Metaboliic disease Flashcards
Glucose-6-phosphate dehydrogenase deficiency
partial protection against malaria
selective advantage
symptoms:
- Neonatal jaundice - caused by oxidation of haem => bilirubin
- Haemolytic anaemia
- Favism (sensitivity to fava beans)
G6PD deficiency make erythrocytes vulnerable for oxidative stress
In erythrocytes the pentose phosphate pathway is the only source of NADPH which is needed for reduction of Glutathione
Oxidising drugs (like anti-malaria drugs), Henna or fava beans => reactive oxygen => haemolysis
G6PD can arise from fructose 6 phosphate or glucose
oxidative stress in G6PD-deficient erythrocytes reduces survival of Plasmodium
But: normal survival in b-thalassemia cells
Most common enzyme deficiency (400 Milli sufferers)
Gene: G6PD
Chromosome: X
Enzyme: Glucose-6-phosphate dehydrogenase mutations have been mapped to:
The dimer interaction srface Close to the where NADP+ binds to enzyme
Homotetramer
Pathway: Pentose phosphate
initiates glucose 6 phosphate generates nicotinamide adenine di- nucleotide phosphate in reduces form
Problem: Lack of NADPH
Treatment: Avoid oxidative drugs/food (fava beans); oxygen and blood transfusion to treat acute anaemia
suffer from anaemia because RBC in the absence of glucose 6 are short of NADPH and cannot detoxify these oxidising agent
toxicity
Lesh-Nyhan syndrome
Rare
when cells are disposed of purines are recycled in production of new cells
if HPRT this enzyme missing
shortage of nucleotides
accumulation of uric acid
degradation product of purines
Gout-like accumulation of uric acid in skin, joints and kidney (kidney stones)
Mental retardation
Self mutilation
Gene: HPRT1
Chromosome: X
telomere end
Enzyme: Hypoxanthine-Guanine-Phosphoribosyltransferase
Homotetramer protein
Pathway: Purine salvage
Problem: Accumulation of uric acid + shortage of nucleotides
Treatment: Control of uric acid levels (allopurinol)
Defect in HPRT activity blocks purine salvage => only purine catabolism to uric acid
Two consequences
Uric acid accumulation => gout
Nucleotide (GTP) deficiency => neurological defects
Next generation sequencing is now being developed to test many hundreds of genes simultaneously
e.g Identification of 4 bp deletion in HPRT1 gene in carrier
Phenylketonuria
Reduced pigmentation
Mental retardation
Hyperactivity
Seizures
normally phenylalanine converted into tyrosine and amino acid transmitters so Large, neutral amino acids are transported by carriers from the blood into the brain
Excess Phenylalanine is neurotoxic
Phenylalanine saturates the carriers => deficiency in other essential amino acids (particularly Tyrosine)
reaction
covert phenylalanine into tyrosine where the PAH enzyme uses tetrahydrobiopterin (co-factor) and an iron atom (intermediate oxygen binding)
to insert the OH group into pheny alanine ring
Oxidisation of tetrahydrobiopterin and iron
Reduction of iron = oxidise phenylalanine
Rare
Gene: PAH
Chromosome: 12
Enzyme: Phenylalanine hydroxylase
Homotetramer
Pathway: Aromatic amino acid synthesis
Problem: Accumulation of phenylalanine is defective
PKU mutations affect amino acids around iron active site
Treatment: avoid phenylalanine diet (no meat, dairy sweetners) To prevent neurological damage, PKU patients must be identified before symptoms occur => screening of newborns
Effective method: tandem mass spectrometry (MS/MS)
1st MS of intact molecule – fragmentation – 2nd MS of fragments
Alternative methods: GC-MS or LC-MS
Haem Biosynthesis
autosomal dominant disorder - 1 parent carriers the defective gene - phenotype is very likely to be expressed
mutations occur in the enzyme that produces porphoboliniogen
porphobilinogen deaminase
catalyses the stitching together of 4 molecules of porpho into linear precursor chain = haem in animals or chlorophyl in plants
amino levilinate dehydrotase -
stitching together 2 molecules to make porphobilinogen
cofactor found in cytochromes haemoglobin oxygen carrier catalase modifications of it in chlorophyll similarity to vitamin B
2 entry routes 1. C4 pathway glycine + succcinyl CoA amino levilinic acid synthesised in animals ala-dehydrogentase catalyses stitching together 2 molecules of ALA to make porpholinogen
PBG deaminase stitches together 4 - linear tetra
synthase into ring order of acidic groups on last ring inverted by to make porphobolyngen
uroporphobolingen carboxylase takes place on short chains to form coproporghyrinogen
coporphory
oxidase decarboxylases it
- C5 pathway in plants
glutamol tRNA = ALA
enzyme blockage effects
Accumulation of upstream substrate - due to inactivation = product is deficient and substate rises (possible toxicity)
Deficiency of downstream product
Production of alternative metabolite after diversion to secondary pathway
mode of inheritance affects the risk of children to contract the disease
analysis of family trees can indicate the mode of inheritance
Carrier diagnosis
Genetic counselling
Newborn/infant screening
Diet
Enzyme replacement therapy
Transplants
Drugs
Gene therapy?
autosomal - not a sex chromosome
recessive x linked mother is carrier unaffect male some of offspring will gain x chromosome from mother rather than father affected son inherits
site of mutation
protein structure
Mutation in the coding sequence
/in splice sites
can affect gene expression
Mutation in enhancers,promoter
Mutation in the 3’ UTR
mutation in transcription factor regulating gene expression
Monogenic disease: One primary metabolic pathway affected
rare, but the mutation usually has an effect = disease
Complex disease: Several metabolic pathways affected more subtly + little
Mutations usually result in partial or complete loss of function = recessive, = one functional copy of the gene is sufficient
How could mutations in enzymes be dominant?
AIP - acute intermittent porphoria
structure of an attack in a n asymptomatic heterozygote - can make all the heam they require to function normally then heam pathway activated by excessAlcohol/drug triggering for cytochrome P450 detox but haem Pathway cannot get past PBGD
Porphobilinogen and aminolaevulinic acid concentration rise.
Aminolaevulinic acid looks like neurotransmitter GABA. that Impacts peripheral and central nervous system involved in pain perception
Severity can be life threatening.
Attack subsides with help from hemin.
Til the next time.
Symptoms
Periodic extreme abdominal pain,
Peripheral neuropathy - limb lose sensation,
Psychiatric disorders= paranoid schizophrenia-like disorder
Tachycardia
Frequency much higher in psychiatric hospitals
Penetrance is 10% in heterozygotes (carriers) = low
homozygote severely damaged.
Trigger: alcohol, drugs(barbiturates,
diclophenac - anti-inflammaetory, analapril - blood pressure drug)
Treatment: intravenous Hemin heam with a cloride moelcue = turns down heam synthesis pathway
structure of porphobolinigin
split into 3 domains
enclosing large cavity lined by arginine residues + covalently attached dipyrromethane cofactor/primer
deamination
atack of cofactor
product release
PBGD can count!
adds molecules of porphobilinogen
to the primer until
six rings are attached to the protein.
then due stationary critical catalytic Residue aspartic 99 four are cleaved off
to leave the primer ready for the next round.
Some mutations interfere with the count to give ES1
ES2, ES3 etc…
structure of porphobolinogin
split into 3 domains
enclosing large cavity lined by arginine residues + covalently attached dipyrromethane cofactor/primer
strcture of
ALADehydrogenase
catalyses the synthesis in porphobolinogen
beta strands surrounded by alpha helices
large loop that comes out the side like an arm
2 subunits come together
deamination
atack of cofactor
product release
PBGD can count!
adds molecules of porphobilinogen
to the primer until
six rings are attached to the protein.
then due stationary critical catalytic Residue aspartic 99 four are cleaved off
to leave the primer ready for the next round.
= elongating intermediate some structural adjustment - pulls perol through residue to participate in each condensation reaction
pirol has acid groups are stabilised by interaction with arginine in the cleft
which can be sites for AIP causing mutations and leads to accumulation of intermediates of extension reaction
enzyme jammed in ES1 2 or 3 state
Point mutations associated
with AIP are widely
dispersed across the
structure but in most cases it is clear why they may be detrimental to the fold or to catalysis.
Classification of metabolic disorders
Monogenic or multi-factorial (complex)
Involvement of small molecules or large/complex molecules
Intoxication or deficiency
Affected pathway
Affected organ
Affected organelle
Age of onset
Lysosome
Formed by fusion of endosomes and transport vesicles
Product of the endo membrane system transport vesicle + endosome = lysosome
Transport vesicles – secretory pathway
are Created in the endoplasmic reticulum and Packaged in the Golgi bodies where they are Loaded with digestive enzymes
Endosomes part of endocytic pathway
are vesicles contains molecules from outside they Fuse with early endosome and Maturation to late endosome => drop in pH
required for lysosomal activity
structure and function
spherical organelle surrounded by single membrane
pH ~4.8 = Optimum pH for the enzymes
Filled with digestive enzymes (for degradation of all major macromolecules)
Digestion of ingested material = Endocytosis+ Hetero phagocytosis
Digestion of cellular material
Autophagy + apoptoseis
Trafficking of acid hydrolases
to deliver all of these degrading enzymes to the right compartment during biosynthesis they are tagged with delivery ntoe on their surface
= Mannose-6-phosphate for lysosome
they are Glycosylated in ER + phosphorylated in Golgi apparatus => M6P
= involved in binding to receptor on the inside of.a transport vesicle
Fusion with early endosome => release of enzyme (by low pH)
Key roles for lysosomes in cell metabolism
Endocytosis Receptor recycling Protein turnover Autophagy Exocytosis Also: Ca2+ and Fe homeostasis, synaptic release
Defective lysosome function => risk of cell damage and apoptosis
Lysosome defects involved in several degenerative diseases
Neurodegeneration, e.g. Alzheimer’s
Huntington’s disease
Lysosomal storage disorders
Common route for enzyme replacement therapy
Binding to M6P receptors on cell surface => endocytosis and delivery to endosome
Lysosomal storage disorders
classified by storage material:
- Sphingolipidoses (Niemann-Pick, Gaucher, Fabry, Tay-Sachs, Sandhoff)
- Mucopolysaccharidoses (Hurler = MPSI, Hunter = MPSII)
- Oligosaccharidoses (α-Mannosidosis)
- Glycoproteinoses
- Lipofuscinoses (Batten)
- Glycogenosis (Pompe)
or by disease mechanism
1. Lysosomal enzyme deficiencies
- Trafficking defect of lysosomal enzymes
- Defects in lysosomal membrane proteins
- Defects in soluble, non-enzymatic lysosomal proteins
Tay-Sachs/Sandhoff diseases - symptoms =
Cellular:
Vacuolisation (lysosome enlargement)
Clinical:
Blindness
Dementia
Paralysis
Seizures
Fatal in early childhood
beta hexosaminidase dfificinecy = geneglioside storage in lysosomes - neuronal damage + microglial activation + reactive gliosis
defect in humans = loss of HEXA or HEXB
Sandhoff mice:
High rate of cell death in the diencephalon
Viral gene therapy has been successful in Sandhoff mouse
Infection of HexB-deficient mice with adeno-associated virus containing the human HEXA and HEXB genes => Hex activity restored in the brain
Virus-mediated HEXA+HEXB expression rescues Sandhoff phenotype
Prevention of progressive neurodegeneration
Injection of neural stem cells leads to improvement of Sandhoff mouse phenotype
.
- Sphingosine + fatty acid = sphingolipid
- Monosaccharides bound to sphingosine = Glycosphingolipids (GSL) like
- Cerebrosides
- Gangliosides
. Degradation in a stepwise process
Several enzymes are involved in digestion particularly of the monosaccharides
Nearly every step in GSL degradation associated with a lysosomal storage disorder
defective GM2 ganglioside degredation =
Tay-Sachs and Sandhoff diseases autosomal
Gene: HEXA (a subunit)
Chromosome: 15
Enzyme: b-Hexosaminidase
able to remove a terminal sugar from end of ganglioside to convert GM2 to GM3 ganglioside
Sandhoff disease ALu related misalignment in replication = deletion Gene: HEXB (b subunit) Chromosome: 5 Enzyme: b-Hexosaminidase A,B
dimer = three isofoms
hexa a. alpha/beta
hexa b beta/beta/
hexa S alpha/alpha
Pathway: Glycosphingolipid degradation
Consequence: Accumulation of GM2 ganglioside (GM2 gangliosidosis)
no Treatment
b-Hexosaminidase remove terminal sugars from larger molecules by breaking glycosidic linkages of terminal b-D-N-acetylglucosamine (GlcNAc) or b-D-N-acetylgalactosamine (GalNAc) residues on glycolipids, glycoproteins,and glycosaminoglycans
Relevant for disease: removal of b-D-N-acetylgalactosamine from GM2 ganglioside:
Only the α/β isozyme (HexA) capable of degrading sialated GM2 thro
its α subunit at significant rate.
HEXA mutations lead to GM2 ↑ in neuronal lysosomes – Tay-Sachs.
HexB (ββ) isozyme only cleaves neutral Globosides but
HEXB mutations inactivate HexA and HexB because HexA is a heterodimer including β– Sandhoff.
Mutations in GM2-activator protein give GM2↑ even if HexA and HexB
are fully active. (Variant AB gangliosidosis)
hex a + hexa b + GM2 activator required to achieve satisfactory degradaion of GM2 ganglioside
beta inactive - sandhoff disease
alpha inactive - tach
GM2 inactive - AB varient of the degredation reaction
function of GM2 activator protein =
- Cleavage of GM2 with Hexa A
- makes GM2 accessible for the enzyme
- latches on head group of GM2 portruding from lipid bilayer + Lifts ganglioside out of the membrane
GM2->GM3 conversion depends on Hexa A (and GM2 activator)
Other Hexaminidase isoforms can compensate for the loss of HEXA in most pathways, but cannot digest GM2
Both Hex subunits are required!
Accumulation of undigested GM2 ganglioside in lysosomes leads to swelling of neurons and axons => neurodegeneration
AIP - acute intermittent porphoria
structure of an attack in a n asymptomatic heterozygote - can make all the heam they require to function normally then heam pathway activated by excessAlcohol/drug triggering for cytochrome P450 detox but haem Pathway cannot get past PBGD
Porphobilinogen and aminolaevulinic acid concentration rise.
Aminolaevulinic acid looks like neurotransmitter GABA. that Impacts peripheral and central nervous system involved in pain perception
Severity can be life threatening.
Attack subsides with help from hemin.
Til the next time.
Symptoms
Periodic extreme abdominal pain,
Peripheral neuropathy - limb lose sensation,
Psychiatric disorders= paranoid schizophrenia-like disorder
Tachycardia
Frequency much higher in psychiatric hospitals
Penetrance is 10% in heterozygotes (carriers) = low
homozygote severely damaged.
Trigger: alcohol, drugs(barbiturates,
diclophenac - anti-inflammaetory, analapril - blood pressure drug)
Treatment: intravenous Hemin heam with a cloride moelcue = turns down heam synthesis pathway
