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

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ind CH3Cbl in the class I “Base-off” mode (Drennan et al. 1994). The last domain 10 (residues 897-1227), termed the activation domain, was shown to bind SAM required for the reductive activation of MS (Dixon et al. 1996, Hall et al. 2000). The mechanism of MS catalysis and reactivation is shown in Figure 1-4. The methyltransferase reaction requires heterolytic cleavage of the carbon-cobalt bond of CH3Cbl resulting in cob(I)alamin and a methyl group. After heterolysis, MS transfers its methyl group to homocysteine, forming methionine and MS-bound cob(I)alamin. During catalysis, additional methyl groups are provided by methyltetrahydrofolate (CH3THF), which regenerates CH3Cbl and releases tetrahydrofolate (THF) (Banerjee and Matthews 1990). Occasionally, the MS-bound cob(I)alamin is oxidized to cob(II)alamin during catalytic turnover, and reductive activation is required to restore the methylation cycle (Fujii et al. 1977, Banerjee et al. 1990, Drummond and Matthews 1993). This involves reduction of cob(II)alamin and methylation by SAM to form CH3Cbl. The reduction of cob(II)alamin for MS activation differs in E. coli and humans. Reduced flavodoxin (FldA) provides the needed electron in E. coli (Fujii and Huennekens 1974). However, humans lack flavodoxin and instead use the dual flavoprotein methionine synthase reductase (MSR) (Olteanu and Banerjee 2001). In humans, a block in the methylation cycle caused by defects in MS results in homocystinuria, a severe and sometimes fatal childhood disease (Fenton and Rosenberg 2000). In addition, partial defects in MS can lead to elevated homocysteine, a major cardiovascular disease risk factor (Refsum et al. 1998). A deficiency in MS also results in the trapping of cellular folate as CH3THF, making it unavailable for other
folate-dependent reactions including purine and pyrimidine biosynthesis (Wilson et al. 1999). Cobalamin Absorption and Transport 11 As mentioned above, humans are incapable of de novo cobalamin synthesis, and require complex precursors in their diet. Suitable precursors (such as HOCbl or CNCbl) can be obtained by the consumption of beef, liver, poultry, fish, eggs, dairy products, and vitamin supplements (Stabler 1999). The cobalt of these cobalamin molecules is in the +3 oxidation state (cob(III)alamin), the form that is recognized for cobalamin absorption and transport. Once ingested, cobalamin molecules that are bound to food proteins are released by the combined action of proteases and acid in the stomach (Del Corral and Carmel 1990). Haptocorrin (a cobalamin carrier protein) binds released cobalamin and transports it from the stomach to the small intestines. Haptocorrin has a high specificity for cobalamin and when bound to cobalamin, protects it from damage by acids in the stomach, until it reaches the small intestines where cobalamin is liberated from haptocorrin via digestion by pancreatic enzymes. After being released from haptocorrin, cobalamin is bound by intrinsic factor (IF), a glycoprotein produced by the parietal cells (gastric glands lining the stomach). The IF-cobalamin complex is resistant to further digestion in the small intestines because of the carbohydrates on the IF. The IF-cobalamin complex recognizes the IF-cobalamin receptor (IFCR) on the epithelial cells of the distal third small intestines (ileum), and is transported into these cells via receptor-mediated endocytosis. In the epithelial cell, IF is degraded by the acidic environment of the lysosome, and cobalamin is released. Transcobalamin II (TCII), a serum-transport protein in the epithelial cells, binds released cobalamin, and transports it out of the cell and into the bloodstream until it is taken up by other cells. The
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: methylmalonic acid. Methylmalonic a
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 and 42: (Watkins et al. 2002). Some patient
Page 43 and 44: Chapter 2 of this dissertation repo
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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