Hexokinase reactions involving

The third indirect route involves the formation of free glucose from glucose 6phosphate by circumventing the hexokinase reaction. This route is catalyzed by... 

Many assays have been described in which the initial product forms the substrate of an intermediary reaction involving auxiliary enzymes. The assay of creatine kinase (EC 2.13.2), for example, involves hexokinase (EC 2.7.1.1) as the auxiliary enzyme and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) as the indicator enzyme ... 

While the entry of some monosaccharides (such as galactose) into the glycolytic pathway is circuitous, for others, alternative sequences of reactions are available. An alternative pathway exists for the metabolism of fructose, apart from its direct conversion into fructose 6-phosphate via hexokinase. This involves the conversion of fructose by way of fructokinase into fructose 1-phosphate,... 

Figure 22.5 An example of reactions involved in an enzyme-catalyzed recycling processes for amplification of the sensitivity. In the left part (A) of the figure the enzyme-pair hexokinase and pyruvate kinase is used for recycling of the coenzyme ATP/ADP. In the right part (B), substrate recycling of pymvate or lactate is accomplished using the enzyme-pair lactate oxidase/lactate dehydrogenase. A multiplication effect is obtained by combination of A and B resulting in a very high sensitivity [27], The calorimetric sensitivity is further inreased by including catalase (cat).

Hexokinase catalyzes the ATP-dependent phosphorylation of a broad spectrum of 6-carbon sugars. In the reaction involving glucose and ATP, glucose-6-phosphate (G6P) and ADP are formed. 

The principle enzymes of glycolysis involved in regulation are hexokinase (reaction 1), phosphofructokinase (reaction 3), and pyruvate kinase (reaction 10) ... 

Hexokinases, other kinases, and many other enzymes that catalyze reactions involving the hydrolysis of ATP require Mg. The Mg forms a complex with the phosphate groups of ATP. Kinases also require K+. 

Recently, a universal enzyme-coupled fluorescence assay for glycosyl transferases was developed. This method is extremely cost-effective and is based on the quantification of nucleotides produced in the glycosyl transfer reaction. The guanosine diphosphate (GDP), uridine diphosphate (UDP), and cytidine monophosphate (CMP) are phos-phorylated with nucleotide kinase in the presence of excess of ATP, generating ADP. Via coupled enzyme reactions involving ADP-hexokinase,glucose-6-phosphate dehydrogenase, and diaphorase, the ADP is utilized for the conversion of resazurin to resorufin, which is then quantified by fluorescence measurement. 

Many enzymes can generate several intermediates as they process a substrate into one or more products. An example is the enzyme chymotrypsin, which we treat in detail in Case study 8.1. Other enzymes act on multiple substrates. An excunple is hexokinase, which catalyzes the reaction between ATP cuid glucose (the two substrates of the enzyme), the first step of glycolysis (Section 4.8). The same strategies developed in Section 8.1 Ccui be used to deal with such complex reaction schemes, and we shall focus on reactions involving two substrates. 

As in so many reactions involving ATP, yeast hexokinase is activated by trace concentrations of a suitable divalent cation, such as magnesium or manganese. Despite this metal requirement, fluoride has no inhibitory effect. 

Yet another difference is that laboratory reactions are often done using relatively small, simple reagents such as Br2, HC1, NaBH4, OO3, and so forth, while biological reactions usually involve relatively complex "reagents" called coenzymes. In the hexokinase-catalyzed phosphorylation of glucose just shown,... 

ATP is involved in many endergonic reactions necessary for cell metabolism, for example, the first stage of glucose utilization catalyzed by hexokinase ... 

Mechanism for Gluconeogenesis. Since the glycolysis involves three energetically irreversible steps at the pyruvate kinase, phosphofructokinase, and hexokinase levels, the production of glucose from simple noncarbohydrate materials, for example, pyruvate or lactate, by a reversal of glycolysis ( from bottom upwards ) is impossible. Therefore, indirect reaction routes are to be sought for. 

Many examples of product inhibition are to found. Some dehydrogenases are inhibited by NADH (a co-product of the reaction), e.g. PDH and isocitrate dehydrogenase (ICD), which are involved with the glycolysis and the TCA cycle are two such examples. Hexokinase isoenzymes in muscle (but not liver) and citrate synthase are inhibited by their products, glucose-6-phosphate and citrate respectively offering a very immediate fine tuning of reaction rate to match cellular requirements and possibly allowing their substrates to be used in alternative pathways. 

Initial phosphorylations The formation of fructose bisphosphate occurs via two phosphorylation reactions that involve ATP hydrolysis, which are catalysed by hexokinase and phosphofructokinase ... 

After the protection of its DNA from damage, perhaps nothing is more important to a cell than maintaining a constant supply and concentration of ATP. Many ATP-using enzymes have Km values between 0.1 and 1 mM, and the ATP concentration in a typical cell is about 5 mM. If [ATP] were to drop significantly, the rates of hundreds of reactions that involve ATP would decrease, and the cell would probably not survive. Furthermore, because ATP is converted to ADP or AMP when spent to accomplish cellular work, the [ATP]/[ADP] ratio profoundly affects all reactions that employ these cofactors. The same is true for other important cofactors, such as NADH/NAD"1" and NADPH/NADP+. For example, consider the reaction catalyzed by hexokinase ... 

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