Enzymes cofactor specificity

The water-soluble vitamins comprise the B complex and vitamin C and function as enzyme cofactors. Fofic acid acts as a carrier of one-carbon units. Deficiency of a single vitamin of the B complex is rare, since poor diets are most often associated with multiple deficiency states. Nevertheless, specific syndromes are characteristic of deficiencies of individual vitamins, eg, beriberi (thiamin) cheilosis, glossitis, seborrhea (riboflavin) pellagra (niacin) peripheral neuritis (pyridoxine) megaloblastic anemia, methyhnalonic aciduria, and pernicious anemia (vitamin Bjj) and megaloblastic anemia (folic acid). Vitamin C deficiency leads to scurvy. 

One type of enzymes removes specifically the methyl group(s) from lysine residues in histones by an oxidative cleavage of the a-carbon bond of the substrate or by the hydrolase of the methyl amine group by the presence of an iron and a-ketoglutarate as cofactor [76, 90, 91]. 

Whole cells are used in stirred tanks with pH control, producing fS )-2-chloropropanoic acid in 50% yield from the racemate (0.3 M) with an enantiomeric excess of over 95%. This approach was selected in preference to other methods of resolution such as acylation of the racemate and then stereoselective hydrolysis. The dehalogenase enzyme is specific for substrates with a carboxyl group and a 2-chloro or bromo substituent. No cofactor or metal ion is required and reaction involves an inversion of configuration. 

The structure of the ALR2 holoenzyme showed that the catalytic site was situated atop the nicotinamide moiety of the NADPH cofactor. The substrate binding site, which would determine the enzyme s specificity and also presumably bind inhibitors, appeared to be composed of a deep cleft (Figures 3 and 4). It extended away from the catalytic site towards the loop composed of residues between (34 and cc4 and the last 20 residues of the carboxy-terminal meander. This hypothesis was supported by the appearance of poorly resolved density that occupied this region, which suggested the presence of an endogenously bound substrate or inhibitor in the structure of the holoenzyme [16]. Subsequent studies indicate that this electron density may be a citrate molecule, one of the components included in the crystallization mixture. Activity studies indicate that citrate is indeed one of the many inhibitors of the enzyme with a K in the millimolar range [23]. 

It has been proposed that 8-aminodcoxyguanosinc is formed from the nitronate tautomer of 2-nitropropane either by base nitrosation followed by reduction, or via an enzyme-mediated conversion of the nitronate anion to hydroxyiam ine-O-sulfonate or acetate, which yields the highly reactive nitrenium ion NHj (Sodum et al., 1993). Sodum et al. (1994) have provided evidence for the activation of 2-nitropropane to an aminating species by rat liver aryl sulfotransferase in vitro and in vivo. Pretreatment of rats with the aryl sulfotransferase inhibitors pentachlorophenol or 2,6-dichloro-4-nitrophenol significantly decreased the levels of nucleic acid modifications produced in the liver by 2-nitropropane treatment. Partially purified rat liver aryl sulfotransferase activated 2-nitropropane and its nitronate at neutral pH to a reactive species that aminated guanosine at the position. This activation was dependent on the presence of the enzyme, its specific cofactor adenosine 3 -phosphate 5 -phosphosulfate, and mercaptoethanol. It was inhibited... 

Folding and posttranslational Specific enzymes, cofactors, and other components for... 

The particular arrangement of an enzyme s amino acid side chains in the active site determines the type of molecules that can bind and react there there are usually about five such side chains in any particular enzyme. In addition, many enzymes have small nonprotein molecules associated with or near the active site that determine substrate specificity. These molecules are called cofactors if they are noncovalently linked to the protein they are called prosthetic groups if covalently bound. In some enzymes a specific metal ion is required for activity. 

During natural evolution, a broad variety of enzymes has been developed, which are classified according to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (IUBMB). Thus, for each type of characterized enzyme an EC (Enzyme Commission) number has been provided (see http // www.expasy.ch/enzyme/). For instance, all hydrolases have EC number 3 and further subdivisions are provided by three additional digits, e.g. all lipases (official name triacylglycerol lipases) have the EC number 3.1.1.3 and are thus distinguished from esterases (official name carboxyl esterases) having the EC number 3.1.1.1. This classification is based on the substrate (and cofactor) specificity of an enzyme only, however often very similar amino acid sequences and also related three-dimensional structures can be observed. 

Superoxide dismutase (SOD) catalyzes the disproportionation of superoxide to peroxide and oxygen according to equation (2). Four different types of SOD are known, containing either Cu and Zn see Copper Proteins with Type 2 Sites), Fe, Mn, or Ni see Nickel Enzymes Cofactors). The Fe and Mn containing SODs have very similar structures and can be further subdivided into metal-specific (i.e. functioning only when the correct metal is bound) and cambialistic (functioning with either Fe or Mn bound to the active site). 

Enzyme Cofactors- In many enzymatic reactions, and in particular biological reactions, a second substrate (i.e., species) must be introduced to activate the enzyme. This substrate, which is referred to as a cofactor or coenzyme even though it is not an enzyme as such, attaches to the enzyme and is most often either reduced or oxidized during the course of die reaction. The enzyme-cofactor complex is referred to as a holoenzyme. The inactive form of the enzyme-cofactor complex for a specific reaction and reaction direction is called an apoenzyme. An example of the type of system in which a cofactor is used is the formation of ethanol from acetaldehyde in the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinuoleotide (NAD) ... 

The conversion of amino acids to keto acids is usually catalyzed by enzymes called aiuinotransferases. Some aminotransferases can recognize a variety of different amino adds as substrates others are more specific in their action. Aminotransferases use vitamin as a cofactor, and the in the figures indicates that the enzyme is a vitamin B -requiring enzyme. Cofactors are small molecules that bind to specific enzymes and participate in the chemistry of cataly.sis. Some... 

Enzymes are catalytically active proteins that are involved in every in vivo transformation. They enhance the rates of biochemical reactions by 10 to 10 2 by reduction of the free energy of activation. Two distinctive properties of enzymes are their high substrate specificity and the narrow range of conditions under which they are effective. They usually catalyze one reaction of a few substrates. Activities are dependent on pH, temperature, the presence of cofactors, as well as concentrations of substrates and products. Enzymes perform specific reactions because they possess cavities in which substrates are oriented white they are transformed (Figure 1). This process involves interaction of the substrate with amino acids of the enzyme. 

Prior to the advent of recombinant DNA technology, the ability to design proteins was limited to chemical modification methods in which specific residues in a protein are modified at the protein level by chemical agents. Different strategies such as atom replacement and segment reassembly have been used to alter enzyme substrate specificity, activity, cofactor requirement, and stability.f These methods can introduce a diverse... 

---