Phosphorylation, adenosine coenzyme

One mechanism that has been proposed to explain the hepatotoxicity of 1,1,2-trichloroethane is the generation of free radical intermediates from reactive metabolites of 1,1,2-trichloroethane (acyl chlorides). Free radicals may stimulate lipid peroxidation which, in turn, may induce liver injury (Albano et al. 1985). However, Klaassen and Plaa (1969) found no evidence of lipid peroxidation in rats given near-lethal doses of 1,1,2-trichloroethane by intraperitoneal injection. Takano and Miyazaki (1982) determined that 1,1,2-trichloroethane inhibits intracellular respiration by blocking the electron transport system from reduced nicotinamide adenine dinucleotide (NADH) to coenzyme Q (CoQ), which would deprive the cell of energy required to phosphorylate adenosine diphosphate (ADP) and thereby lead to depletion of energy stores. 

The thiol used in biological systems for the formation of thioesters is coenzyme A. The compound is written CoASH to emphasize that the thiol group is the reactive part of the molecule. CoASH is composed of a decarboxylated cysteine (an amino acid), pantothenate (a vitamin), and phosphorylated adenosine diphosphate. 

FIGURE 18.8 Coenzyme A is derived from a phosphorylated adenosine diphosphate (ADP) and pantothenic acid bonded by an amide bond to aminoethanethiol, which contains the — SH reactive part of the molecule. 

Organisms of all biological kingdoms convert 64 into the cys-teamine derivative phosphopantetheine (65) using L-cysteine as substrate. 65 is converted to coenzyme A (66) by attachment of an adenosine moiety via a pyrophosphate linker and phosphorylation of the ribose moiety. Phosphopantetheine can be attached covalently to serine residues of acyl carrier proteins that are parts of fatty acid synthases and polyketide synthases. 

The answer is c. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121—138. Wilson, pp 287-320.1 The almost universal carrier of acyl groups is coenzyme A (CoA). However, acyl carrier protein (ACP) also functions as a carrier ol acyl groups. In fatty acid synthesis, ACP carries the acyl intermediates. The reactive prosthetic group of both ACP and CoA is a phosphopantetheine sulfhiydryl. In ACP, the phosphopantetheine group is attached to the 77-residue polypeptide chain via a serine hydroxyl. In CoA, the phosphopantetheine is linked to the 5 -phosphate of adenosine that is phosphorylated in its 3 -hydroxyl. 

Energy from fuel oxidation is converted to the high-energy phosphate bonds of adenosine triphosphate (ATP) by the process of oxidative phosphorylation. Most of the energy from oxidation of fuels in the TCA cycle and other pathways is conserved in the form of the reduced electron-accepting coenzymes, NADH and FAD(2H). The electron transport chain oxidizes NADH and FAD(2H), and donates the electrons to O2, which is reduced to H2O (Fig. 21.1). Energy from reduction 0/O2 is used for phosphorylation of adenosine diphosphate (ADP) to ATP by ATP synthase (FgFjATPase). The net yield of oxidative phosphorylation is approximately 2.5 moles of ATP per mole of NADH oxidized, or 1.5 moles of ATP per mole of FAD(2H) oxidized. 

Obviously, the elucidation of the enzymic mechanism required the preliminary purification of at least one of the transaminases. An 85-90% pure glutamic aspartic transaminase was obtained and found to contain 2 moles of pyridoxal phosphate per mole of enzyme. But pyridoxal is not the active coenzyme. Gunsalus, Bellamy, and Umbreit discovered that the addition of pyridoxal to a culture medium of a strain of Streptococcus faecalis grown on a pyri-doxal-deficient medium has little effect on the ability of the bacteria to decarboxylate tyrosine. When the culture was supplemented with pyridoxal and adenosine triphosphate, or with phosphorylated derivatives of pyridoxal, the tyrosine decarboxylation activity was greatly enhanced. It was later established that... 

In a still later paper " Cohen and McGilvery purified the enzyme further and then found that ATP in the absence of oxidizable metabolite promoted p-aminohippuric acid synthesis anaerobically, that adenosine-monophosphate was active even aerobically only under conditions in which it was phosphorylated to ATP, that AT-phosphoglycine was inactive, as were coenzymes I and II, cocarboxylase, and pyridoxalphosphate. 

The coenzyme adenosine triphosphate (ATP) acts as the central link between energy-yielding metabolic pathways and energy expenditure on physical and chemical work. The oxidation of metabolic fuels is linked to the phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP), while the expenditure of metabolic energy for the synthesis of body constituents, transport of compounds across cell membranes and the contraction of muscle results overall in the hydrolysis of ATP to yield ADP and phosphate ions. The total body content of ATP + ADP is under 350 mmol (about 10 g), but the amount of ATP synthesized and used each day is about 100 mol — about 70 kg, an amount equal to body weight. 

The evidence supporting this concept is derived from experiments on yeast extracts and pigeon liver preparations but further joint studies by Lipmann and Lynen make the formation of phosphoryl-ated or pyrophosphorylated coenzyme A improbable and suggest the formation of enzyme-bound adenosine phosphate and coenzyme A. In the presence of purified yeast enzymes isotopic pyrophosphate was found to exchange the isotope with ATP in the absence of coenzyme A. This excludes the latter as an obligatory participant. Isotopic acetate also exchanged readily with acetyl coenzyme A in the presence of this yeast enzyme. The following scheme of 3 reactions fits the facts ... 

FIGURE 2 Some important reactions in metabolism. Shown are the phosphorylation of ADP to ATP, NAD+, NADH, FAD, FADH2 acetate, CoA, and acetyl CoA. For clarity, just the parts of the larger molecules that undergo reaction are shown. NAD+, nicotinamide adenine dinucleotide NADH, nicotinamide adenine dinucleotide (reduced form) FAD, flavin adenine dinucleotide FADH2, flavin adenine dinucleotide (reduced form) CoA, coenzyme A AMP, adenosine monophosphate. 

In food, thiamin occurs mainly as phosphate coenzymes and the predominant form is TDP (also called thiamin pyrophosphate and cocarboxylase). The phosphate coenzymes are broken down in the gut by phosphatases to give free thiamin for absorption. Thiamin is absorbed mainly from the upper intestine, and less thiamin is absorbed on an empty stomach than when taken with a meal. The latter could be due to the alkaline conditions in the duodenum, which are prevented by the presence of food. Absorption of up to 2 mg per meal occurs by an active saturable process involving a sodium-dependent adenosine triphosphatase and against a concentration gradient. During absorption, thiamin is phosphorylated to the monophosphate ester (TMP). Thiamin is absorbed via the portal venous system. Further phosphorylation to TDP occurs on entry into all tissues. TDP can cross the blood-brain barrier, where a portion is converted to TTP, although even in the brain, TDP is the predominant form of thiamin. A second passive absorption process operates when intakes of thiamin are >5 mg but the maximum that can be absorbed from an oral dose is 2-5 mg. 

This means that the acetyl-(CoA) species are very effective in transferring acetyl groups and are therefore called acetyl carriers, or acetyl transmitters. Coenzyme A is said to be an acyl carrier, so its biological function is similar to ATP (adenosine triphosphate), a phosphoryl carrier. Thus, the biological functions of (CoA) and ATP are similar. 

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