Magnesium ions enzyme activators

These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to remove specific functional groups. Examples include decarboxylases, which remove carboxylic acid groups as carbon dioxide, dehydrases, which remove water, and aldolases. The decarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in the presence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiamine pyrophosphate and magnesium ions for activity. 

For each enzyme, an optimized buffer solution is provided. Most of the buffers are based on Tris-HCl and have a pH range of 7.4-8.0. All contain magnesium ions as activators for the enzymes. The ionic strength of the buffers is important and different enzymes require different ionic strengths for optimal activity. In general, three strengths of buffer are used low, medium and high, which contain 10, 50 and 100 mol l-1 sodium or potassium chloride respectively. 

The function of magnesium in enzyme activity may either be to form a complex with the substrate, as in the magnesium-ATP complex formed in creatine kinase and phosphofhictokinase, or to bind to the enzyme and either produce an allosteric activation or play a direct role in catalysis. If an enzyme is known to utilize a nucleotide as one of its substrates, it can be assumed that magnesium is also required for catalysis. The magnesium ion possibly acts as an electrostatic shield. The enzyme pyravate kinase, described earlier, and shown in Figure 1, requires both magnesium and potassium ions for maximal activity. 

The use of alkali and alkaline earth group metal ions, especially those of sodium, potassium, magnesium, and calcium, for maintenance of electrolyte balance and for signaling and promotion of enzyme activity and protein function are not discussed in this text. Many of these ions, used for signaling purposes in the exciting area of neuroscience, are of great interest. In ribozymes, RNAs with catalytic activity, solvated magnesium ions stabilize complex secondary and tertiary molecular structure. Telomeres, sequences of DNA at the ends of chromosomes that are implicated in cell death or immortalization, require potassium ions for structural stabilization. 

Farnesyltransferase is a heterodimeric protein composed of an a- and a /1-sub-unit, and zinc and magnesium ions are required for its activity [7,8]. A closely related heterodimeric enzyme is geranylgeranyltransferase I (GGTase I). It also recognizes proteins with a CAAX-box when X is leucine. Both enzymes share the same a-subunit and the requirement for Zn2+ and Mg2+ but the /1-subunits are different. The similarity of these enzymes highlights the importance of selectivity of farnesyltransferase (FT)-inhibitors. 

While having three metal ions in an enzyme active site is uncommon, it is not unique to PLCBc. The well-known alkaline phosphatase from E. coli (APase) contains two zinc ions and a magnesium ion [67], whereas the a-toxin from Clostridiumperfringens [68]. and the PI nuclease from Penicillium citrinum [69] each contain three zinc ions. Indeed, the zinc ions and coordinating ligands of PI nuclease bear an uncanny resemblance to those of PLCBc as the only differ-... 

Larsen, T.M., Wedeking, J.E., Rayment, I. and Reed, G.H. (1996) A carboxylate oxygen of the substrate bridges the magnesium ions at the active site of enolase structure of the yeast enzyme complexed with the equilibrium mixture of 2-phosphoglycerate and phosphoenolpyruvate at 1.8 A resolution, Biochemistry, 30, 4349-4358. 

The enzyme is activated by magnesium ions, has an optimum pH of about 7.5 and offers a very sensitive assay method for ATP but has much wider applications when used in coupled assays with ATP-converting enzymes. 

Another enzyme used for the measurement of glucose is hexokinase (EC 2.7.1.1) which catalyses the phosphorylation of glucose to produce glucose-6-phosphate with adenosine triphosphate as the phosphate donor and magnesium ions as an activator. The rate of formation of glucose-6-phosphate can be linked to the reduction of NADP by the enzyme glucose-6-phosphate dehydrogenase (EC 1.1.1.49). This indicator reaction can be monitored spectrophotometrically at 340 nm or fluorimetrically ... 

Magnesium ion is usually involved (for charge neutralization ) where high-energy phosphate is moved from one molecule to another by an enzyme, i.e., the metabolically active form of ATP is usually the magnesium chelate. 

Many proteins, including many enzymes, contain hghtly bound metal ions. These may be inhmately involved in enzyme catalysis or may serve a purely structural role. The most common tightly bound metal ions found in metalloproteins include copper (Cu+ and Cu +), zinc (Zn +), iron (Fe + and Fe +), and manganese (Mn +). Other proteins may contain weakly bound metal ions that generally serve as modulators of enzyme activity. These include sodium (Na+), potassium (K+), calcium (Ca +), and magnesium (Mg +). There are also exotic cases for which enzymes may depend on nickel, selenium, molybdenum, or silicon for activity. These account for the very small requirements for these metals in the human diet. 

For example, Bachelard used [Mgtotai]/[ATPtotai ] = 1 in his rate studies, and he obtained a slightly sigmoidal plot of initial velocity versus substrate ATP concentration. This culminated in the erroneous proposal that brain hexokinase was allosterically activated by magnesium ions and by magnesium ion-adenosine triphosphate complex. Purich and Fromm demonstrated that failure to achieve adequate experimental control over the free magnesium ion concentration can wreak havoc on the examination of enzyme kinetic behavior. Indeed, these investigators were able to account fully for the effects obtained in the previous hexokinase study. ... 

Magnesium is bound in the active site of D-xylose isomerase (Carrell et al., 1984, 1989 Farber et al., 1987 Key etal., 1988 Henrick a/., 1989). Here, the site at which two metals [from among Mg(II), Mn(ll), and Co(II)] bind is similar to that found for Fe(ll) in ribonucleotide reductase (Nordlund et al., 1990). The active site of xylose isomerase is shown in Fig. 28. Magnesium ions are preferred in ATP-utilizing enzyme reactions (Mildvan, 1987). 

Primer Activity. Biologically active DNA will replicate itself in vitro if supplied with an enzyme, DNA polymerase, the triphosphates of adenosine, guanosine, cytidine, and thymidine, magnesium ion and the proper conditions of temperature and pH. The same DNA (or other samples of biologically active DNA) if supplied with another enzyme,... 

Since then, a considerable amount of structural and mechanistic information has been collected and yeast enolase is probably the best understood sequential enzyme to date. It is a homodimer and requires two Mg + ions per active site for catalytic activity under physiological conditions, although magnesium can be replaced with a variety of divalent metal ions in vitro. During a catalytic turnover, the metal ions bind to the active site in a kinetically ordered, sequential manner with differential binding affinities. The mode of action of yeast enolase is illustrated in Figure 26 and is unusually well understood since several solid-state structures for each intermediate identified with kinetic methods have been determined. 

The magnesium ion has a smaller radius than Ca2+, a fact that may account for its more ready entry into cells. Mg2+ can often be replaced by Mn2+ with full activity for enzymes that require it. On the other hand, high concentrations of Ca2+ are often antagonistic to Mg2+. This antagonism is clearly seen in the effect of the two ions on irritability of protozoa. Both deficiency of magnesium and excess of calcium in the surrounding medium cause in-... 

The main physiological function of both TAs and LTAs is to collect the divalent cations, particularly magnesium ions, from the surrounding medium, and to flux them through the cell wall to the membrane region where their concentration is kept at the proper level, i.e. from 10 to 15 mM, by the polyphosphate chains of LTAs. The proper concentration of Mg2 + ions is required for the activity of enzymes associated with the membrane. 

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