B12 METABOLISM IN HUMANS By NICOLE AURORA LEAL A ...

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26 degradation of propanediol suggests that this compound serves an important purpose in the lifestyle of Salmonella (Roth et al. 1996). Salmonella uses propanediol as a carbon and energy source in a B12-dependent manner. The proposed pathway of propanediol degradation was determined based on biochemical studies (Figure 1-6). The pathway begins with the conversion of 1,2-propanediol to propionaldehyde, a reaction catalyzed by the AdoCbl-dependent diol dehydratase (Toraya et al. 1979, Obradors et al. 1988). Propionaldehyde is then disproportionated to propanol and propionic acid by a reaction series thought to involve propanol dehydrogenase, phosphotransacylase, and propionate kinase (Toraya et al. 1979, Obradors et al. 1988). Aerobic growth of S. enterica on 1,2-propanediol requires addition of CNCbl (or other complex cobalamin precursors) to growth media. Research Overview Coenzyme B12-dependent processes are essential for human health. Methylmalonyl-CoA mutase and methionine synthase, the only two reported B12-dependent enzymes in humans, are crucial for propionate metabolism and methionine synthesis. Inborn errors of cobalamin metabolism can block these pathways resulting in methylmalonic aciduria and homocystinuria, serious diseases that are often fatal in newborns. Cobalamin metabolism has been studied in both bacterial and eukaryotic systems, and has been found to be quite similar. Progress in identifying human genes involved in B12 metabolism has been slow due to difficulties in purifying the corresponding enzymes and the lack of facile genetic techniques. To circumvent these problems, S. enterica was used as a model system for studies of B12 metabolism in humans.
Chapter 2 of this dissertation reports the identification of the human 27 ATP:cob(I)alamin adenosyltransferase involved in AdoCbl metabolism. An S. enterica mutant deficient in ATR activity allowed for the isolation of a bovine ATR cDNA. Subsequent sequence similarity searching was used to identify a homologous human cDNA. Both the bovine and human cDNAs were independently cloned and overexpressed in E. coli. Enzyme assays showed that both the bovine and human enzymes had ATR activity in vitro. Subsequent studies showed that the human cDNA clone complemented an ATR-deficient S. enterica strain for AdoCbl-dependent growth on 1,2-propanediol in vivo. In addition, Western blots were used to show that ATR expression is altered in cell lines derived from cblB methylmalonic aciduria patients compared with cell lines from normal individuals. Chapter 3 of this dissertation reports the biochemical properties of the human ATR and its interaction with methionine synthase reductase. Two common polymorphic variants of the ATR, which are found in normal individuals, were expressed in E. coli and purified to apparent homogeneity. Purified ATR variants were used for kinetic studies and the catalytic properties of both ATR variants including the Vmax and the Km for ATP and cob(I)alamin were determined. Investigations also showed that the purified methionine synthase reductase in combination with purified ATR can convert cob(II)alamin to AdoCbl in vitro. In this system, MSR reduced cob(II)alamin to cob(I)alamin which was adenosylated to AdoCbl by the ATR enzyme and results indicated that MSR and ATR interact in such a way that the highly reactive intermediate (cob(I)alamin) was sequestered. The finding that MSR can reduce cob(II)alamin to cob(I)alamin for AdoCbl synthesis (in conjunction with the prior finding that MSR
Page 1 and 2: B12 METABOLISM IN HUMANS By NICOLE
Page 3 and 4: The work in this dissertation is de
Page 5 and 6: TABLE OF CONTENTS Page ACKNOWLEDGME
Page 7 and 8: General Molecular and Protein Metho
Page 9 and 10: LIST OF TABLES Table page 1-1. Aden
Page 11 and 12: 3-7. Stoichiometry of the MSR-ATR s
Page 13 and 14: DTT dithiothreitol E. coli Escheric
Page 15 and 16: Abstract of Dissertation Presented
Page 17 and 18: CHAPTER 1 INTRODUCTION History of C
Page 19 and 20: 3 (DMB). The DMB moiety is covalent
Page 21 and 22: 5 and deoxyadenosine. After hydroge
Page 23 and 24: 7 homocysteine recycling and methio
Page 25 and 26: methylmalonic acid. Methylmalonic a
Page 27 and 28: folate-dependent reactions includin
Page 29 and 30: tetanomorphum, P. shermanii, Euglen
Page 31 and 32: enzyme. This suggests that cob(II)a
Page 33 and 34: at the 5’-C of ATP by cob(I)alami
Page 35 and 36: iochemical studies and complementat
Page 37 and 38: 1997). Both cases of the cblD disor
Page 39 and 40: 23 The cblB complementation group i
Page 41: (Watkins et al. 2002). Some patient
Page 45 and 46: Corrin Ring Dimethyl benzimidazole
Page 47 and 48: Classes Eliminases Dehydratases OH
Page 49 and 50: A) B) CH 3THF 33 MS-cob(I)alamin Me
Page 51 and 52: propanol dehydrogenase (PduQ) propa
Page 53 and 54: or from mutations that impair the a
Page 55 and 56: 1989). 5-bromo-4-chloro-3-indolyl-
Page 57 and 58: 41 5’-GCCGCCGGTACCGATGACGACGACAAG
Page 59 and 60: 43 Growth of Adenosyltransferase Ex
Page 61 and 62: anti-rabbit IgG (γ-chain specific)
Page 63 and 64: sequence similarity to a known ATR
Page 65 and 66: ATRs were produced as fusion protei
Page 67 and 68: 51 of the lacI repressor (not shown
Page 69 and 70: in which proteins bind B12, the "ba
Page 71 and 72: libraries may be particularly usefu
Page 73 and 74: 57 Figure 2-1. Multiple sequence al
Page 75 and 76: Table 2-2. Specific activities of b
Page 77 and 78: 61 1 2 3 4 5 25 kDa Figure 2-4. Wes
Page 79 and 80: 63 (cyanocobalamin, CNCbl) and hydr
Page 81 and 82: 65 5’- triphosphate (ATP), and di
Page 83 and 84: extracts of ATR 239K (160 mg protei
Page 85 and 86: Milford, MA). Samples (200 µl) wer
Page 87 and 88: Linearity of the ATR reaction 71 Th
Page 89 and 90: 73 reactions was characteristic of
Page 91 and 92: 75 MSR Produces little Cob(I)alamin
Page 93 and 94:
Johnson et al. 2001, Saridakis et a
Page 95 and 96:
AdoCbl by the MSR-ATR system. Exper
Page 97 and 98:
propionyl-CoA PCC (2S)-methylmalony
Page 99 and 100:
kDa 97 66 45 31 21 14 7 83 1 2 3 4
Page 101 and 102:
85 Table 3-3. The MSR-ATR system se
Page 103 and 104:
Absorbance 0.5 0.4 0.3 0.2 0.1 0.0
Page 105 and 106:
Wavelength, (525 nm) 0.24 0.23 0.22
Page 107 and 108:
Activity, (nmol min-1 Activity, (nm
Page 109 and 110:
al. 1988). The aldehyde is then dis
Page 111 and 112:
CoA-acylating propionaldehyde dehyd
Page 113 and 114:
97 previously published DNA sequenc
Page 115 and 116:
mM CHES (2-[N-cyclohexylamino]ethan
Page 117 and 118:
101 For the conjugation step, strai
Page 119 and 120:
103 from New England Biolabs (Bever
Page 121 and 122:
105 Propionaldehyde Dehydrogenase A
Page 123 and 124:
107 fraction. Following centrifugat
Page 125 and 126:
109 antiserum obtained was specific
Page 127 and 128:
111 biochemical evidence was presen
Page 129 and 130:
113 Table 4-1. Bacterial strains Sp
Page 131 and 132:
115 Table 4-3. Propionaldehyde dehy
Page 133 and 134:
kDa 209 80 49 35 29 117 1 2 3 4 Fig
Page 135 and 136:
CHAPTER 5 CONCLUSIONS In humans, co
Page 137 and 138:
121 substitutions that are common p
Page 139 and 140:
123 Studies also indicated that hum
Page 141 and 142:
LIST OF REFERENCES Aberg, A., S. Ha
Page 143 and 144:
127 Bradford, M. 1976. A rapid and
Page 145 and 146:
129 Fenton, W. A. and L. E. Rosenbe
Page 147 and 148:
131 Johnson, C. L. V. J., E. Pechon
Page 149 and 150:
133 Mellman, I., H. F. Willard and
Page 151 and 152:
135 Rossi, M., J. P. Glusker, L. Ra
Page 153 and 154:
137 Vlasie, M., S. Chowdhury and R.
Page 155 and 156:
BIOGRAPHICAL SKETCH Nicole Aurora L
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