Tryptophan-activating enzyme specificity

A transfer of the adenylyl group i.e., a P—0 split has not been obtained so far (or maybe has not been looked for) with the adenylates in the absence of enzyme. In the presence of the activating enzymes, on the other hand, the adenylyl group is veiy readily transferred onto pyrophosphate to give ATP 116, 169, 164, 177). This reaction, however, is not specific, since tryptophan activating enzyme, for example, can synthetize ATP from all the amino acid adenylates tested 116) as well as from the adenylate of D-tiyptophan 169). The same transfer of the AMP moiety onto PP does, of course, also occur in the amino acid catalyzed ATP-PP exchange reaction. 

At One time it was thought that women taking oral contraceptives were at risk for B deficiency. This notion seem-S to have been in error. The error was due to a misinterpretation of the tryptophan load lest. As mentioned earlier, a deficiency in vitamin B(,can induce the accumulation of specific intermediates of the tryptophan catabolic pathway and enhanced excretion in the urine. Oral contraceptives can also induce ar increase in the formation and excretion of specific intermediates by stimulating the activity of specific enzymes of the tryptophan catabolic pathway, This stimulation was responsible for the false indications of deficiency. Independently of the tryptophan load test, there continues to be some evidence for risk associated with the use of oral contraceptives. Oral contraceptive use may result in lowered levels of plasma vitamin Bf, Tlicsc lowered levels may result in a vitamin deficiency when coupled with pregnancy and lactation. 

In addition to the striking effects of sub-unit interaction upon catalytic activity and specificity which have been noted for the L-tryptophan synthetase from Escherichia coli the enzyme and its a and 2 sub-units have been shown to catalyse a variety of other reactions not entirely connected with L-tryptophan biosynthe-... 

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

Not a great deal is known about factors that actually activate tryptophan hydroxylase. In particular, the relative contribution of tryptophan supply versus factors that specifically modify enzyme activity under normal dietary conditions is unknown. However, removal of end-product inhibition of tryptophan hydroxylase has been firmly ruled out. Also, it has been established that this enzyme is activated by electrical stimulation of brain slices, even in the absence of any change in tryptophan concentration, and so other mechanisms are clearly involved. 

CNTs can be functionalized with protein via non-covalent bond (Li et al., 2005 Kim et al., 2003 Mitchell et al., 2002). For example, (beta-lactamase I, that can be immobilized inside or outside CNTs, doesn t change enzyme s activity (Vinuesa and Goodnow, 2002). Taq enzyme can attach to the outside of CNT, and doesn t change its activity (Cui et al., 2004). Peptide with Histidine and Tryptophan can have selective affinity for CNT(Guo et al., 1998). Monoclonal antibody can attach to SWNTs. Protein-modified CNTs can be used to improve its biocompatibility and biomolecular recognition capabilities (Um et al., 2006). For example, CNTs functionalized with PEG and Triton X-100 can prevent nonspecific binding of protein and CNTs. Biotin moiety is attached to the PEG chains Streptavidin can bind specifically with biotin-CNT (Shim et al., 2002). 

Estimation of the Binding Site. Tryptophan-108 shows a specific reaction with iodine, distinguishing it from other tryptophan residues of lysoz3mie. When try-108 is selectively oxidized by iodine, lysozyme completely loses its activity. Nevertheless, the lysozyme still shows the ability to form an enzyme-substrate complex with CM-chitin. This observation contributes to the conclusion that try-62 is an essential binding site for a complex formation (13). All ozonized lysozymes formed strong complexes with CM-chitin and could only be eluted by 0.2N HA (Fig. 6). This further confirms that two tryptophan residues (108 and 111) are indispensible for the hydrolytic action of lysozyme, and that inactivation by ozone cannot be attributed to inhibition of substrate binding capability. 

A key structural and mechanistic feature of lactate and malate dehydrogenases is the active site loop, residues 98-110 of the lactate enzyme, which was seen in the crystal structure to close over the reagents in the ternary complex.49,50 The loop has two functions it carries Arg-109, which helps to stabilize the transition state during hydride transfer and contacts around 101-103 are the main determinants of specificity. Tryptophan residues were placed in various parts of lactate dehydrogenase to monitor conformational changes during catalysis.54,59,60 Loop closure is the slowest of the motions. 

Some of the above features can be seen in the spectra of cathepsin D (Fig. B3.5.10), where the intact single-chain bovine enzyme is compared with the same material cleaved at an exposed loop but without dissociation. The latter has 50% of the specific activity of the intact molecule. Phenylalanine residues can be seen to be present in specific environments in both forms. The fine structure of the Lb transition of tryptophan is superimposed on the broad peak of the La transition, which is apparently more intense in the intact enzyme. Alternatively, there could be a greater contribution from disulfide bonds, but the absence of ellipticity above 320 nm favors the former assignment and the CD is therefore consistent with a limited increase in dynamics of the molecule as a result of the chain... 

The biosynthesis of tryptophan. Tryptophan is synthesized in five steps from chorismate. Each step requires a specific enzyme activity. The four intermediates between chorismate and tryptophan serve no function other than as precursors of tryptophan. The detailed steps of each reaction in this sequence are described in chapter 21. 

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