MICHAELIS CONSTANTS FOR
THE METHIONINE PATHWAY
The Michaelis
constant (Km) is the half‑maximally saturating concentration of substrate for a
given enzyme. The higher the Km when
compared to blood or tissue levels of substrate, the more the enzyme can be
"pushed" by increasing levels of substrate. If the enzyme is already near saturation by
normal levels of substrate, increasing substrate levels will not increase
enzyme activity. When two or more
enzymes compete for the same substrate, the enzyme with a relatively low Km is
favored at low substrate concentrations, and the enzyme with a relatively high Km
is favored at high substrate concentrations.
Source Enzyme
Km
Substrate Levels
Human Aminoacyl
Human blood
placenta Methionyl: 50 uM
Methionine:
Transfer RNA (Methionine) 13-37 uM/L
synthetase
EC 6.1.1.10
Human skin Si = 10.8
fibroblasts nanomoles/mg protein/hr
This enzyme
functions to place methionine on the ribosome during protein synthesis. It is constitutive, in that it does not
respond to increasing levels of methionine.
Human Methionine:
Human plasma
liver Adenosine
650 uM
Methionine:
Transferase (Methionine) 13-37 uM/L
EC 2.5.1.6
Human plasma
Human Si = 30-130 SAM
levels:
liver nanomoles/mg
protein/hr
38-50 nM/L
Rat liver Si = .282
male Units/ gm liver
Rat liver Si = .465
female Units/ gm liver
Function: This enzyme combines methionine and adenosine
to form S‑Adenosyl‑Methionine (SAM), the primary methyl donor in the body. Inhibited by excess SAH. Induced by methionine. Magnesium dependent. Glutathione dependent. 50% inhibition by androgen in rats.
Source Enzyme Km Substrate level
All S-Adenosyl methionine- < 100 uM
tissues dependent Transmethylases
Human SAM-Phospholipid Km = ?
liver Methyltransferase
Si = 14.4 nanomoles/
mg protein/hr
Function: to transfer a methyl group from SAM to a
myriad of acceptors. Leaves SAH.
Inhibited by
SAH. There are more than 100 of these
transmethylases.
Human S‑Adenosyl‑
Human Plasma
Monocytes Homocysteine 32‑39 uM
Hydrolase
EC 3.3.1.1 S‑Adenosyl‑HCY levels:
40 nmol/L
62 uM
(rat
liver)
Si = ?
Function: Removes adenosine from SAH, leaving
homocysteine. Inhibited by excess
homocysteine.
Human Methyl‑THF: Human blood HCY:
liver Homocysteine
60 uM
0‑20uM/L (normal)
Methyl Transferase (Homocysteine) 20‑300+ uM/L
EC 2.1.1.13 (abnormal)
Human Si = 1.30 Human Si
= 13.0
liver nanomoles/mg protein/hr fibroblasts nanomoles / mg protein / hr
Human Si = 2.34 Human Si = 1.37
kidney nanomoles/mg protein/hr brain nanomoles / mg protein / hr
Function: This enzyme remethylates homocysteine back to
methionine, with the methyl group obtained from serine, by way of B2, B6, B12,
and folate. Inhibited by excess
methionine.
Rat Choline
5000 uM Human liver tissue
liver dehydrogenase (Choline)
Choline: 60 uM
EC 1.1.99.1
Human serum
Choline: 10 uM
Human Si = 78 nmoles / mg wet tissue / hr
liver = 84
gm/24 hr liver only
Rat liver
Si = 1332 nmoles / mg wet tissue / hr
Function: Forms betaine aldehyde and trimethylamine
from choline.
Source Enzyme
Km
Substrate levels
Human Betaine
Human liver tissue
liver aldehyde
Km
= ? Betaine aldehyde: ?
dehydrogenase Human serum betaine:
EC 1.2.1.8 18-73 uM/L
Rat liver Si = 5.7 umoles NADH / min / mg protein
Function: Forms betaine from betaine aldehyde.
Human Betaine: 2200 uM Human blood Betaine:
liver Homocysteine (Betaine) 18-73 uM/L
Methyl Human blood HCY:
Transferase 4 uM 0‑20 uM/L
EC 2.1.1.5 (Homocysteine) Human
liver tissue
Betaine:
?
Human Si = 32.1 Human liver tissue
liver nanomoles/mg protein/hr Homocysteine: ?
Human Si = 14.1
kidney nanomoles/mg
protein/hr
Human Si = 0.0
brain nanomoles/mg
protein/hr
Function: Remethylates homocysteine, with the methyl
group obtained from triglycerides, by way of choline and betaine. Serves as an alternate source of methyl
groups when glucose is scarce. Activated
by excess homocysteine. Inhibited by
excess methionine. Forms dimethylglycine. Found primarily in the liver and
kidney.
Serves to
conserve the sulfur backbone of homocysteine when methionine is scarce.
Rat Dimethyl Human serum level
liver glycine Km
= ? Dimethylglycine=
dehydrogenase 1.4-5.3
uM/L
EC 1.5.99.2
Si = ?
Function:
Forms sarcosine from dimethylglycine.
Source Enzyme
Km
Substrate levels
Rat Sarcosine Human
serum glycine:
liver dehydrogenase Km = ? 147-422 uM/L
EC 1.5.99.1 (Sarcosine)
Si = ?
Function:
Forms glycine from sarcosine.
Rat SAM:Glycine
130 uM
Rat liver
liver N-Methyltransferase (Glycine) Glycine:
EC 2.1.1.20 1 uM/gm
Rat Si = 1.87 nmol
sarcosine
liver /mg protein/min
Function: Forms methylated glycine (sarcosine), which
is then oxidized to CO2, and glycine.
Serves as an excess methyl group removal pathway. Activated by excess methionine and SAM.
Human Cystathionine 4000 uM Human blood Serine:
liver beta‑synthase (Serine) 95-230 uM/L
E.C. 4.2.1.22
25000 uM Human blood HCY:
Human Si = 98
(Homocysteine)
0-20 uM/L
liver nanomoles/mg protein/hr
Function: Condenses serine and homocysteine to form
cystathionine. Provides sulfur for cysteine formation. Irreversibly removes serine and homocysteine
from the methionine remethylation cycle.
Activated by excess methionine and homocysteine. Dependent on sufficient serine.
Human Cystathionase 1200 uM
Human blood
liver E.C. 4.4.1.1 (Cystathionine) Cystathionine:
0‑1uM/L
Human Si = 126
liver nanomoles/mg protein/hr
Function: Cleaves cystathionine to form cysteine and
homoserine. Activated by excess
cystathionine.
Source Enzyme Km
Substrate Levels
Rat Cysteine
450 uM
Human blood
liver Dioxygenase
(Cysteine) Total Cystine:
E.C. 1.13.11.20 200-361 uM/L
Free
Cysteine: 12 uM/L
Rat Si = 60 nanomoles/mg
protein/hr
liver on low
protein diet
Si = 1820
nanomoles/mg protein/hr
on low
protein diet + 5% methionine
Function: Cleaves cysteine, forming cysteinesulfinic acid, which either transaminates to sulfite (95%),
or is decarboxylated to hypotaurine (5%).
Regulates cysteine levels.
Activated by excess cysteine,
inhibited by cysteine deficiency.
Rat Cysteine
Sulfinate Km = 9.4 mM Human liver: ?
liver Transaminase (Cysteinesulfinate)
E.C. 2.6.1.1
Rat Si = 47.8 mM
brain / mg protein / hr
Rat Si = 13.68 mM
liver / mg protein / hr
Function: Transaminates cysteinesulfinate to
b-sulfinylpyruvate, which then nonenzymatically transforms to sulfite and
pyruvate. Also known as Aspartate
Aminotransferase.
Human Sulfite Km = 17 uM Human
liver
liver Oxidase (Sulfite) Sulfite: ?
E.C. 1.8.3.1
Human plasma
sulfate
: 0.3 mM
Human Si = 0.66
liver nanomoles of
sulfate/mg protein/min
Function: Oxidizes sulfite to sulfate. Molybdenum dependent.
Source Enzyme Km
Substrate
Cysteinesulfinic
acid Km = 120 uM Tissue levels = ?
Human Decarboxylase (Cysteine-sulfinate)
liver E.C. 4.1.1.29
Human Si = 0.32
liver nanomoles/ mg
protein/ hr
Rat Si = 468
liver nanomoles/ mg
protein/ hr
Function: Decarboxylates CSA to hypotaurine, which then
spontaneously oxidizes to taurine. Vitamin B6
dependent. Rate limiting for taurine
formation in humans.
Source Enzyme Km
Substrate Levels
Human Phosphoserine
36 uM Human brain
brain Phosphatase
(Serine) Serine: 80 uM/L
E.C. 3.1.3.3
Si = 600 nanomoles/mg protein/hr
Human brain (1350 gm) = serine/24 hr
Si in uM/
gm liver/hr for:
chicken= 25
410 uM
Human plasma
pigeons= 18
(Serine) Serine: 95-230 uM/L
rats = 26
guinea pigs= 18
humans = ?
18 uM Serine(M.W.=105) / 1500 gm
liver / 24hr = 68 gm/day in man
Function: Removes phosphate from phosphoserine, formed
from glucose, glutamate, and ATP, leaving serine. Inhibited by
excess serine.
Human Serine 6200 uM
Human brain
brain hydroxymethyl (Serine) Serine: 80 uM/L
transferase
E.C. 2.1.2.1
Human Si = 32 uM/
brain gm protein/hr = 22 gm serine/ 24 hr
Human Si = 1834 uM/ Human serum Serine:
liver gm
protein/hr 95-230 uM/L
1834 uM Serine / 300 gm liver
protein / 24hr = 1368 gm/day in man
Function: Removes hydroxymethyl group from serine,
leaving glycine, and places the methyl group on Tetrahydrofolate. Primary source of endogenous methyl groups in
the brain. Primary source of methyl groups for purine and pyrimidine
synthesis. Primary source of methyl
groups for endogenous synthesis of methionine.
Activated by serine. Inhibited by
excess glycine. B6 dependent.
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