Transcript Sulfur Containing Amino Acids

#4 Sulfur Containing Amino Acids
1.Uses of MET/CYS
2.Active Methyl Cycle & Trans-Sulfuration
Pathway
3.Defects
- Vitamin B6 responsive
- Homocystinurea
- Cystinurea
- Cystinosis
Sulfur Containing Amino Acids
• 2AA: MET & CYS, MET is converted to
CYS through trans-sulfuration pathway
• Cystine: dimer of CYS
• Homocysteine: not a primary AA
Methionine
MET - thioether derivative (contains S)
- EAA (3.5% animal protein, 1.5% plant protein)
IMPORTANCE:
(i) as a constituent of P, as well as the initiating AA in P
synthesis
(ii) source methyl groups for  PL, DNA, RNA,
hormones via conversion of MET to S-adenosyl MET
(iii) source S in CYS via trans-sulfuration pathway
(iv) polyamine
Cysteine
CYS IMPORTANCE:
(I) protein synthesis
(ii) catalytic site of enzymes where SH is a nucleophile
(iii) synthesis of compounds  CoA, glutathione, taurine
(iv) provides SO4 on complete oxidation
CYSTINE:
(I) dimer of CYS
(ii) formed through oxidation mediated by O2, Cu2+, Mn2+ or Fe2+
(iii) little free in cells
(iv) important in disulfide bond formation in intact proteins to
provide bridges and stabilize conformation of proteins.
Synthesis of CYS/Metabolism of MET
2 pathways: Active Methyl Cycle and the TransSulfuration Pathway.
Active Methyl Cycle
MET
SAM
SAH
H – Cys
Cys
First step: Activation of MET
→ SAM (a high energy compound)
Most Rxns with SAM are Methyl
transferases
Methyl transferred SAM → several
compounds where Methyl linked to O or
N in acceptor molecule
eg, Methylation RNA, DNA, cathecholamines
adrenaline synthesis
creatine
phosphatidyl choline
Second step: Free energy loss:
SAM → SAH
SAH: no longer has a charged sulfonium atom
(thioether  MET), rxn irreversible
This represents a branch point in MET
metabolism (direction depends on
physiological needs of organism).
(I) if MET limited  homoCYS remethylated to
MET (closed pathway of active methyl cycle)
(ii) If CYS needed and SAM adequate then
homoCYS  trans-sulfuration pathway.
(ii) If both CYS and MET are adequate, then 
other pathways
Active Methyl Cycle
(i) MET is an EAA
(ii) But mammals can synthesize MET from homoCYS if available
(can replace MET in diet) because it is the “homoCYS” that
cannot be made
(iii) Can transfer a methyl group back to homo CYS from betaine or
(primarily) 5 methyl THF (tetrahydrofolate) Therefore regenerate
MET  P synthesis, methylation and CYS synthesis
(iv) Crossroads for 2 important vitamins : folic acid and vitamin B12
(Vitamin B12 deficiency  folate deficient state)
(v) One of the major determinants whether homoCYS  MET is
level of 5 methyl THF. SAM blocks 5 methyl THF production
therefore decreased MET production when [SAM] increases
Trans-Sulfuration Pathway
Trans - Sulfuration pathway is
analagous to transamination for
AA
(i) Major degradation pathway
for MET in mammals
(ii) End Product is CYS
(iii) Two RXNs, both use
pyridoxal phosphate as a
cofactor (as with
transamination)
FATE OF ATOMS OF MET
CH3

S

 methyl transfer
CH2


converted to propionyl CoA  succinyl CoA
CH2


CH— NH3

COO-
 CYS
 NH4+
CO2
 glucose
Inborn Errors of Sulfur - containing AA Metabolism
(i) Two of these defects - homocystinuria (type I) & cystathioninuria involve
enzymes of trans-sulfuration pathway (homo Cys  Cys)
(ii) Homocystinuria (2nd most common genetic AA disease) named due to 
homocystine urine (CYS dimer)
 homocystine& MET & homoCYS blood which spills over into urine.
Dimer (homocystine) forms spontaneously in tissue
(iii) 4 types of homoCYS ( see table)
Type I = defect in cystathionine synthase (enzyme homoCYS  CYS:
first step) Also with less homoCYS  MET therefore increase
homoCYS
Type II: defect in methylene THF reductase decrease [5 methyl THF]
which is necessary to methylate homoCYS
Type III: Decrease in B12
Type IV: malabsorption of B12
Type I Homocystinuria
B6 responsive - (responsive to vitamin therapy)
- 50% Type I
- doses B6 up to 1 g / day
Remember defect in cystathionine synthase
enzyme that converts homoCYS  CYS (Step I)
pyridoxal phosphate is a cofactor
and that B6  Ppal
 Km mutants: do not bind cofactor as avidly therefore
need much more cofactor to increase enzyme activity
Note: overly large dose (4-5 g / day)  nervous system
dysfunction.
Type I Homocystinuria
B6 unresponsive
- enzyme mutation that does not involve cofactor
binding site
therefore  dietary therapy
(i) add betaine  Why? Enhance alternate pathway
(ii) keep MET (use plant P like soybean/lentil which
has half the MET of animal P)
(iii) CYS  conditionally essential because MET
cannot be converted to CYS
(iv) Start therapy early due to serious clinical
symptoms
Symptoms
- Mental defects
- Skeletal malformations (osteoporosis) due to defective
collagen formation (homoCYS interferes with
corsslinking collagen)
-Dislocation ocular lens(abnormal lens ligaments)
- Thromboembolism and vascular occlusion (decreased
life expectancy) with damage to lining of blood vessels
 major cause morbidity and mortality.
Note:Increased homoCYS is a recognized risk factor for
heart attack / stroke in adults
TREATMENT:Dietary folic acid which decreases
homoCYS (via trans-sulfuration pathway)
Cystathioninuria
- much rarer than Type I
- due to a defect in cystathion(in)ase (Step 2 CYS
synthesis)
- less clinical abnormalities than Type I
- accumulation cystathionine in blood/urine (not
detectable normally)
- responds to B6 supplementation in some cases
(B6  Ppal  cofactor to enzyme) which also
suggests a Km mutant
#4 Sulfur-Containing Amino Acids:
Homocystinuria Case Discussion
A 6-year-old girl was brought to the hospital with vision problems. She
was found to have a downward dislocation of the left lens. Her mother
indicated that the girl’s birth was normal, but that she lagged in
development. She was unable to crawl until 1-year-old and did not walk
until 2 years. Speaking was also delayed. She had long, thin bones; on
roentgenographic examination the lower femur showed signs of
osteoporosis. An older brother had similar symptoms, but had been
diagnosed as having Marfan’s syndrome. A simple cyanide-nitroprusside
test of the patient’s urine was positive, suggesting homocystinuria. This
was confirmed by amino acid analysis of the plasma, which revealed
homocystine, an abnormally high methionine level, and other sulfurcontaining compounds that were derivatives of homocysteine. The patient
was treated with a low-methionine diet supplemented with folic acid and
pyridoxine.
Symptoms / clinical characteristics
1. Downward dislocation left lens
Slow development - crawling / walking /
speaking.
Osteoporosis, long thin bones
2. Cyanide/nitro prusside urine test = +ve
3. Plasma AA   homoCYS, MET, homoCYS
derivatives
Suggests that homoCYS  due to decrease
conversion to CYS. MET also  therefore no
problem in conversion homoCYS MET
Discussion
1. What is the origin of the homocystine excreted in this disease?
2. What are some of the metabolic substances formed by the
enzymatic reactions that use S-adenosylmethionine as the
methylating agent?
1. Origin of homocystine: dimer of homoCYS,
product of excess homoCYS, overflows
into urine
2. These include RNA, DNA, amino terminal
groups P’s, precursors of melatonin,
creatine, epinephrine,
phosphatidylcholine, methyl cobalamin
(B6).
Discussion
3. What are some causes of homocystinuria in humans?
4. How would one test for a deficiency of cystationine
βsynthase in this patient?
3. Any part of pathway that impedes MET  homoCYS pathway 
homoCYS
- decrease cystathionine B synthase (Step 1 for CYS)
- decreased syn of MET from homoCYS (5 MeTHF or B12 or enzyme
) genetic or nutritional
- Also 6 azauridine administration: anticancer agent which inh PPal
enzymes therefore decreases both step 1 and 2
- bacterial action on cystathionine in urine
4. Test of enzyme: Measure enzyme in cells (human skin
fibroblasts)to define hetero or complete mutation. Test addition of
pyridoxal phosphate in vitro since  with effectiveness of therapy. (ie
B6 sensitivity)
Discussion
5. Explain why pyridoxine is useful in the treatment of some
patients with homocystinuria.
6. What effect would a diet low in folate have on this patient?
5. (i) pyridoxine and/or B6  improvement in behaviour
and IQ
(ii) Diet decreased MET, add smaller meals to prevent
MET overload
(iii) add Vitamin B12 and folic acid
6. Diet decrease folate: Good or Bad? BAD Seriously
effect 5 MeTHF rxn therefore  homoCYS even worse (no
alternate pathway)
Discussion
7. What might account for the homocystinuria of an apparently normal
infant (not in this case) with severe megaloblastic anemia and who was
exclusively breast-fed by a strict vegetarian mother?
8. Describe the genetics of homocystinuria (cystathionine βsynthase
deficiency).
7. HomoCYS  B12 deficiency
- vegetarians: no eggs, no dairy, scrubbed vegetables 
decrease cobalamin therefore not enough vitamin B12  synthesize
cofactor
- therefore no homoCYS  MET
Can result in permanent neurological damage
8. Autosomal Recessive (1:45,000), Defects in gene chromosome 21,
Heterogeneous