Sulfhydryl groups, metal binding

Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4 90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101... 

Heavy metals with no known biological function, such as aluminum, arsenic, lead, and mercury, are nonessential metals.4-5 These metals are toxic because they can irreversibly bind to enzymes that require metal cofactors. Toxic metals readily bind to sulfhydryl groups of proteins.6-7 In fact,... 

The vast majority of biochemical processes in which a metal plays a role involve a only a relatively small number of metals. Those metals include Na, K, Mg, Ca, Mo, or the first-row transition metals from V to Zn. Only molybdenum could be considered as a heavy metal. It should also be observed that the metal ions constitute those that can be considered as hard or borderline in hardness. It is a general property that ions of heavy metals having low charge (that is to say "soft") are toxic. These include Hg, Pb, Cd, H, and numerous others. Some heavy metals bind to groups such as the sulfhydryl (-SH) group in enzymes, thereby destroying the ability of the enzyme to promote the reaction in a... 

Many metals bind to the sulfur of sulfhydryl groups in proteins. Metals that bind sulfur in preference to oxygen not only form strong cr bonds with the readily polarizable ligands, but also tt bonds by back-donation of electrons from metal drr to ligand dir orpir orbitals. The electronegativity... 

Inhibition by a variety of metal-binding agents competitive with respect to phosphoryl substrates (118-120) has suggested that an enzyme-bound divalent cation (other than Mg2+) may participate also in the binding of phosphate substrates. Observed inhibition by p-chloro-mercuriphenyl sulfonate and iodoacetate suggests the possibility that sulfhydryl groups may also be involved at, or near, the active enzymic site (119, 120). 

In response to the presence of detrimental Cd +, Hg +, Pb +, and other heavy metal ions, the human hver and kidneys synthesize more metallothionein, an unusual small protein in which approximately one-third of the 61 amino acid residues are cysteine see Metallothioneins). The frequency and juxtaposition of sulfhydryl groups provide strong binding sites for several heavy metal ions. Though not as profusely as metallothionein, many proteins contain sulfhydryl groups that may become metalated by toxic heavy metal ions such as Cd +, Hg +, and Pb +, and it is widely believed that this complex formation explains the toxicity of these metal ions. The exact proteins where the most consequential damage occurs remain uncertain. 

Treatment for these poisons is the administration of sulfhydryl reagents with adjacent sulfhydryl groups to compete with the dihydrolipoyl residues for binding with the metal ion, which is then excreted in the urine. Indeed, 2,3-dimercaptopropanol (see Figure 17.20) was developed after World War I as an antidote to lewisite, an arsenic-based chemical weapon. This compound was initially called BAL, for British anti-lewisite. 

The metal-binding proteins metallothionein and ceruloplasmin are recurring themes in the study of zinc and copper Metallothionein is a small protein with a protein binds zinc and copper ions, as well as nonnutritive heavy metals. The protein consists of about 60 amino acids, and has a molecular wreight of about 7000. One third of these amino acids (20 of them) are cysteine. The 20 sulfhydryl groups of these cysteine residues can bind a total of 7 bivalent metal ions, i.e., 7 zinc atoms or 7 copper atoms (l agi and Schaffer, 1988). 

After inhalation, 70-80% of metallic vapor is retained and absorbed. Little is taken up in the gastrointestinal tract, and less than 10% is absorbed. In the body, it is oxidized to mercuric mercury, which binds to reduced sulfhydryl groups. The kidney is the main depository following exposure to both metallic and mercuric mercury. In addition to other organs, it passes into the brain and fetus. 

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