Dehydrogenases three-substrate reactions

A three-substrate (A, B, and C), two-product (P and Q) enzyme reaction scheme in which all substrates and products bind and are released in an ordered fashion. Glyceraldehyde-3-phosphate dehydrogenase has been reported to have this reaction scheme. The steady-state and rapid equilibrium expressions, in the absence of products and abortive complexes, are identical to the ordered Ter Ter mechanism. See Ordered Ter Ter Mechanism... 

A three-substrate, three-product enzyme reaction scheme in which all substrates bind and all products are released in an ordered sequence. Glutamate dehydrogenase at a pH 8.8 has been reported to have this reaction scheme. 

A three-substrate, three-product enzyme-catalyzed reaction scheme in which the three substrates (A, B, and C) and three products (P, Q, and R) can bind to and be released in any order. A number of enzymes have been reported to have this mechanism for example, adenylosuccinate synthetase , glutamate dehydrogenase, glutamine synthetase , formyltetrahydrofolate synthetase, and tubulin tyrosine ligase . See Multisubstrate Mechanisms... 

Enzymes that catalyze redox reactions often require a coenzyme such as NAD+ or FADH2 in addition to a substrate. These are all multiple substrate enzymes. Each substrate and coenzyme will have its own Km value. The substrates for glutamate dehydrogenase, an enzyme with three substrates in both forward and reverse directions, are shown in Scheme 4.10 with their K values.9... 

The sensitivity and versatility of bioluminescent reactions has led to a wide range of applications (Table 3). Three bioluminescent reactions, firefly luciferase, marine bacterial luciferase, and the aequorin reaction, dominate and account for more than 90% of the applications. The versatility of bioluminescent reactions stems from the dependence of the reactions on key substances such as ATP (firefly reaction), NAD/NADH (marine bacterial reaction), and metal ions such as calcium (aequorin), and the fact that ATP and NAD can in turn be coupled to kinases and dehydrogenases to measure either the enzyme or its substrate. 

Multisubstrate enzymes enzymes that require two or more substrates in order to catalyse a particular reaction. Accordingly, the enzyme forms a ternary (two substrates), quaternary (three substrates), etc. complex. Many enzymes are of this type, e.g. NAD-dependent dehydrogenases must bind both the substrate and NADt See Cleland s short notation. 

The pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes operating in concert to catalyze successive steps in the conversion of pyruvate to acetyl-CoA. The active sites of ail three enzymes are not far removed from one another, and the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution. The overall reaction (see A Deeper Look Reaction Mechanism of the Pyruvate Dehydrogenase Complex ) involves a total of five coenzymes thiamine pyrophosphate, coenzyme A, lipoic acid, NAD+, and FAD. 

Figure 8-11. Representations of three classes of Bi-Bi reaction mechanisms. Horizontal lines represent the enzyme. Arrows indicate the addition of substrates and departure of products. Top An ordered Bi-Bi reaction, characteristic of many NAD(P)H-dependent oxidore-ductases. Center A random Bi-Bi reaction, characteristic of many kinases and some dehydrogenases. Bottom A ping-pong reaction, characteristic of aminotransferases and serine proteases.

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

The NADPH is produced from glucose 6-phosphate in the first three reactions in the pentose phosphate pathway (see below). Hence the pentose phosphate pathway is essential in the erythrocyte and glycolysis provides the substrate glucose 6-phosphate. Individuals with a reduced amount of glucose 6-phosphate dehydrogenase can suffer from oxidative damage to their cells and hence haemolysis. 

Initial rate patterns for Escherichia coli NAD+-dependent coenzyme A-linked aldehyde dehydrogenase (Reaction NAD+ + CoA-SFI + acetaldehyde = NADFI + acetyl-S-CoA + FI+). The results of each of three experiments are shown as a single double-reciprocal plot, and the nonvaried substrate concentrations for each curve are indicated above the data points. 

The last three steps of this four-step sequence are catalyzed by either of two sets of enzymes, with the enzymes employed depending on the length of the fatty acyl chain. For fatty acyl chains of 12 or more carbons, the reactions are catalyzed by a multienzyme complex associated with the inner mitochondrial membrane, the trifunctional protein (TFP). TFP is a heterooctamer of 4/34 subunits. Each a subunit contains two activities, the enoyl-CoA hydratase and the /3-hydroxyacyl-CoA dehydrogenase the /3 subunits contain the thiolase activity. This tight association of three enzymes may allow efficient substrate channeling from one active site to the... 

If an uncoupler somehow caused the breakdown of an intermediate form of an electron carrier, the electron carrier would be set free and electron transport to 02 could continue. However, something evidently is different about oxidative phosphorylation compared with the substrate-level phosphorylation catalyzed by 3-phosphoglyceraldehyde dehydrogenase (see chapter 12), because uncouplers have no effect on the latter reaction. Nor do they affect other soluble enzymes that make or use ATP. On the other hand, a molecule that acts as an uncoupler at any one of the three coupling sites of oxidative phosphorylation invariably has a similar effect at the other two sites. This suggests that uncouplers cause the breakdown of something that is generated at all three sites. 

Enantioselective oxidation of racemic alcohols as well as reduction of racemic ketones and aldehydes have been widely applied to obtain optically active alcohols.25 27 The enzymes catalyzing these reactions are alcohol dehydrogenase, oxidases, and reductases etc. Coenzymes (NADH, NADPH, flavine etc) are usually necessary for theses enzymes. For example, for the oxidation of alcohols, NAD(P)+ are used. The hydride removed from the substrate is transferred to the coenzyme bound in the enzyme, as shown in Figure 24. There are four stereochemical patterns, but only three types of the enzymes are known. 

Four of the Krebs cycle reactions are considered irreversible citrate synthase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, and succinyl-CoA synthase. In two of these, COz is evolved. In one reaction, succinyl-CoA synthase, a substrate-level phosphorylation takes place, in which a high-energy compound, GTP, is generated. Note that in three of the reactions NADH is... 

---