Enzymes that Catalyze Reactions Involving Phosphate

Enzymes that Catalyze Reactions Involving Phosphate 

EcoRV (EC 3.1.24) and myosin (EC 3.6.4.1), which catalyze the hydrolysis of phosphodiester and ATP, respectively. 

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Enzymes that Catalyze Reactions Involving Phosphate... 

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+. 

Vitamin B6 is a collective term for pyridoxine, pyridoxal, and pyridox amine, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring (Figure 28.10). Pyridoxine occurs primarily in plants, whereas pyridoxal and pyridoxamine are found in foods obtained from animals. All three compounds can serve as precur sors of the biologically active coenzyme, pyridoxal phosphate. Pyridoxal phosphate functions as a coenzyme for a large number of enzymes, par ticularly those that catalyze reactions involving amino acids. 

In 1978 Cohn and Hu (1) demonstrated the isotopic effect by 180 on the Ip-nmr spectrum of inorganic phosphate. For each that replaces an 1 0, an upfield shift of 0.021 ppm results. Thus the chemical shift for HP O 58 is 0.084 ppm downfield from Hpl 04=. Based on this observation it is obvious that any chemical event that potentially involves substitution or exchange of an 0 for 16() in phosphorus containing compounds can be followed by monitoring the Ip-nmr spectrum throughout the course of the reaction. This article describes experiments from my laboratory on two enzymes that catalyze phosphoryl transfer reactions and our use of the [1 0/1 0] lp-nmr methodology to detect intermediates in the enzymic reactions. 

There are two other cofactors that can participate in redox processes these are /lavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate (NADP+). both of which are shown in Fig. 11-2. FAD accepts 2H s and is thereby reduced to FADH2, whereas NADP+ accepts H and is reduced to NADPH and H +. Both of these reduced cofactors can be oxidized, thereby donating their H s (or reducing equivalents), similar to the oxidation of NADH. The enzymes that catalyze those reactions involving an oxidation or a reduction are usually very selective toward a particular cofactor (NAD or NADP) in a particular oxidation state. 

The final part in the construction of adenosylcobalamin synthesis involves the transfer of a-ribazole 5 -phosphate to adenosyl-GDP-cobinamide with displacement of the GDP moiety to generate adenosylcobalamin 5 -phosphate (Figure 27). The enzyme that catalyzes this reaction is CobV in P. denitrificans and Cobs in S. The enzyme from P. denitrificans is able to accept a range of substrates including... 

Mammalian tissues contain enzymes that catalyze the nonoxidative deamination of serine, threonine, and homoserine. Since the postulated reaction mechanism involves a dehydration before the deamination, these enzymes are called dehydrases. L-Serine, L-threonine, and L-homoserine dehydrases have been partially purified and all are specific for the L-amino acid. Serine and threonine dehydrases require pyridoxal phosphate, ATP, and glutathione for activity. Pyridoxal phosphate requires the homoserine enzyme, but the need for ATP and glutathione has not been demonstrated. The reaction is likely to involve the formation of a Schiff base. The homoserine dehydrase has been... 

Since thiol estem are the immediate end products of various oxidative reactions, the phosphorolysis of these intermediates constitutes a mechanism by which the sulfhydryl enzymes and coenzymes involved are constantly regenerated with the simultaneous formation of more generally useful energy-rich acyl-phosphate compounds. In the presence of specific enzymes the phosphoryl groups of the acyl phosphates are transferred to ADP with the formation of ATP. For example, many bacteria contain an enzyme that catalyzes the synthesis of ATP by a reaction between acetyl-P and ADP [reaction (15)]. The enzyme catalyzing this reaction... 

Metal-mediated reactions involving water are essential to life and catalytic industrial processes [1-3]. In biological systems, metalloenzymes containing various divalent metal ions catalyze the hydrolysis of amide, carboxylic ester and phosphate ester bonds using both mono- and multinuclear active-site structural motifs [4—6], Mononuclear metal centers are also found within the active sites of enzymes that catalyze the hydration, or the addition of water, to CO2 [Zn(II)] and nitriles [Co(III)/Fe(III)j [7-10]. In many of these processes, formation of a metal hydroxide moiety via deprotonation of a metal-coordinated water molecule is a key proposed step in the reaction pathway. Thus, a substantial amount of research over the past several years has been directed at dehneating how the structural and electronic environments of biological metal ions influence the pKa of a metal-bound water molecule. In this regard, studies directed at the preparation, characterization and elucidation of the reactivity of discrete metal aqua and hydroxo complexes have been paramoimt [11-13]. 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

Phosphates of pharmaceutical interest are often monoesters (Sect. 9.3), and the enzymes that are able to hydrolyze them include alkaline and acid phosphatases. Alkaline phosphatase (alkaline phosphomonoesterase, EC 3.1.3.1) is a nonspecific esterase of phosphoric monoesters with an optimal pH for catalysis of ca. 8 [140], In the presence of a phosphate acceptor such as 2-aminoethanol, the enzyme also catalyzes a transphosphorylation reaction involving transfer of the phosphoryl group to the alcohol. Alkaline phosphatase is bound extracellularly to membranes and is widely distributed, in particular in the pancreas, liver, bile, placenta, and osteoplasts. Its specific functions in mammals remain poorly understood, but it seems to play an important role in modulation by osteoplasts of bone mineralization. 

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

An example of an a-ketol formation that does not involve decarboxylation is provided by the reaction catalyzed by transketolase, an enzyme that plays an essential role in the pentose phosphate pathway and in photosynthesis (equation 21) (B-77MI11001). The mechanism of the reaction of equation (21) is similar to that of acetolactate synthesis (equation 20). The addition of (39) to the carbonyl group of (44) is followed by aldol cleavage to give a TPP-stabilized carbanion (analogous to (41)). The condensation of this carbanionic intermediate with the second substrate, followed by the elimination of (39), accounts for the observed products (B-7IMIHOO1). 

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