بیوشیمی لینجر به زبان انگلیسی فصل ۱۸

بیوشیمی لینجر به زبان انگلیسی فصل ۱۸

اسلاید 1: AMINO ACID OXIDATION AND THE PRODUCTION OF UREAI chose the study of the synthesis of urea in the liverbecause it appeared to be a relatively simple problem.—Hans Krebs, article in Perspectivesin Biology and Medicine, 1970

اسلاید 2: We now turn our attention to the amino acids, the finalclass of biomolecules that, through their oxidativedegradation, make a significant contribution to thegeneration of metabolic energy. The fraction of metabolicenergy obtained from amino acids, whether theyare derived from dietary protein or from tissue protein,varies greatly with the type of organism and with metabolicconditions. Carnivores can obtain (immediately followinga meal) up to 90% of their energy requirementsfrom amino acid oxidation, whereas herbivores may fillonly a small fraction of their energy needs by this route.Most microorganisms can scavenge amino acids fromtheir environment and use them as fuel when requiredby metabolic conditions. Plants, however, rarely if everoxidize amino acids to provide energy; the carbohydrateproduced from CO2 and H2O in photosynthesis is generallytheir sole energy source. Amino acid concentrationsin plant tissues are carefully regulated to just meetthe requirements for biosynthesis of proteins, nucleicacids, and other molecules needed to support growth.Amino acid catabolism does occur in plants, but its purposeis to produce metabolites for other biosynthetic

اسلاید 3: In animals, amino acids undergo oxidative degradationin three different metabolic circumstances:1. During the normal synthesis and degradation ofcellular proteins (protein turnover; Chapter 27),some amino acids that are released from proteinbreakdown and are not needed for new proteinsynthesis undergo oxidative degradation.2. When a diet is rich in protein and the ingestedamino acids exceed the body’s needs for proteinsynthesis, the surplus is catabolized; amino acidscannot be stored.3. During starvation or in uncontrolled diabetes mellitus,when carbohydrates are either unavailable ornot properly utilized, cellular proteins are used as fuel. Under all these metabolic conditions, amino acids losetheir amino groups to form -keto acids, the “carbonskeletons” of amino acids. The -keto acids undergo oxidationto CO2 and H2O or, often more importantly, providethree- and four-carbon units that can be convertedby gluconeogenesis into glucose, the fuel for brain,skeletal muscle, and other tissues.

اسلاید 4: The pathways of amino acid catabolism are quitesimilar in most organisms. The focus of this chapter ison the pathways in vertebrates, because these have receivedthe most research attention. As in carbohydrateand fatty acid catabolism, the processes of amino aciddegradation converge on the central catabolic pathways,with the carbon skeletons of most amino acids findingtheir way to the citric acid cycle. In some cases the reactionpathways of amino acid breakdown closely parallelsteps in the catabolism of fatty acids (Chapter 17).

اسلاید 5: One important feature distinguishes amino aciddegradation from other catabolic processes described tothis point: every amino acid contains an amino group,and the pathways for amino acid degradation thereforeinclude a key step in which the -amino group is separatedfrom the carbon skeleton and shunted into thepathways of amino group metabolism (Fig. 18–1). Wedeal first with amino group metabolism and nitrogenexcretion, then with the fate of the carbon skeletonsderived from the amino acids; along the way we see howthe pathways are interconnected.