Enzyme, adaptation amino acid, specific

Cultured fibroblasts or amniocytes can be probed with FAO substrates and carnitine. Cell cultures deficient of an FAO enzyme will accumulate specific acylcarni-tine species when incubated with substrates such as palmitate, allowing for the diagnosis of FAO disorders [28-37]. Modifications of this assay system have also been developed for the diagnosis of defects affecting the metabolism of branched-chain amino acids [20, 31, 34]. Recently, this approach was also adapted for the study of peripheral blood mononuclear cells [38]. 

Fig. 7.9. The Y2H system can be used to evolve peptides that bind target proteins with high affinities. A. Brent and co-workers adapted the Y2H system to select for short peptides (20 amino acids) that bind specifically to cyclin-dependent kinase 2 (Cdk2), in hopes of finding an inhibitor to this enzyme [76], E. coli thioredoxin (TrxA) is fused to an AD and used as a rigid scaffold for displaying the peptide libraries. After one round...

The adaptation of these enzymes to extreme environments is related to different factors that influence their 3D structure. Among them changes in specific amino acid residues (mainly glycine), increased number of ion pairs formed, increase in the extent of secondary structure formation and truncated amino and carboxyl termini, increased hydrophobic interaction at subunit interfaces, reduction in the size of loops and in the number of cavities, and an excess of negatively charged amino acids on the protein surface [435,436]... 

Figura 8a TTiioicmplate mechanism (A) and modified diiotemplate mechanism (B) for pep tide biosynihesis. (A) Activated amino acids (AAl, AA2, AA3,. ..) are bound to thiol groups cysteine residues at the corresponding active centers (ACl, AC2, AC3,. . . A central 4 -phos-phopantetheine arm ( H) acts as carrier to build up the peptide chain via transthiolation reac-cihns. E stands for emyrne. (B) Each active center carries its own 4 pho5phopBntetheine residue ( H) at which the corresponding activated amino acid ts bound and transported to each next active center for elongation of the peptide chain (1,2,4)- The 4 -pho phopanteiheine arms themselves ate bound covalently over specific serine residues (denoted 0) to the enzyme. In this model (3), a central 4 -phosphopantetheine arm is not needed ( ). (Adapted from Ref. 71.)...

Induced cells contain a mechanism for concentrating inducers within the cells, and this mechanism appears to play a part in the induction process. The formation of enzyme depends on the continued presence of inducer. The rate of enzyme synthesis with adequate amoimts of inducer is proportional to the growth of the bacteria. When the inducer is removed (by suspending the centrifuged bacteria in fresh medium), enzyme synthesis stops abruptly. The enzyme already formed, however, is stable, and persists unchanged for many generations. Sulfur-labeled amino acids have been used to demonstrate that the induced enzyme is formed directly from free amino acids, and that proteins already in the bacteria do not contribute amino acids to the new enzyme. In the absence of the inducer, the adaptive enzyme retains its label. Some properties of inducers were found in a study of penicillinase production by BadUus cereusJ With this system it was shown that in a brief exposure a small amount of penicillin is specifically bound within the cells, and is not hydrolyzed, but stimulates the production of several equivalents of penicillinase. 

The carboxylases have for their coenzyme pyridoxal phosphate which acts according to the mechanism described on p. 174. A whole series of decarboxylases exists, each being specific for the L-form of a given amino add. Certain of them have been isolated from animal tissues such as liver and kidney, but the majority have been isolated from micro-organisms in which the enzymes appear if their specific substrate is present in the culture medium. In micro-organisms therefore these decarboxylases are adaptive enzymes. The amines produced by the decarboxylation of amino acids (Table XIII) often possess pharmacological activity this is the case for histamine, the product of the decarboxylation of histidine. 

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