Hexokinase substrates

Hexokinase (substrate-level regulation by glucose-6-phosphate)... 

Retention, too, is highly tissue-specific. Sometimes, the extraction mechanism is also the retention mechanism, as for Tc-sestamibi, which is retained in mitochondria as long as transmembrane potentials remain intact. Others are separate. F-2-Fluorodeoxyglucose enters the cell by the same pathway as glucose, but is trapped because it is not a substrate for hexokinase, preventing further intracellular metabohsm. 

T"he extraordinary ability of an enzyme to catalyze only one particular reaction is a quality known as specificity (Chapter 14). Specificity means an enzyme acts only on a specific substance, its substrate, invariably transforming it into a specific product. That is, an enzyme binds only certain compounds, and then, only a specific reaction ensues. Some enzymes show absolute specificity, catalyzing the transformation of only one specific substrate to yield a unique product. Other enzymes carry out a particular reaction but act on a class of compounds. For example, hexokinase (ATP hexose-6-phosphotransferase) will carry out the ATP-dependent phosphorylation of a number of hexoses at the 6-posi-tion, including glucose. 

In most animal, plant, and microbial cells, the enzyme that phosphorylates glucose is hexokinase. Magnesium ion (Mg ) is required for this reaction, as for the other kinase enzymes in the glycolytic pathway. The true substrate for the hexokinase reaction is MgATP. The apparent K , for glucose of the animal... 

In the kidney and in muscle tissues, fructose is readily phosphorylated by hexokinase, which, as pointed out above, can utilize several different hexose substrates. The free energy of hydrolysis of ATP drives the reaction forward ... 

S] + K )] for the hexokinase-catalyzed phosphorylation reactions of 2DG and D-glucose, respectively [S (substrate) + E (enzyme) — ES— -I- P (product)]. This constant (LC) accounts for the ratio of the arteriovenous extraction fraction (by transport and phosphorylation) of 2DG to that of D-glucose (LC= 1) under steady-state conditions. This concept can be directly applied to the case of 2DFG by employing the LC (-0.5) for 2DFG. 

Deoxy-3-fluoro-D-glucose (see Section 11,2), a weak substrate for yeast hexokinase, is phosphorylated enzymically - to give the 6-phosphate 588, which is transformed into 2-deoxy-2-fluoro-D-arabinose 5-phos-phate (589) by lead tetraacetate oxidation. 

It may be identified as a nonequilibrium reaction in which the of the enzyme is considerably lower than the normal substrate concentration. The first reaction in glycolysis, catalyzed by hexokinase (Figure 17-2), is such a flux-generating step because its for glucose of 0.05 mmol/L is well below the normal blood glucose concentration of 5 mmol/L. 

Computer models showing the shape of hexokinase (a) without and (b) with bound glucose. The enzyme folds around the substrate to bind it and isolate it from its aqueous environment. 

Luckily, water is not a good substrate for hexokinase. Otherwise, the ATP hydrolysis would... 

Figure 30. A medium complexity model of yeast glycolysis [342], The model consists of nine metabolites and nine reactions. The main regulatory step is the phosphofructokinase (PFK), combined with the hexokinase (HK) reaction into a single reaction vi. As in the minimal model, we only consider the inhibition by its substrate ATP, although PFK is known to have several effectors. External glucose (Glc ) and ethanol (EtOH) are assumed to be constant. Additional abbreviations Glucose (Glc), fructose 1,6 biphosphate (FBP), pool of triosephosphates (TP), 1,3 biphosphogly cerate (BPG), and the pool of pyruvate and acetaldehyde (Pyr).

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 ... 

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. 

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