Amino acid 5-carbon, interconversion

These are the energy producers within the cell. They generate energy in the form of Adenosine Tri-Phosphate (ATP). Generally, the more energy a cell needs, the more mitochondria it contains. Site for Kreb s Citric Acid Cycle Electron transport system and Oxidative Phosphorylation Fatty acid oxidation Amino acid catabolism Interconversion of carbon skeletons. 

The citric acid cycle is not only a pathway for oxidation of two-carbon units—it is also a major pathway for interconversion of metabolites arising from transamination and deamination of amino acids. It also provides the substtates for amino acid synthesis by transamination, as well as for gluconeogenesis and fatty acid synthesis. Because it fimctions in both oxidative and synthetic processes, it is amphibolic (Figure 16—4). 

The citric acid cycle is amphibolic, since in addition to oxidation it is important in the provision of carbon skeletons for gluconeogenesis, fatty acid synthesis, and interconversion of amino acids. 

Further complications may arise with the larger amino acids such as isoleucine, where the R side-chain itself contains a chiral carbon atom [R = CH3CH2C H(CH)3, where the asterisk denotes the second chiral centre]. This molecule is an example of a diastereomer - a molecule with more than one chiral centre. Diastereomers have different physical and chemical properties, and their interconversion is more complicated, and is termed epimerization. 

The interconversion of fructose-6-phosphate and fructose-1,6 bis phosphate is a control point in glycolysis and gluconeogenesis. Gluconeogenesis is a pathway which allows carbon atoms from substrates such as lactate, glycerol and some amino acids to be used for the synthesis of glucose, so it is in effect physiologically the opposite of... 

These one-carbon groups, which are required for the synthesis of purines, thymidine nucleotides and for the interconversion some amino acids, are attached to THF at nitrogen-5 (N5), nitrogen-10 (N10) or both N5and N10. Active forms of folate are derived metabolically from THF so a deficiency of the parent compound will affect a number of pathways which use any form of THF. 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

The metabolic flux distributions around the intermediate pyruvate for different strains and environmental conditions are summarised in Fig. 12. This part of the metabolism has been shown to be an important node for the interconversion between glycolytic C3 metabolites and C4 metabolites of the tricarboxylic acid (TCA) cycle. The different anaplerotic reactions are of special importance for the production of recombinant proteins as they provide precursors, such as oxaloace-tate, for amino acid biosynthesis. Due to that, the flux distribution is noticeably affected by both the cultivation conditions and the carbon source used which indicates flexible adaptation to the environmental situation. The flux from pyruvate to oxaloacetate through the reaction catalysed by pyruvate carboxylase was found to be the main anaplerotic pathway in B. megaterium. 

In Fig. 11, at high concentrations of ethylene carbonate, the rate constants ks[EC] and kR[EC] for insertion into the EBTHI zirconaaziridine 17q are much greater than kSSR and ksss and insertion occurs more rapidly than the equilibrium can be maintained. The product ratio reflects the equilibrium of 17q, where Keq is 17.2 (Eq. 32) [21]. Beak has called this limit a dynamic thermodynamic resolution pathway [66]. In contrast, at the lowest concentration of ethylene carbonate in Fig. 10, the first-order rate constants kSSR and ksss for diastereomer interconversion are comparable to the effective first-order rate constants for insertion. As Keq is known to be 17.2, ks/kR can be calculated the 53% ee of (S)-amino acid ester 19q (Scheme 9) implies that kslkR<0.19 (Eq. 33) and that the rate constant for insertion kR[EC] into the minor diastereomer is at least five times faster than ks[EC] into the major diastereomer. 

R,R-diphenyl ethylene carbonate CR,R-DPEC)) with a racemic zirconaaziridine. (C2-symmetric, cyclic carbonates are attractive as optically active synthons for C02 because optically active diols are readily available through Sharpless asymmetric dihydroxylations [67].) Reaction through diastereomeric transition states affords the two diastereomers of the spirocyclic insertion product protonolysis and Zr-mediated transesterification in methanol yield a-amino acid esters. As above, the stereochemistry of the new chiral center is determined by the competition between the rate of interconversion of the zirconaaziridine enantiomers and the rate of insertion of the carbonate. As the ratio of zirconaaziridine enantiomers (S)-2/(R)-2 is initially 1 1, a kinetic quench of their equilibrium will result in no selectivity (see Eq. 32). Maximum diastereoselec-tivity (and, therefore, maximum enantioselectivity for the preparation of the... 

All tissues have some capability for synthesis of non-essential amino acids, through the interconversion (transamination) of amino acids and their keto-acid carbon skeletons (Figure 1.10). 

Succinyl CoA is a point of entry for some of the carbon atoms of methio-nine, isoleucine, and valine. Propionyl CoA and then methylmalonyl CoA are intermediates in the breakdown of these three nonpolar amino acids (Figure 23.26). The mechanism for the interconversion of propionyl CoA and methylmalonyl CoA was presented in Section 22.3.3. This pathway from propionyl CoA to succinyl CoA is also used in the oxidation of fatty acids that have an odd number of carbon atoms (Section 22.3.2). 

A transferase that also has aldolase activity and has been used to prepare a number of chiral compounds is the enzyme serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1). This enzyme, also known as threonine aldolase, catalyzes the physiological reaction of the interconversion of serine and glycine with pyridoxal phosphate (PLP) and tetrahydrofolate (FH4) as the shuttling cofactor of the C-1 unit. It also catalyzes a number of other reactions, some of which are independent of PLP and FH4 [72]. The SHMT-catalyzed aldolase reaction generates two stereocenters, which it does stereospecifically at the (/.-carbon, whereas it is less strict at the [l-carbon (Scheme 13). Nevertheless, this enzyme from porcine liver, Escherichia coU and Candida humicola (threonine aldolase) has been used to prepare a number of P-hydroxy-a-amino acids [73-76],... 

Quantum mechanical calculations on the conformational properties of norepinephrine have also been reported. Interconversion of ephedrine and pseudo-ephedrine to a slight extent under y-irradiation has been observed. Absorption of carbon dioxide by cupric ephedrinates can be accounted for by carbamate formation rather than formation of metal-carbon dioxide bonds. -Acylation of /3-phenethylamines by protected amino-acids, e.g. N-CBZ-leucine, has been reported. ... 

Norton s group has demonstrated that the insertion of chiral C2-symmetric diphenylethylene carbonate into the Zr-C bond of a zirconaaziridine led to the asymmetric synthesis of an amino acid methyl ester after a subsequent treatment with NaOH in methanol. Since the zirconaaziridine enantiomers interconverted, the reaction was a DKR. It was shown that the efficiency of this process was determined by the balance between the rate of enantiomer interconversion and the rate of insertion. A slow addition of the inserting enan-tiopure carbonate was often required to maximise the stereoselectivity of the reaction, allowing a diastereoselectivity of up to 90% de combined with a quantitative yield to be obtained, as shown in Scheme 1.59. 

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