Amino Acid Metabolism
Amino acid metabolism refers to the processes by which the body synthesizes and breaks down amino acids. This includes the conversion of amino acids into energy, the production of new proteins, and the elimination of excess nitrogen. Key pathways involved in amino acid metabolism include transamination, deamination, and the urea cycle. These processes are essential for maintaining proper cellular function and overall health.
8 Key excerpts on "Amino Acid Metabolism"
eBook - ePub- Antonio Blanco, Gustavo Blanco(Authors)
- 2017(Publication Date)
- Academic Press(Publisher)
...Chapter 16 Amino Acid Metabolism Abstract Amino acids (AA) from the diet join those generated by the degradation of endogenous proteins to form a common metabolic pool in the body. Once at the end of their life, proteins are degraded in lysosomes and in the ubiquitin–proteasome system. Nitrogen balance is the equilibrium that exists in normal well-fed adults between the intake of nitrogen (represented mainly by the protein content of the diet) and the nitrogen excreted via the urine and feces. AA are used in the synthesis of body proteins and other nitrogenous compounds or, if energy is needed, they are catabolized. The amine group is separated by transamination. This reaction transfers the α-amino group of AA to an α-keto acid. The reaction is catalyzed by transaminases or aminotransferases. Except for lysine and threonine, all AA participate in transamination reactions with pyruvate, oxaloacetate, or α-ketoglutarate to form alanine, aspartate, or glutamate, respectively, and the α-keto acids corresponding to the original AA. In turn, alanine and aspartate undergo transamination with α-ketoglutarate, with the amine groups of all AA converging to glutamate. Deamination of glutamate produces α-ketoglutarate and ammonia, which is converted to urea. Another mechanism of removal of ammonia is the formation of glutamine. Urea is formed in liver by a metabolic cycle and eliminated by the kidneys. The C skeletons of AA are metabolized to produce pyruvate or intermediates of citric acid cycle (glucogenic) and others, acetyl-CoA or acetoacetate (ketogenic). Other general mechanisms for AA degradation include decarboxylation, which leads to formation of biogenic amines. Transfer of monocarbon groups from AA (methyl, OH-methyl, formyl, and CO 2) are used in various processes of syntheses, catalyzed by specific methyltransferases. Each AA has different metabolic pathways that catabolize them producing energy or different compounds...
eBook - ePubBiochemistry Explained
A Practical Guide to Learning Biochemistry
- Thomas Millar(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
...8 Metabolism of Amino Acids In this chapter you will learn: that the catabolism of the amine group and carbon skeletons of amino acids are by quite different pathways how the amine group of amino acids is processed to form urea the structure of urea the relationship between urea and uric acid the urea cycle how α-keto acids are formed from of glutamate, aspartate and alanine transamination and deamination of amino acids the importance of the B vitamins in Amino Acid Metabolism how the breakdown pathways of the amino acids lead to the formation of elements of the TCA cycle or ketones strategies for learning about amino acid breakdown about the synthesis of amino acids assimilation of ammonia formation of the carbon skeletons of amino acids the essential and non-essential amino acids. Amino acid degradation Amino acids are the building blocks of proteins (Chapter 4). In our bodies, proteins are continually being broken down into amino acids, and some of these amino acids are used for making new proteins, but the remainder are broken down further. In this chapter, we will examine the pathways for the breakdown of amino acids. The degradation of amino acids is traditionally regarded as a difficult topic to study. Although it is not simple, it is impossible if you do not know the structures of the 20 amino acids that make up proteins. You must know their structures and to almost know their structures is not good enough for learning about their metabolism. Every time you read about an amino acid, look up its structure if you do not know it and spend a minute trying to commit it to memory. Without this foundation you miss out on one of the most exhilarating pieces of learning – the degradation of amino acids. It really is a thrill to see how these amino acids relate to each other and fit into other biochemical pathways...
eBook - ePub- Susan A. Lanham-New, Thomas R. Hill, Alison M. Gallagher, Hester H. Vorster, Susan A. Lanham-New, Thomas R. Hill, Alison M. Gallagher, Hester H. Vorster(Authors)
- 2019(Publication Date)
- Wiley-Blackwell(Publisher)
...Other useful pathways include amino acids irreversibly transformed to a variety of other compounds. Oxidation is the catabolism of amino acid to urea and ammonia generating ATP and CO 2, while de novo formation is the synthesis of amino acids from other amino acids, glucose, and other nitrogen sources (e.g., urea and ammonia). Those pathways within the dotted area identified as the minimum nutritional demand are discussed further below. Figure 7.2 Protein metabolism and the metabolic demand for amino acids:‐a simplified scheme. The body protein pool The body protein pool, about 11 kg for a 70 kg adult man, which is 20% of the fat free mass, is distributed between the cellular mass (75%), extra cellular solids (bone, cartilage, tendons, fascia) 23%, and a minor part in the extracellular fluid (2%). Of the cellular mass within the organs, skeletal muscle protein accounts for about 50%. Protein turnover All intracellular proteins and many extracellular proteins continually turn over, i.e., they are hydrolyzed to their constituent amino acids by proteolytic enzymes in all cells and replaced by new synthesis, a process which accounts for a significant part of cellular energy expenditure. The idea that cells undertake continual replacement had been first suggested by the French physiologist Francois Magendie in Paris early in the nineteenth century. However, this was not pursued until a century later when Rudolf Schoenheimer and David Rittenberg in New York, in the late 1930s, early 1940s, developed stable isotope tracer techniques with the newly discovered isotopes [ 2 H] (deuterium) and [ 15 N]...
eBook - ePub- David A Bender, Shauna M C Cunningham(Authors)
- 2021(Publication Date)
- CRC Press(Publisher)
...In the fed state, amino acids in excess of immediate requirements for protein synthesis are oxidized, and the carbon skeletons are used mainly for the synthesis of fatty acids. Overall, for an adult in nitrogen balance, the total amount of amino acids being metabolized will be equal to the total absorption of amino acids from dietary proteins. Amino acids are also required for the synthesis of a variety of metabolic products, including: • purines (synthesized from glycine) and pyrimidines (synthesized from aspartate) for nucleic acid synthesis • heme, synthesized from glycine • the catecholamine neurotransmitters, dopamine, noradrenaline and adrenaline, synthesized from tyrosine • the thyroid hormones thyroxine and tri-iodothyronine, synthesized from tyrosine (Section 11.16.3.3) • melanin, the pigment of skin and hair, synthesized from tyrosine • the nicotinamide ring of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), synthesized from tryptophan (Section 11.8.2) • the neurotransmitter serotonin (5-hydroxytryptamine), synthesized from tryptophan • the neurotransmitter histamine, synthesized from histidine • the neurotransmitter γ-aminobutyric acid (GABA) synthesized from glutamate (Figure 5.19) • carnitine (Section 5.5.1), synthesized from lysine and methionine • creatine (Section 3.2.3.1), synthesized from arginine, glycine and. methionine • the phospholipid bases ethanolamine and choline (Section 4.3.1.2), synthesized from serine and methionine. Acetyl choline is a neurotransmitter • taurine, synthesized from cysteine. In general, the amounts of amino acids required for synthesis of these products are small compared with the requirement for maintenance of nitrogen balance and protein turnover. 9.3.1 Metabolism of the Amino Nitrogen The initial step in the metabolism of amino acids is the removal of the amino group (–NH 3 +), leaving the carbon skeleton of the amino acid...
eBook - ePubMedical Biochemistry
Human Metabolism in Health and Disease
- Miriam D. Rosenthal, Robert H. Glew(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
...CHAPTER 20 AMINO ACIDS 20.1 FUNCTIONS OF Amino Acid Metabolism Since 20 common amino acids, some with cyclic and branched structures, are utilized for protein synthesis, the synthesis and catabolism of amino acid carbon skeletons can be a complex and daunting subject with a myriad of details. Nonetheless, there are a number of common themes that are of major importance in understanding the overall metabolism of the body. 20.1.1 Synthesis of Amino Acids Some of the 20 common amino acids can be synthesized in the body. The amino-transferase reactions that remove the amino group from most of these amino acids are readily reversible and can therefore be utilized to synthesize amino acids. For example, aspartate aminotransferase can be used to synthesize aspartate from the TCA-cycle intermediate oxaloacetate: Since the glutamate dehydrogenase reaction, too, is reversible, it can be used to incorporate NH 4 + into α-ketoglutarate, generating glutamate. Glutamate, in turn, can donate its amino group for the synthesis of other amino acids: Thus, humans can synthesize a particular amino acid if they can synthesize its corresponding α-ketoacid carbon skeleton. 20.1.1.1 Essential Amino Acids. Certain amino acids, however, cannot be synthesized in the body; these are the essential amino acids and they must be obtained from the diet. The amino acids that are essential in adults are listed in Table 20-1. Two other amino acids, tyrosine and cysteine, can be synthesized from the essential amino acids phenylalanine and methionine, respectively. In addition, although arginine is not an essential amino acid in adults, its rate of synthesis in neonates is not adequate to meet their requirements for optimal growth...
eBook - ePubAmino Acids
Biochemistry and Nutrition
- Guoyao Wu(Author)
- 2021(Publication Date)
- CRC Press(Publisher)
...10 Regulation of Amino Acid Metabolism A metabolic pool of free amino acids (AAs) in organs (e.g., skeletal muscle, liver, and kidneys), tissues (e.g., the blood, white adipose tissue, and skin), cells (e.g., hepatocytes, macrophages, and lymphocytes), and intracellular organelles (e.g., cytoplasm and mitochondria) is relatively constant in healthy individuals at a given developmental stage (Figure 10.1). This reflects the fine balance between the supply of AAs (exogenous and endogenous) and their utilization. In the postabsorptive state, the concentrations of AAs [including nutritionally essential AAs (EAAs) and so-called nutritionally non-essential AAs (NEAAs)] in the plasma and tissues do not fluctuate substantially so as to maintain desirable concentration gradients between the plasma and tissues or cells (Hou et al. 2020 ; Jungas et al. 1992 ; Li et al. 2020 ; Wester et al. 2015). As illustrated in studies with rapidly growing pigs, concentrations of most AAs in cells and tissues (except for arginine in the mammalian liver) are much greater than those in the plasma (Table 10.1). It is noteworthy that concentrations of free AAs in the plasma and tissues vary with species, developmental stage, nutritional state, endocrine status, physical activity, time of the day, and diseased condition (Brosnan 2003 ; Gilbreath et al. 2021 ; Gill et al. 1989 ; Li et al. 2021 ; Wheatley et al. 2014). FIGURE 10.1 Regulation of homeostasis of free amino acids by their input and utilization in a metabolic pool, which can be an organ, a tissue, a cell, and an intracellular organelle. The free pool of amino acids is relatively constant in healthy individuals at a given developmental stage...
eBook - ePub- J. G. Salway(Author)
- 2011(Publication Date)
- Wiley-Blackwell(Publisher)
...Part 7: Metabolism of amino acids and porphyrins 44 Urea cycle and overview of amino acid catabolism Catabolism of Amino Acids Produces Ammonium Ions (NH 4 +) Proteins are hydrolysed in the stomach by pepsin to form amino acids. Further hydrolysis occurs in the intestine. The amino acids are absorbed. Any amino acids in excess of those needed to replace the wear and tear of tissues, and for biosynthesis to hormones, pyrimidines, purines, etc., are used for gluconeogenesis, or for energy metabolism. However, catabolism of amino acids generates ammonium ions (NH 4 +), which are very toxic. Accordingly, NH 4 + is disposed of by conversion to urea which is non-toxic and is readily excreted via the kidney. Ammonium Ions Are Metabolised to Urea in the Urea Cycle Figure 44.1 shows that catabolism of amino acids generates either NH 4 + directly or glutamate, which is subsequently deaminated to form NH 4 +. Ammonium ion reacts with bicarbonate ion (HCO 3 −) and two molecules of ATP in a reaction catalysed by carbamoyl phosphate synthetase I (CPS I) to form carbamoyl phosphate. This now reacts with ornithine to form citrulline in the presence of ornithine transcarbamoylase (OTC). Aspartate (the vehicle for the second amino group) reacts with citrulline to form argininosuccinate, which is cleaved to produce fumarate and arginine. Finally, the arginine is hydrolysed to form urea and in the process generates ornithine which is now available to repeat the cycle. Figure 44.1 An overview of amino acid catabolism and the detoxification of NH 4 + by forming urea. Access a high quality version of this image at http://booksupport.wiley.com. NB Do not confuse the CPS I mentioned here with CPS II which is involved in the synthesis of pyrimidines (Chapter 58). Disorders of the Urea Cycle: OTC Deficiency There are several rare disorders of the urea cycle. However, the most common is OTC deficiency, which is an X-linked disease...
eBook - ePub- M. J. Hill(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
...Chapter 7 Metabolism Of Ammonia, Urea, And Amino Acids, And Their Significance In Liver Disease Angela J. Vince TABLE OF CONTENTS I. Introduction II. Protein III. Peptides IV. Amino Acids A. Amino Acid Uptake B. Amino Acid Synthesis C. Amino Acid Degradation 1.Deamination 2. Decarboxylation V. Ammonia A. Uptake of Ammonia B. Ammonia Assimilation 1. Mechanism of Ammonia Assimilation C. Bacteria Producing Ammonia from Amino Acids and Peptides D. Factors Affecting the Production and Use of Ammonia 1. pH 2. Ammonia Concentration 3. Viability 4. Antibiotics 5. Substrate Effect E. Sources of Intestinal Ammonia 1. Urea 2. Nonurea F. Fate of Intestinal Ammonia VI. Urea A. Bacteria Producing Ureases B. Properties and Control of Urease C. Inhibition of Urea Hydrolysis 1. Antibiotics 2. Acetohydroxamic Acid 3. Immunization 4. Limiting Ammonia Available to Form Urea VII. Liver Disease, Role of Bacteria in Portal-Systemic Encephalopathy (PSE) A. Toxins Incriminated in the Pathogenesis of PSE 1. Ammonia 2. Methionine 3. Tryptophan 4. Fatty Acids 5. False Neurochemical Transmitters 6. Amino Acids B. Treatment 1. Diet 2. Antibiotics 3. Lactulose a. Effect on Fecal Flora b. Cathartic Effect c. Trapping of NH + 4 at Low pH d. Substrate Effect e. Other Effects of Lactulose Acknowledgments References I. Introduction Interest in ammonia formation in the human intestine is centered on its role as a toxin in the pathogenesis of portal-systemic encephalopathy. To this end it has been important to determine first the sources of intestinal ammonia and second the means of preventing its release. Evidence has been accumulating recently that proteins and their derivatives constitute a major source of ammonia formed in the intestine. This displaces the long-standing belief that urea is the single major source of intestinal ammonia...
