Krebs-citric acid cycle

Kay J, Weitzman PDJ (editors) Krebs Citric Acid Cycle—Haifa Century and Still Turning. Biochemical Society, London, 1987. 

Fig. 3. Krebs citric acid cycle. Enzymes involved (1) Condensing enzyme (2) aconitase (3) isocitric acid (4) a-ketoglucaric acid dehydrogenase (4) a succinic acid thiokinasc (5) succinic acid dehydrogenase (6) fumarasc (7) malaic acid dehydrogenase. Abbreviations CA = citric acid ACOM = eij-aconitic acid KG = a-ketoglutaric acid SIC = succinic acid FA = fumaric acid MA = malic acid OA = oxalaceiic acid...

The conversion of fumaric acid to malic acid is an important biological hydration reaction. It is one of a cycle of reactions (Krebs citric acid cycle) involved in the metabolic combustion of fuels (amino acids and carbohydrates) to C02 and H20 in a living cell. 

A second example, provided by one of the steps in metabolism by way of the Krebs citric acid cycle (see Section 20-1 OB), is the oxidation of L-2-hydroxy-butanedioic (L-malic) acid to 2-oxobutanedioic (oxaloacetic) acid. This enzyme functions only with L-malic acid ... 

A perusal of current biochemistry texts suggests that the use of the squiggle has largely died out, but the concept of a group potential and the importance of these phosphate anhydride bonds in energy storage and transfer is universally accepted. Lipmann shared the Nobel Prize in Physiology/Medicine in 1953 with Hans Krebs of the Krebs (citric acid) cycle. His Nobel citation read in part,... 

Melendez-Hevia, E., T.G. Waddell, and M. Cascante (1996). The puzzle of the Krebs citric acid cycle Assembling the pieces of chemically feasible reactions and opportunism in the design of metabolic... 

Evans, Buchanan, and Arnon (41a) have recently found that the ferredoxin-dependent pyruvate and a-ketoglutarate synthesizing reactions function in a new carbon cycle for the photosynthetic fixation of C02. The new cycle was named the reductive carboxylic acid cycle, and apart from pyruvate and a-ketoglutarate synthases, it includes certain of the enzymes associated with Krebs citric acid cycle, operating in the synthetic direction. Photoreduced ferredoxin and ATP, formed by photo-... 

Some Aa are transaminated or deaminated to ketoacids, which are then metabolised by many pathways, including the Krebs-citric acid cycle (Figure 2.3). Others are metabolised to ammonia and urea by the Krebs-Fienseleit urea cycle (Figure 2.4). 

The Krebs-citric acid cycle is the final common pathway for the oxidation of fuel molecules amino acids, fatty acids and carbohydrates. Most fuel molecules enter the cycle as a breakdown product, acetyl coenzyme A (acetyl CoA), which reacts with oxaloacetate (a four-carbon compound) to produce citrate (a six-carbon compound), which is then converted in a series of enzyme-catalysed steps back to oxaloacetate. In the process, two molecules of carbon dioxide and four energy-rich molecules are given off, and these latter are the precursors of the energy-rich molecule ATP, which is subsequently formed and which acts as the fuel source for all aerobic organisms. 

Cobalt most often depresses the activity of enzyme including catalase, amino levulinic acid synthetase, and P-450, enzymes involved in cellular respiration. The Krebs citric acid cycle can be blocked by cobalt resulting in the inhibition of cellular energy production. Cobalt can replace zinc in a number of zinc-required enzymes like alcohol dehydrogenase. Cobalt can also enhance the kinetics of some enzymes such as heme oxidase in the liver. Cobalt interferes with and depresses iodine metabolism resulting in reduced thyroid activity. Reduced thyroid activity can lead to goiter. 

Kay, J. Weitzman, P.D.J. (eds) (1987) Krebs Citric Acid Cycle Haifa Century and Still Tkiming, Biochemical Society Symposium 54, The Biochemical Society, London. 

Another important concept, recently demonstrated, is the interrelationship of protein, fat, and carbohydrate oxidative mechanisms and the final common pathway through the Krebs citric acid cycle (Chapter 16). 

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