Cysteine determination

GSH [ 17 ]. The physiological regulation of GCS activity relies on the availability of L-cysteine [ 18] and on the feedback inhibition by GSH [4,14,19]. The intracellular cysteine concentration approximates the apparent Ku, value of GCS for cysteine (0.1-0.3 mM in rats and humans), whereas intracellular glutamate levels are several fold higher than the value of GCS for glutamate (1.8 mM) [20,21]. Consequently, the availability of intracellular cysteine determines the rate of GSH synthesis under physiological conditions. 

Hassan, S.S.M.,E1-Baz, A.E., Abd-Rabboh, H.S.M., 2007. A novel potentiometric biosensor for selective L-cysteine determination using L-cysteiine-desulfhydrase producing Tridiosporonjirovecii yeast cells coupled with sulfide electrode. Anal. Chim. Acta 602, 108—113. 

Zhang, Z.-Y., Dixon, J. E. Active site labeling of the yersinia protein tyrosine phosphatase The determination of the pKa of active site cysteine and the function of the conserved histidine 402. Biochem. 32 (1993) 9340-9345. 

Sanger also determined the sequence of the A chain and identified the cysteine residues involved m disulfide bonds befween fhe A and B chains as well as m fhe disulfide linkage wifhin fhe A chain The complefe insulin sfruefure is shown m Figure 27 11 The sfruefure shown is fhaf of bovine insulin (from cattle) The A chains of human insulin and bovine insulin differ m only fwo ammo acid residues fheir B chains are identical except for the ammo acid at the C terminus... 

The primary structure of a peptide is given by its ammo acid sequence plus any disulfide bonds between two cysteine residues The primary structure is determined by a systematic approach m which the protein is cleaved to smaller fragments even individual ammo acids The smaller fragments are sequenced and the mam sequence deduced by finding regions of overlap among the smaller peptides... 

In the case of ACSO it was found also that N20 addition reduces the yield of S-allyl-L-cysteine (ACS), indicating that this product is formed by eaq - but not by OH radicals. As a result it can be expected that KBr addition will not reduce the ACS yield. It was found that KBr not only does not reduce the yield of ACS, but it rather increases i ts formation. This is explained as due to ACS formation by reaction of eaq" with ACSO, and its disappearance by reaction with OH radicals to give back ACSO as it is known for the reaction with sulfides. The authors suggest the same reactions for PCSO and PCS (propyl-L-cysteine) although the yield of PCS was not determined. 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

While the redox potentials of Rieske clusters are above -1-100 mV at pH 7, values between 100 and 150 mV have been determined for the redox potentials of Rieske-type clusters (Table XI). Several 4-cysteine coordinated [2Fe-2S] clusters have redox potentials similar to those of Rieske-type clusters, for example, the [2Fe-2S] clusters of the dioxygenase reductases [compilation in (104)]-, therefore, the redox potential is not useful for distinguishing between Rieske-type and ferredoxin-type clusters. 

The structure of phthalate dioxygenase reductase that transfers electrons directly from NADPH to phthalate dioxygenase has been determined by X-ray crystallography (119). In class II or class III dioxygenases, the ferredoxin obligately transfers electrons from the reductase to the terminal dioxygenase (64a) it can be either a Rieske-type ferredoxin or a ferredoxin containing a 4-cysteine coordinated [2Fe-2S] cluster. 

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

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