S cofactors

In 1958 Barker (20) isolated a red, heat stable, light labile, cofactor which was required for the metabolism of glutamate in cell-extracts of Clostridium tetanomorphum. Subsequently this cofactor was crystallized. X-ray crystallography identified Barker s cofactor as the coenzyme form of Vitamin B12 (15, 21). 

Sulfite and nitrite reductase, heme and Fe S cofactors Ostrowski et al. (1989)... 

The conditions used in an enzyme assay depend on what is to be accomplished by the assay. There are two primary applications of an enzyme assay procedure. First, it may be used to measure the concentration of active enzyme in a preparation. In this circumstance, the measured rate of the enzyme-catalyzed reaction must be proportional to the concentration of enzyme stated in more kinetic terms, there must be a linear relationship between initial rate and enzyme concentration (the reaction is first order in enzyme concentration). To achieve this, certain conditions must be met (1) the concentrations of substrate(s), cofactors, and other requirements must be in excess (2) the reaction mixture must not contain inhibitors of the enzyme and (3) all environmental factors such as pH, temperature, and ionic strength should be controlled. Under these conditions, a plot of enzyme activity (p-rnole product formed/minute) vs. enzyme concentration is a straight line and can be used to estimate the concentration of active enzyme in solution. 

Mann KG, Jerry RJ, Krishnaswamy S. Cofactor proteins in the assembly of blood clotting enzyme complexes. Annu Rev Biochem 1988 57 915-956. 

Figure 2 shows the 9x9 transition matrix that is constructed from these conditional probabilities. It should be noted that all initial and final states are dyads in this case. Since it would be difficult to use Price s cofactor method (5) with such a large matrix, the matrix multiplication method we have described previously (1) was used to evaluate unconditional dyad probabilities (e.g., P(AB), P(CC), etc.). Thus, repeated multiplication of the transition matrix by itself causes it to converge to the matrix shown in Figure 3, which contains unconditional dyad probabilities as its elements. These are then added to obtain monomer probabilities, viz., ... 

Severe hypertriglyceridaemia in a child may be caused by a decrease in lipoprotein lipase activity. This may result from genetic defects in the enzyme itself or in the enzyme s cofactor, apolipoprotein ClI. Lipoprotein lipase is essential for the normal catabolism of chylomicrons and VLDL. 

In order to transfer electrons directly between the electrodes and enzymes, an electron relay that transfers redox equivalents (electrons) from the active site of GOD s cofactor to the electrode surface is needed. The choice for an artificial electron relay depends on a molecule s ability to reach the reduced flavin adenine dinucleotide, FADH2 (in close proximity to the GOD active site), undergo fast electron transfer, and then transport electrons to the electrodes as rapidly as possible. Surridge and co-workers have carried out electron-transport rate studies on an enzyme electrode for glucose using interdigited array electrodes [16]. 

In contrast to mediated electron transfer (MET), the ability for electrons to transfer between an enzyme s cofactor and an electrode (acting as an... 

The OCP of a BFC is determined when there is no current flowing across the device C/max = 0) and thus, no bioelectrocatalysis is taking place at either the bioanode or the biocathode. As previously discussed, the use of an electron mediator requires a potential difference between the electron mediator and the enzyme s cofactor. This therefore can result in lower OCPs of BFCs, since the potential at both the bioanode and the biocathode is determined by the onset potential of the anodic electron mediator (F J and cathodic electron mediator (F ) 7). In... 

Although FeMo-cofactor is clearly knpHcated in substrate reduction cataly2ed by the Mo-nitrogenase, efforts to reduce substrates using the isolated FeMo-cofactor have been mosdy equivocal. Thus the FeMo-cofactor s polypeptide environment must play a critical role in substrate binding and reduction. Also, the different spectroscopic features of protein-bound vs isolated FeMo-cofactor clearly indicate a role for the polypeptide in electronically fine-tuning the substrate-reduction site. Site-directed amino acid substitution studies have been used to probe the possible effects of FeMo-cofactor s polypeptide environment on substrate reduction (163—169). Catalytic and spectroscopic consequences of such substitutions should provide information concerning the specific functions of individual amino acids located within the FeMo-cofactor environment (95,122,149). 

Protein S. Protein S is a single-chain molecule of approximately 78,000 daltons that contains 10 y-carboxy glutamic acid residues in the NH -terminal portion of the molecule. Protein S is a regulatory vitamin K-dependent protein. In plasma 40% of this protein circulates free and 60% circulates bound to C4b binding protein. Free Protein S functions as a nonenzymatic cofactor that promotes the binding of Protein C to membrane surfaces (22—25). 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

What enzyme cofactor is associated with each of the following kinds of reaction s ... 

The luciferin-luciferase reaction of fireflies was first demonstrated by Harvey (1917), although the light observed was weak and short-lasting. Thirty years after Harvey s discovery, McElroy (1947) made a crucial breakthrough in the study of firefly bioluminescence. He found that the light-emitting reaction requires ATP as a cofactor. The addition of ATP to the mixtures of luciferin and luciferase... 

The protein-C pathway is one of the most important anticoagulant mechanisms. It is activated by thrombin. Thrombin binds to a cofactor in the membrane of endothelial cells, thrombomodulin (TM). TM bound thrombin no longer activates clotting factors or platelets but becomes an effective protein C (PC) activator. Activated PC (APC) forms a complex with Protein S, which inactivates FVIIIa and FVa. Hereby generation of Flla by the prothrombinase complex is inhibited (Fig. 9). Thus, the PC-pathway controls thrombin generation in a negative feedback manner. 

S-adenosyl-L-methionine (AdoMet, SAM) is a cofactor and the most important donor of the methyl (CH3-) group for methyltransferases, including COMT. When the methyl-group has been transferred, the remaining demethylated compound is called S-adenosyl-L-homo-cysteine. 

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