Sulphydryl residues

In the elucidation of the X-ray structure of hCP by the method of isomorphous replacement, gold and mercury heavy atom derivatives were utilized. In the case of the mercury derivative, p-chloromercury-benzoate, the heavy atom bound to the free sulphydryl residue, C221, but for the gold cyanide derivative the gold atom was found to bind in the vicinity of the trinuclear copper cluster. A realistic explanation of this... 

All the complexes consist of several subunits (Table 2) complex I has a flavin mononucleotide (FMN) prosthetic group and complex II a flavin adenine dinucleotide (FAD) prosthetic group. Complexes I, II, and III contain iron-sulphur (FeS) centers. These centers contain either two, three, or four Fe atoms linked to the sulphydryl groups of peptide cysteine residues and they also contain acid-labile sulphur atoms. Each center can accept or donate reversibly a single electron. 

In total the analysis of protein complexes indicates that methionine is the most likely group to bind to [PtCU]2 and [PtenClfc] probably by a displacement reaction. However a specific attachment to another residue such as histidine or sulphydryl can not be ruled out. Furthermore the antitumour activity is that of cis- [Pt (NH 3) 2CI2] and the di-functional character of this reagent has not been revealed so far by the above studies. In fact no attempt has been made to uncover the difference between [PtCl2(NH3)2] in its cis and fraMS-forms. 

Cytochromes are electron-transfer proteins having one or several haem groups. Cytochrome c binds to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochrome c has been proved to be a useful model system for studying the relationship between protein structure and thermostability due to the availability of its three-dimensional structure from a wide variety of organisms, both mesophiles and thermophiles. 

Active Site Structure of Rubredoxin There are several non-heme iron-sulphur proteins that are involved in electron transfer. They contain distinct iron-sulphur clusters composed of iron atoms, sulphydryl groups from cysteine residues and inorganic or labile sulphur atoms or sulphide ions. The labile sulphur is readily removed by washing with acid. The cysteine moieties are incorporated within the protein chain and are thus not labile. The simplest type of cluster is bacteria rubredoxin, (Cys-S)4 Fe (often abbreviated FelSO where S stands for inorganic sulphur), and contains only non labile sulphur. It is a bacterial protein of uncertain function with a molecular weight of 6000. The single iron atom is at the centre of a tetrahedron of four cysteine ligands (Fig.). 

Hey is a sulphydryl amino acid of molecular weight 135.2. It oxides readily to its disulphide form homocystine, and can be converted to the thiolactone form in acid solution. In normal plasma, Hey exists in various forms the sulphydryl form (approximately 1%), bound to the cysteine residues of proteins (approximately 70%), and bound to free cysteine as cysteine-Hcy mixed disulphide (approximately 30%). When levels are elevated, the disulphide form homocystine is formed. All of these forms can be converted to Hey by chemical reduction and then measured as tHcy. 

Thiols exist in biological systems in various oxidised and reduced forms. Very little Hey in the free sulphydryl form is found in plasma and it exists mainly as cysteine-Hcy mixed disulphide, bound to cysteine residues of plasma proteins, or as its disulphide homocystine, but the latter only when levels are high. Routinely measured tHcy constitutes all of these forms together and is produced by chemical reduction. 

The production of sulphur compounds is believed to be very important in the development of Cheddar cheese flavour. Residual sulphydryl oxidase activity may play a role in initially reoxidizing sulphydryl groups exposed upon heating cheesemilk the sulphydryl groups thus protected may be reformed during the ripening process. 

Some of the most important consequences of the heat denaturation of whey proteins are due to the fact that these proteins contain sulphydryl and/or disulphide residues which are exposed on heating (Figure 9.17). They are important for at least the following reasons ... 

Martin (1971) found that the transport of choline across the red cell membrane was inhibited by N-ethylmaleimide (NEM), a sulphydryl reagent which covalently modifies cysteine residues in proteins. The extent of the inhibition depended on... 

The interaction of a non-enzymatic degradation product, D-benzylpenicillenic acid (formed by cleavage of the thiazolidine ring of benzylpenicillin in solution see Fig. 10.3B), with sulphydryl or amino groups in tissue proteins, to form hapten-protein conjugates, is also of importance. In particular, the reaction between D-benzylpenicillenic acid and the e-amino group of lysine (a,e-diamino- -caproic acid, NH2(CH2)4.CH(NH2).COOH) residues... 

Both compounds interfere with purine biosynthesis at one of the three stages where glutamine is required. Addition of either of these two inhibitors to cells leads to an accumulation of formylglycineamide ribotide. The inhibition of the enzyme is reversible for a short period but soon becomes irreversible as a result of the inhibitor alkylating the enzyme, probably through a sulphydryl group of a cysteine residue. 

The amino acid composition is shown in Table 4.4. a-La is relatively rich in tryptophan (four residues per mole). It is also rich in sulphur (1.9%) which is present in cystine (four intramolecular disulphides per mole) and methionine it contains no cysteine (sulphydryl groups). The principal a-la s contain no phosphorus or carbohydrate, although some minor forms may contain either or both. The isoionic point is c. pH 4.8 and minimum solubility in 0.5 M NaCl is also at pH 4.8. 

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