Oxidative Deamination

As shown in figure 5.15, glutamic acid can be formed through transamination by the combination of an amine group with a-ketoglutaric acid. Glutamic acid is also produced in the liver from the ammonia that is generated by intestinal bacteria and carried to the liver in the hepatic portal vein. Since free ammonia is very toxic, its removal from the blood and incorporation into glutamic acid is an important function of a healthy liver.

Table 5.2 The Essential and Nonessential

Amino Acids

Essential

Nonessential

Amino Acids

Amino Acids

Lysine

Aspartic acid

Tryptophan

Glutamic acid

Phenylalanine

Proline

Threonine

Glycine

Valine

Serine

Methionine

Alanine

Leucine

Cysteine

Isoleucine

Arginine

Histidine (in children)

Asparagine

Glutamine

Glutamic acid Oxaloacetic acid a-Ketoglutaric acid Aspartic acid

HO O

a-Ketoglutaric acid Alanine blue indicate the parts of the molecules that a aspartate transaminase; ALT = alanine transaminase. The amino acids are identified in boldface.)

H\ 1 ^N-c-H

C=0 + |

H —C—H

H-C—H

H —C—H I

H

HO^ ^0

Glutamic acid

Pyruvic acid

If there are more amino acids than are needed for protein synthesis, the amine group from glutamic acid may be removed and excreted as urea in the urine (fig. 5.15). The metabolic pathway that removes amine groups from amino acids—leaving a keto acid and ammonia (which is converted to urea)—is known as oxidative deamination.

A number of amino acids can be converted into glutamic acid by transamination. Since glutamic acid can donate amine groups to urea (through deamination), it serves as a channel through which other amino acids can be used to produce keto

Amino acid a-Ketoglutaric acid

Amino transfer

Keto acid Glutamic acid

Amino acid a-Ketoglutaric acid

Amino transfer

Keto acid Glutamic acid

Urea cycle in liver

Urea cycle in liver

Urea

H

O il

H

\

II

/

N

— C —

N

/

\

H

H

■ Figure 5.15 Oxidative deamination. Glutamic acid is converted to a-ketoglutaric acid as it donates its amine group to the metabolic pathway that results in the formation of urea.

acids (pyruvic acid and Krebs cycle acids). These keto acids may then be used in the Krebs cycle as a source of energy (fig. 5.16).

Depending upon which amino acid is deaminated, the keto acid left over may be either pyruvic acid or one of the Krebs cycle acids. These can be respired for energy, converted to fat, or converted to glucose. In the last case, the amino acids are eventually changed to pyruvic acid, which is used to form glucose. This process—the formation of glucose from amino acids or other noncarbohydrate molecules—is called gluconeogenesis, as mentioned previously in connection with the Cori cycle.

The main substrates for gluconeogenesis are the three-carbon-long molecules of alanine (an amino acid), lactic acid, and glycerol. This illustrates the interrelationship between amino acids, carbohydrates, and fat, as shown in figure 5.17. Recent experiments in humans have suggested that, even in the early stages of fasting, most of the glucose secreted by the liver is derived through gluco-neogenesis. Findings indicate that hydrolysis of liver glycogen (glycogenolysis) contributes only 36% of the glucose secreted during the early stages of a fast. At 42 hours of fasting, all of the glucose secreted by the liver is being produced by gluconeogenesis.

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Responses

  • hamid
    Why is Glutamic Acid: important in oxidative deamination?
    8 years ago
  • Patrick White
    Can aspartic acid be used for synthesis of glucose during early stages of fasting?
    8 years ago
  • ilse niinist
    How is the deamination process related to the Kreb’s cycle?
    7 years ago
  • heike
    When amine group is removed from amino acid leaving ammonia and keto acid , it is called what?
    7 years ago
  • giacinto palerma
    Is oxidative deamination before the krebs cycle?
    5 years ago
  • Jennifer
    What is reduced during oidative deanimation?
    4 years ago
  • milo
    What goes to the kreb's cycle or to fat or glucose in oxidative deamination?
    4 years ago
  • Aimee
    What are the biochemical importance of oxidative deamination?
    2 years ago

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