Allosteric control of enzymes

What is the minimum size of synzymes And is there any control effect observed that is analogous to the allosteric control of enzyme activities ... 

Pawlyk AC, Pettigrew DW. Transplanting allosteric control of enzyme activity by protein-protein interactions coupling a regulatory site to the conserved catalytic core. Proc. Natl. Acad. Sci. U.S.A. 2002 99 11115-11120. 

Organs serve as reservoirs or sites for synthesis of the metabolites required for homeostasis at all levels homeostasis is regulated by the endocrine system. Fluxes of metabolites through biochemical pathways are regulated by stoichiometric need, allosteric control of enzyme activity, modification of regulatory enzymes, and changes in their rate of enzyme synthesis or degradation. Hormones often influence these processes and, in turn, metabolites modulate the secretion of hormones. Thus, many... 

BIOSENSORS USING THE APTAMERIC ENZYME SUBUNIT THE USE OF APTAMERS IN THE ALLOSTERIC CONTROL OF ENZYMES... 

The logic in favor of "storing" enzymes indirectly as DNA sequences does not exclude inactivation-reactivation as an important regulatory event. Rather, this event must be restricted to a limited number of enzymes and is most appropriate as a response to brief environmental changes. The processes of inactivation and reactivation include allosteric control of enzyme activities and have been treated in detail in Chapter 4. 

ALLOSTERIC HORMONAL MECHANISMS ARE IMPORTANT IN THE METABOLIC CONTROL OF ENZYME-CATALYZED REACTIONS... 

Fructose 2,6-bisphosphate is formed by phosphorylation of fructose 6-phosphate by phosphofructoki-nase-2. The same enzyme protein is also responsible for its breakdown, since it has fructose-2,6-hisphos-phatase activity. This hifrmctional enzyme is under the allosteric control of fructose 6-phosphate, which stimulates the kinase and inhibits the phosphatase. Hence, when glucose is abundant, the concentration of fructose 2,6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting... 

There are many examples of phosphorylation/dephosphorylation control of enzymes found in carbohydrate, fat and amino acid metabolism and most are ultimately under the control of a hormone induced second messenger usually, cytosolic cyclic AMP (cAMP). PDH is one of the relatively few mitochondrial enzymes to show covalent modification control, but PDH kinase and PDH phosphatase are controlled primarily by allosteric effects of NADH, acetyl-CoA and calcium ions rather than cAMP (see Table 6.6). 

The third type of inhibition is called allosteric inhibition, and is particularly important in the control of intermediary metabolism This refers to the ability of enzymes to change their shape (tertiary and quaternary structure, see Section 13.3) when exposed to certain molecules. This sometimes leads to inhibition, whereas in other cases it may actually activate the enzyme. The process allows subtle control of enzyme activity according to an organism s demands. Further consideration of this complex phenomenon is outside our immediate needs. 

A. Many metabolic pathways are regulated by allosteric control of key enzymes catalyzing the rate-limiting step of the pathway (Figure 5-1). 

In allosteric enzymes, the activity of the enzyme is modulated by a non-covalently bound metabolite at a site on a protein other than the catalytic site. Normally, this results in a conformational change, which makes the catalytic site inactive or less active. Covalent modulated enzymes are interconverted between active and inactive forms by the action of other enzymes, some of which are modulated by allosteric-type control. Both of these control mechanisms are responsive to changes in cell conditions and typically the response time in allosteric control is a matter of seconds as compared with minutes in covalent modulation. A third type of control, the control of enzyme synthesis at the transcription stage of protein synthesis (see Appendix 5.6), can take several hours to take effect. 

The concept of control of metabolic activity by allosteric enzymes or the control of enzyme activity by ligand-induced conformational changes arose from the study of metabolic pathways and their regulatory enzymes. A good example is the multi-enzymatic sequence catalysing the conversion of L-threonine to L-isoleucine shown in Fig. 5.32. 

Allosteric Enzymes Typically Exhibit a Sigmoidal Dependence on Substrate Concentration The Symmetry Model Provides a Useful Framework for Relating Conformational Transitions to Allosteric Activation or Inhibition Phosphofructokinase Allosteric Control of Glycolysis Is Consistent with the Symmetry Model Aspartate Carbamoyl Transferase Allosteric Control of Pyrimidine Biosynthesis Glycogen Phosphorylase Combined Control by Allosteric Effectors and Phosphorylation... 

Thus, regulation of ADPGlc PPase, at both the transcriptional level and by allosteric control of the enzyme, modulates the rate of ADPglucose synthesis and starch synthesis. 

The answer is that allosteric binding sites are important in the control of enzymes. A biosynthetic pathway to a particular product involves a series of enzymes, all working efficiently to convert raw materials into final product. Eventually, the cell will have enough of the required material and will want to stop production. Therefore, there has to be some sort of control which says enough is enough. The most common control mechanism is one known as feedback control, where the final... 

A different control mechanism results from allosteric interactions between distinct sites this is a central regulatory mechanism employed by proteins. While there are numerous examples of allosteric control of the activity of enzymes, receptors, and transport proteins, regulation of ET in redox enzymes has rarely been documented, and no kinetic analysis of such processes has been performed. Interestingly, site-site interactions involved in control of electron... 

These enzymic reactions are essential to all living cells in that they provide the monomeric precursors for the de novo synthesis of DNA. The production of the 2 -deoxyribonucleoside phosphates required for DNA synthesis is carefully regulated through allosteric control of the enzyme by 2 -deoxynucleoside triphosphates and ATP, which regulate both overall activity and substrate specificity. 

The main differences between direct allosteric control of an enzyme (allosteric regulator interacts directly with the enzyme to be regulated) and control through covalent modification are ... 

Glycogen synthase is subject to covalent modification and to allosteric control. The enzyme is active in its phosphorylated form and inactive when dephosphorylated. AMP is an allosteric inhibitor of glycogen synthase, whereas ATP and glucose-6-phosphate are allosteric activators. 

The control of enzyme activity by allosteric regulation of key enzymes is of immense benefit to an organism because it allows the concentration of cellular products to be maintained within very narrow limits. 

However, almost none of appropriate enzyme models have ever been reported for the reaction path control or the allosteric control in homogeneous aqueous solution(2). In biological systems, both control mechanisms are very frequently and significantly operating as in the case of the reaction path control of the amino acid metabolism by individual enzyme(3) or the allosteric control of levels of many important compounds by glutamine synthetase. 

Compounds that act as allosteric activators of enzymes are often precursors of the pathway, so this is a mechanism for feed-forward activation, increasing the activity of a controlling enzyme in anticipation of increased availability of substrate. 

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