The composition and properties of the tricarboxylic acid cycle of the micro
aerophilic human pathogen Helicobacter pylori were investigated in situ and
in cell extracts using [H-1] and [C-13]-NMR spectroscopy and spectrophotom
etry. NR LR spectroscopy assays enabled highly specific measurements of som
e enzyme activities, previously not possible using spectrophotometry, in in
situ studies with PI. pylori, thus providing the first accurate picture of
the complete tricarboxylic acid cycle of the bacterium. The presence, cell
ular location and kinetic parameters of citrate synthase, aconitase, isocit
rate dehydrogenase, alpha-ketoglutarate oxidase, fumarate reductase, fumara
se, malate dehydrogenase. and malate synthase activities in H. pylori are d
escribed. The absence of other enzyme activities of the cycle including alp
ha-ketoglutarate dehydrogmase, succinyl-CoA synthetase, and succinate dehyd
rogenase also are shown. The H. pylori tricarboxylic acid cycle appears to
be a noncyclic, branched pathway, characteristic of anaerobic metabolism, d
irected towards the production of succinnte in the reductive dicarboxylic a
cid branch and alpha-ketoglutarate in the oxidative tricarboxylic acid bran
ch. Both branches were metabolically linked by the presence of or-ketogluta
rate oxidase activity. Under the growth conditions employed, H. pylori did
not possess an operational glyoxylate bypass, owing to the absence of isoci
trate lyase activity: nor a gamma-aminobutyrate shunt, swing to the absence
of both gamma-aminobutyrate transaminase and succinic semialdehyde dehydro
genase activities. The catalytic and regulatory properties of the H. pylori
tricarboxylic acid cycle enzymes are discussed by comparing their amino ac
id sequences with those of other, more extensively studied enzymes.