Herring enzyme-catalyzed reactions

Alcohol dehydrogenase, a cytosolic enzyme, catalyzes the oxidation of ethanol to acetaldehyde with the generation of NADH. The energy from this NADH is transferred to mitochondria, primarily by means of the malate-aspartate shuttle. When an alcoholic consumes over 50% of his or her energy in the form of alcohol, this shuttle becomes vastly important. Most of the acetaldehyde is oxidized in the mitochondria by mitochondrial aldehyde dehydrogenase, though a cytosolic version of the enzyme also exists. These reactions are shown in Figure 4.69. 

Mary Ellen Jones and her colleagues set out to purify the enzyme or enzymes involved in these two reactions. Their main goal was to determine whether the two reactions are carried out by one protein or more than one. Their findings indicated that the two reactions were both catalyzed by the same enzyme, consisting of a single polypeptide chain. To demonstrate this fact, it was necessary to monitor both enzyme activities at each step in the purification and show that both activities copurified. For this purpose, Jones used specific enzyme assays for both enzyme activities. All fractions were assayed for both enzymatic activities at each stage of the purification. 

Banko et al. [11] speculated that the putative dual-enzyme process was instead catalyzed by a single, multifunctional enzyme. Their data clearly showed that in cell-free extracts of C. acremonium the rate of ACV synthesis was significantly greater when the precursor amino acids were provided than it was when AC and valine were used as substrates for the reaction [11], Jensen and her coworkers [12] demonstrated a similar result using cell-free extracts of S. clavuli-gerus. 

Hatano, 1995b) from Amycolaptosis sp. is known to catalyze more than one different chemical reaction using a substantially different substrate (Palmer et al., 1999). Investigation of this catalytic flexibility in the context of the enolase superfamily raises the question of whether this enzyme may represent an example of nature s present-day reengineering of the superfamily scaffold for an entirely new function. Other examples of catalytically promiscuous enzymes from other superfamilies have been observed, as reviewed by O Brien and Herschlag (O Brien and Her-schlag, 1999). 

Joan B. Broderick was born in 1965. She received a Bachelor of Science in Chemistry from Washington State University and a Ph.D. from Northwestern University, where she was a National Science Foundation Graduate Fellow. She was an American Cancer Society postdoctoral fellow at MIT before joining the faculty at Amherst College as assistant professor in 1993. She moved to Michigan State University in 1998 and to Montana State University in 2005, where she is currently Professor of Chemistry and Biochemistry. Her research interests are in mechanistic bioinorganic chemistry, with a particular focus on enzymes utilizing iron-sulfur clusters to catalyze radical reactions. 

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