Nitrogen balance and amino acid oxidation. The manner in which nitrogen balances vary as a function of protein-N intake in adults is illustrated in the upper. Carnivores can obtain (immediately following a meal) up to 90% of their energy requirements from amino acid oxidation, whereas herbivores may fill only a. Amino Acid Oxidation and the Production of Urea. Amino acids, derived largely from protein in the diet or from degradation of intracellular proteins, are the final class of biomolecules whose oxidation makes a significant contribution to the generation of metabolic energy.


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Nitrogen ranks fourth, amino acid oxidation carbon, hydrogen, and oxygen, in its contribution to the mass of living cells. Atmospheric nitrogen, N 2is abundant but is too inert for use in most biochemical processes.

Amino acid oxidation and food intake

Because only a few microorganisms can convert N 2 to biologically useful forms amino acid oxidation as NH 3amino groups are used with great economy in biological systems. An overview of the catabolism of ammonia and amino groups in vertebrates is provided in Figure p.

Amino acids amino acid oxidation from dietary proteins are the source of most amino groups. Most of the amino acids are metabolized in the liver.

Some of the ammonia that is generated is recycled and used in a variety of biosynthetic processes; amino acid oxidation excess is either excreted directly or converted to uric acid or urea for excretion, depending on the organism.

Excess ammonia generated in other extrahepatic tissues is transported to the liver in the form of amino groups, as described below for conversion to the appropriate excreted form.

With these reactions we encounter the coenzyme pyridoxal phosphate, the functional form of vitamin B 6 and a coenzyme of major importance in nitrogen metabolism.

Figure Amino acid oxidation of the catabolism of amino acids.

Chapter 17

The separate paths amino acid oxidation by the carbon skeleton and the amino groups are emphasized by the orange divergent amino acid oxidation. Figure Overview of amino group catabolism in the vertebrate liver shaded. The amino acids glutamate and glutamine play especially critical roles in these pathways Fig.

Excess ammonia generated in most other tissues is converted to the amide nitrogen of glutamine, then transported to liver mitochondria. In most tissues, one or both of these amino acids are found in elevated concentrations relative to other amino acids.

Chapter 17 : Amino Acid Oxidation and the Production of Urea

Amino acid oxidation muscle, excess amino groups are generally transferred to pyruvate to form alanine. Alanine is another important molecule in the transport of amino groups, conveying them from muscle to the liver.

We begin with a discussion of the breakdown of dietary proteins to amino acids, then turn to a general description of the metabolic fates of amino groups.

  • Amino Acid Oxidation And Urea Cycle
  • Pathways of Amino Acid Degradation

Dietary Protein Is Enzymatically Degraded to Amino Acids In humans, the amino acid oxidation of ingested proteins into their constituent amino acids occurs in the gastrointestinal tract. Entry of protein into the stomach stimulates the gastric mucosa to secrete the hormone gastrin, which in turn stimulates the secretion of hydrochloric acid amino acid oxidation the parietal cells of the gastric glands Fig.

The steady-state levels of N-acetylglutamate are determined by the concentrations of glutamate and acetyl-CoA the substrates for N-acetylglutamate synthase and arginine an activator of N-acetylglutamate synthase, and thus an activator of the urea cycle.

Energy Balance If we consider the urea cycle in isolation, we see that the synthesis of one molecule of urea requires four highenergy phosphate groups Fig.


Each NADH molecule can generate up to 2. Genetic Defects People with genetic defects in any enzyme involved in urea formation cannot tolerate proteinrich diets. Amino acids ingested in excess of the minimum daily requirements for protein synthesis are deaminated in the liver, producing free ammonia that cannot be converted to urea and exported into the bloodstream, and, as we have seen, ammonia is highly toxic.

The absence of a urea cycle enzyme can result in hyperammonemia or in the build-up of amino acid oxidation or more urea cycle intermediates, depending on the enzyme that is missing. Although the breakdown of amino acids can have serious health consequences in individuals with urea cycle deficiencies, a protein-free diet is not a treatment option.

Humans are incapable amino acid oxidation synthesizing half of the 20 common amino acids, and these essential amino acids must be provided in the diet. In the urea cycle, ornithine combines with ammonia, in the form of carbamoyl amino acid oxidation, to form citrulline.

A second amino group is transferred to citrulline from aspartate to form arginine —the immediate precursor of urea. Arginase catalyzes hydrolysis of arginine to urea and ornithine; thus ornithine is regenerated amino acid oxidation each turn of the cycle.

The urea cycle results in a net conversion of oxaloacetate to fumarate, both of amino acid oxidation are intermediates in the citric acid cycle. The two cycles are thus interconnected.