino acid catabolism?

Excess amino acids are stored

Amino acids are converted into glucose

Urea is formed from glutamate

α-keto acids are metabolized into common intermediates (correct)

Which substance acts as the donor for the amino group in transamination?

α-ketoglutarate Signup and view all the answers

What happens to amino acids after complete digestion in the gastrointestinal tract?

They are absorbed and then hydrolyzed to free amino acids Signup and view all the answers

Which metabolic fate is NOT associated with the catabolism of amino acids?

Conversion to nitrogen gas Signup and view all the answers

What is the main product of oxidative deamination?

NH4+ Signup and view all the answers

What is the role of aminotransferase in amino acid metabolism?

Catalyzing transamination reactions Signup and view all the answers

What role do aminotransferases play in amino acid metabolism?

They catalyze the transfer of amino groups between amino acids and keto acids. Signup and view all the answers

Which amino acids do not participate in transamination reactions?

Threonine and serine Signup and view all the answers

What is the function of aspartate in the urea cycle?

It provides a nitrogen source for urea synthesis. Signup and view all the answers

What is the combined action of aminotransferase and glutamate dehydrogenase called?

Transdeamination Signup and view all the answers

Which vitamin is a crucial cofactor for all aminotransferases?

Pyridoxal phosphate (Vitamin B6) Signup and view all the answers

Plasma AST is also known as which of the following?

Serum glutamate-oxaloacetate transaminase (SGOT) Signup and view all the answers

What is the primary role of oxidative deamination in amino acid metabolism?

To remove amino groups from amino acids, resulting in the formation of ammonia. Signup and view all the answers

Which of the following statements about the equilibrium of transamination reactions is accurate?

The equilibrium constant for most transamination reactions is near 1. Signup and view all the answers

What role does N-acetylglutamate play in the urea cycle?

It is an allosteric activator of carbamoyl phosphate synthetase I. Signup and view all the answers

Where does the first two reactions of urea synthesis occur?

Mitochondria Signup and view all the answers

Which of the following statements about carbamoyl phosphate synthetase II is correct?

It participates in the biosynthesis of pyrimidines. Signup and view all the answers

What is the effect of protein-rich meals on urea synthesis?

It increases the concentration of N-acetylglutamate. Signup and view all the answers

What is the fate of urea produced in the liver?

It diffuses to the kidneys for excretion in urine. Signup and view all the answers

Which compound is produced from the oxidative deamination of glutamate?

Ammonia Signup and view all the answers

What happens to ammonia if it is not used in synthesizing new amino acids?

It is eliminated from the body due to its toxicity. Signup and view all the answers

How is urea related to the citric acid cycle?

The urea cycle and citric acid cycle are linked biochemically. Signup and view all the answers

What is the primary disposal route for ammonia in the body?

Formation of urea Signup and view all the answers

Which branched-chain amino acid is NOT metabolized by the liver?

Threonine Signup and view all the answers

What is the consequence of a deficiency in branched-chain α-ketoacid dehydrogenase?

Accumulation of α-keto acids Signup and view all the answers

Which amino acids are classified as exclusively ketogenic?

Leucine and Lysine Signup and view all the answers

What type of amino acids contributes to glucose production?

Glucogenic amino acids Signup and view all the answers

What is produced from the catabolism of Isoleucine?

Acetyl CoA + Succinyl CoA Signup and view all the answers

Which of the following is NOT an end product of amino acid catabolism?

Glucosamine Signup and view all the answers

How is ammonia stored non-toxically in the body?

As glutamine Signup and view all the answers

What effect does a high-protein meal have on ammonia production?

Increases ammonia production Signup and view all the answers

Which coenzymes act as inhibitors of glutamate dehydrogenase?

ATP and GTP Signup and view all the answers

Which statement accurately describes the role of glutamine in ammonia transport?

Glutamine serves as a non-toxic transport form of ammonia. Signup and view all the answers

What is the primary function of the enzyme glutaminase?

To convert glutamine into glutamate and ammonium Signup and view all the answers

Which compound provides nitrogen for the urea cycle alongside free NH4+?

Aspartate Signup and view all the answers

How does ammonia released from glutamate dehydrogenase contribute to amino acid metabolism?

It serves as a substrate for the synthesis of non-essential amino acids. Signup and view all the answers

What role does alanine play in the glucose-alanine cycle?

Transports ammonia from muscles to the liver Signup and view all the answers

In which cellular location does oxidative deamination of glutamate primarily occur?

Mitochondria of liver and kidney cells Signup and view all the answers

Study Notes

Nitrogen Metabolism

Nitrogen disposal is a key function
Urea cycle is crucial
Catabolism of the carbon skeleton is involved
Nitrogen-containing substances are processed

Amino Acid Metabolism: Disposal of Nitrogen

Amino acids cannot be stored; excess are catabolized and degraded
Catabolism of amino acids has two phases.
Phase 1: removal of the amino group to form ammonia and a-keto acid. This involves transamination and oxidative deamination.
Amino group is recycled or excreted in urine.
Phase 2: the carbon skeleton of a-keto acids converted into intermediates for energy production.
The most important route for nitrogen disposal is urea

The fate of N₂ in different organisms

Ammonotelic animals: excrete ammonia (fishes, larvae of amphibians)
Ureotelic animals: excrete urea (most land vertebrates, some sharks)
Uricotelic animals: excrete uric acid (birds, reptiles)

Digestion of Dietary Proteins

Dietary proteins are broken down into amino acids by enzymes in the stomach, pancreas, and small intestine.
Enzymes are involved include pepsin, chymotrypsin, elastase, carboxypeptidases etc.
The breakdown results in a chain of oligopeptides and amino acids
Free amino acids and dipeptides are absorbed by epithelial cells
Dipeptides are hydrolyzed into amino acids in the cytosol.

Overall Nitrogen Metabolism

Amino acids are precursors of nitrogen-containing compounds.
Amino acid catabolism is part of nitrogen metabolism in the body.
Nitrogen (N2) from food is converted to amino acids (a.a) and exits as urea then small amounts of $NH_4^+$

Metabolic fates of amino groups: Transamination and Oxidative deamination

The first step in amino acid catabolism is the transfer of the amino group to a-ketoglutarate.
This process is called transamination and is mediated by aminotransferase.
Transamination funnels amino groups to glutamate.
Oxidative deamination releases ammonia.

Substrate Specificity of Aminotransferases

Each aminotransferase is specific for one or few amino acids.
The acceptor is typically $\alpha$-ketoglutarate.
The equilibrium constant for transamination reactions is near 1.

Mechanism of action of aminotransferases

All aminotransferases need pyridoxal phosphate derivatives of vitamin B6.
All amino acids except threonine and serine participate in transamination.
The text describes enzyme reactions in detail.

Threonine and Serine Dehydratase Enzyme

Threonine and serine are amino acids.
Reactions involving dehydratase enzymes in detail.
Some reactions involve the use of a pyridoxal phosphate (PLP) intermediate.

Two Important Transferases

Alanine aminotransferase (ALT) or Glutamate-Pyruvate transferase (GPT): transfers amino group from alanine to produce pyruvate and glutamate.
Aspartate aminotransferase (AST) or Glutamate-Oxaloacetate transferase (GOT): transfers amino group from glutamate to produce aspartate.

Oxidative Deamination

Amino groups of many amino acids are collected in the liver in the form of glutamate.
Glutamate is transported to mitochondria where it undergoes oxidative deamination catalyzed by glutamate dehydrogenase.
Oxidative deamination produces ammonia.

Transdeamination

The combined action of aminotransferase and glutamate dehydrogenase is referred to as transdeamination.

Regulation of Oxidative Deamination

The direction of the reaction depends on the relative concentrations of glutamate, a-ketoglutarate, and ammonia.
Allosteric regulation plays a role, with ATP and GTP as inhibitors, ADP and GDP as activators.

Oxidative Deamination:

The enzyme glutamate dehydrogenase is present in the mitochondrial matrix and can use either NAD+ or NADP+ as oxidants.
Oxidative deamination releases ammonia.
This happens primarily in the mitochondria of liver and kidney.

Glutamine Transports Ammonia

Many tissues (brain) produce ammonia from metabolic processes.
Glutamine transports ammonia in a non-toxic form.
Glutaminase in the liver and kidneys converts glutamine into glutamate and ammonia.

"Glucose-alanine cycle"

The cycle connects muscle glycolysis with liver gluconeogenesis.
Amino acids are degraded in the muscles; the amino groups are collected in the form of glutamate by transamination.
A-amino group can be transferred to pyruvate by alanine aminotransferase.
Alanine is reconverted to pyruvate in the cytosol, enters gluconeogenic pathway.

Urea Cycle

Urea is the major form of nitrogen disposal from amino acids.
One nitrogen comes from ammonium and the other from aspartate.
Glutamate is a key intermediate.
Urea is produced in the liver; transported to kidneys for excretion.

Formation of Carbamoyl Phosphate

The enzyme carbamoyl phosphate synthetase I has an absolute requirement for N-acetylglutamate as an activator.
Carbamoyl phosphate synthetase II is for pyrimidine synthesis, independent of N-acetylglutamate.

Overall Stoichiometry of the Urea Cycle

The reaction summarizing aspects of the urea cycle.

The Citric Acid and Urea Cycles Are Linked

The citric acid cycle and urea cycle intermediates are connected.
Fumarate produced by the urea cycle is an intermediate in the Citric acid cycle.

Fate of Urea

Urea diffuses from liver and transported to the kidneys and excreted into urine or lost in feces.
Bacterial ureases in the intestine act on urea.

Metabolism of Ammonia

Ammonia is toxic; the body removes it.
Sources of ammonia include amino acid catabolism, renal glutaminase activity, bacterial action, and purine/pyrimidine catabolism.

Transport of Ammonia in the Circulation

Ammonia is continuously produced by tissues.
It's removed rapidly.
Urea is the primary pathway for ammonia removal, transported from the liver to kidneys.
Glutamine provides non-toxic transport of ammonia.

Catabolism of the Branched Chain Amino Acids

Branched-chain amino acids (BCAAs) are typically metabolized in skeletal muscle.
Steps in BCAA catabolism include transamination, oxidative decarboxylation, and dehydrogenation.
A deficiency in the enzyme can cause maple syrup urine disease.

Catabolism Carbon Skeleton Amino Acids

Amino acids are classified as glucogenic or ketogenic based on their metabolic end products.
Glucogenic amino acids are converted to glucose precursors, whereas ketogenic amino acids are converted to ketone body precursors.
The catabolism of 10-15% of energy comes from proteins.

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Description

This quiz explores key concepts related to amino acid metabolism, including nitrogen disposal, transamination, and oxidative deamination. Test your knowledge on the functions of aminotransferases and the urea cycle. Perfect for students studying biochemistry.