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

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DMB ligand by a histidine residue on the α-subunit of MCM. DMB is still attached to the corrin ring through a nucleotide loop; however, in the class I interaction, the coordinate bond of DMB to cobalt is displaced and substituted by His610 of the α-subunit. The exact role of the His610 residue is not fully understood, but current 8 studies suggest that its major role is in DMB displacement and organization of the active site for AdoCbl binding and enzyme catalysis (Vlasie et al. 2002). The isomerization of methylmalonyl-CoA to succinyl-CoA by MCM requires homolytic cleavage of the carbon-cobalt bond of enzyme-bound AdoCbl (Padmakumar and Banerjee 1997). Homolysis of the organometallic bond occurs once methylmalonyl-CoA binds MCM. This results in the formation of an adenosyl radical and cob(II)alamin. The carbon-cobalt bond of free AdoCbl is weak and in solution has a rate of homolysis of 4x10 -10 sec -1 . With the addition of MCM, the rate of homolysis is increased to higher than 600 sec -1 , suggesting that MCM promotes the homolysis of AdoCbl for this rearrangement reaction (Padmakumar and Banerjee 1997). The adenosyl radical generated by homolysis abstracts hydrogen from the methyl-group of methylmalonyl-CoA, forming a primary-substrate radical and 5’-deoxyadenosine. The primary-substrate radical undergoes an intramolecular 1,2-rearrangement resulting in the formation of a secondary product radical that abstracts hydrogen from deoxyadenosine. Lastly, there is recombination of the adenosyl radical and cob(II)alamin, regenerating AdoCbl and releasing succinyl-CoA that can flow into the tricarboxylic acid (TCA) cycle. In humans, defects in the enzyme MCM cause a block in propionyl-CoA metabolism, resulting in a buildup of methylmalonyl-CoA that is hydrolyzed to
methylmalonic acid. Methylmalonic acid is excreted into the urine, resulting in methylmalonic aciduria, an inherited disorder that is often fatal in newborns. Methionine synthase MS catalyzes the conversion of CH3THF and homocysteine to THF and methionine. This enzyme is found in both bacteria and humans. In E. coli, two MS enzymes occur and they differ in their requirement for CH3Cbl (Drummond and Matthews 1993). In humans, only CH3Cbl-dependent MS is present. The E. coli and human genes encoding CH3Cbl-dependent MS have been cloned and encode large 9 monomeric proteins of 136 and 141 kDa, respectively (Banerjee et al. 1989, Leclerc et al. 1996). The human gene was mapped to chromosome 1 at position q43 (Leclerc et al. 1996). The bacterial and the human enzymes share 55% identity in amino acid sequence, and have established similarities in the mechanism of catalysis (Hall et al. 2000). However, only the E. coli CH3Cbl-dependent MS has been studied extensively. Methionine synthase from E. coli is a modular enzyme consisting of four domains, all of which are crucial for enzyme catalysis (Goulding et al. 1997). The N-terminal domain (residues 2-353) has been cloned, expressed, and shown to be involved in homocysteine binding, and methyl group transfer from CH3Cbl to homocysteine (Goulding et al. 1997). It contains three cysteine residues used to coordinate a zinc ion essential for homocysteine binding, which are conserved in other MS enzymes including Homo sapiens, Caenorhabditis elegans, and Synechocystis species (Goulding and Matthews 1997). The second domain (residues 354-649) was shown to catalyze methyl transfer from CH3THF to cob(I)alamin (Goulding et al. 1997). The third domain (residues 650-896) was lacking enzymatic activity; however, it was shown to be essential for CH3Cbl binding (Banerjee et al. 1989). This fragment was crystallized and shown to
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: 7 homocysteine recycling and methio
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 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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