AA Metabolism (Biochem) Flashcards
1
Q
How does the body get rid of excess N
A
- through the urine
- most excess N is converted to urea in the liver and goes through the kidney where it is eliminated in urine
- the kidney adds small quantities of ammonium ion to the urine in part to regulate acid-base balance, but also to eliminate N
- amino groups released by deamination runs form ammonium ion (NH4+) which must not escape into peripheral blood
- hyperammonemia has toxic effects on the brain (cerebral edema, convulsions, coma, death)
- most tissues add excess N to the blood as glutamine by attaching ammonia to the gamma-carboxyl group of glutamate
- Muscle sends N to the liver as alanine and smaller # of other aa including glutamine
2
Q
Glutamine Synthetase
A
- in most tissues
- captures excess N by laminating glutamate to form glutamine
- IRREVERSIBLE Rxn
- Glutamine (relatively nontoxic) is the major carrier of excess N from tissues
3
Q
Glutaminase
A
- in the kidney, intestine, and small amounts in the liver
- allows it to deaminate glutamine (arriving from the blood)
- and to eliminate the amino group as ammonium ion (NH4+) in the urine
- IRREVERSIBLE Ron
- KIDNEY glutaminase induced by chronic acidosis
- (where excretion of ammonium may become the major defense mechanism)
- High levels of Glutaminase in the intestine (where ammonium ion from deamination can be sent directly to the liver via the portal blood and used for urea synthesis)
- the intestinal bacteria and glutamine from dietary protein contribute to the intestinal ammonia entering the portal blood
4
Q
Aminotransferases (aka transaminases)
A
- in both liver and muscle
- do not release the amino groups as free ammonium
- transfer the amino group from 1 AA to another (usually alpha-ketoglutarate, a CAC intermediate)
- Pyridoxal phosphate (PLP) derived from Vitamin B6 is required to mediate the transfer
- named by the AA donating the amino group to alpha-ketoglutarate.
- Ex) alanine aminotransferase (ALT)
- Ex) aspartate aminotransferase (AST)
- in pathologic conditions may leak into blood = indicate liver or muscle damage
5
Q
Glutamate Dehydrogenase
A
- found in many tissues
- REVERSIBLY deaminates Glutamate
- produces alpha-ketoglutarate (CAC intermediate)
6
Q
What is the major carrier of excess N from tissues
A
- Glutamine (relatively nontoxic)
- most tissues add excess N to the blood as glutamine by attaching ammonia to the gamma-carboxyl group of glutamate
- Muscle sends N to the liver as alanine and smaller # of other aa including glutamine
7
Q
What are the sources of N for the urea cycle?
A
NH3 and aspartate
8
Q
Urea cycle
A
- urea contains 2 N and is synthesized in the liver from aspartate and carbamoyl P
- carbamoyl P: made from ammonium ion (NH4+) and CO by mito carbamoyl phosphate synthase (requires N-acetylglutamate as an activator)
- N-acetylglutamate is only produced when free AA are present
- rate-limiting enzyme: mito carbamoyl phosphate synthase
- acts catalytically
- small # of the intermediates can synthesize large # of urea
- occurs partially in the mito and partially in the cytoplasm
- citrulline enters cytoplasm and ornithine returns to the mitochondria (mitochondrial ornithine transcarbamoylase)
9
Q
if gluconeogenesis is active, fumarate can be converted to
A
glucose
10
Q
the product of the urea cycle (urea) is formed in the
A
cytoplasm and enters the blood of delivery to the kidney
11
Q
Carbamoyl Phosphate Synthetase Deficiency
- remember, this is a mitochondrial enzyme
A
- increased [NH4+]; hyperammonemia
- increased blood glutamine
- decreased BUN
- No orotic aciduria* (excretion of orotic acid in urine)
- AR*
- cerebral edema (lethargy, convulsions, coma, death)
12
Q
Ornithine Transcarbamoylase Deficiency
- remember, this is a mitochondrial enzyme
A
- increased [NH4+]; hyperammonemia
- increased blood glutamine
- decreased BUN
- orotic aciduria** (excretion of orotic acid in urine)
- X-Linked Recessive**
- cerebral edema (lethargy, convulsions, coma, death)
- Without OTC, Carbamoyl P accumulates in mito and leaks into the cytoplasm
- Carmaboyl P is first substrate in pathway of orotic acid metabolism
13
Q
Phenylalanine Hydroxylase Deficiency (Phenylketonuria aka PKU)
A
- 1:15,000
- normal at birth. If untreated: slow development,
- severe mental retardation, autistic symptoms
- loss of motor control
- musty odor to urine bc there’s an increase in phenyl-ketones
- kids have pale skin and white-blonde hair
- sx from high levels pf he and not to the phenylketones. Tx: diet LOW in phe (but still need some bc essential AA)
- avoid aspartame
- diet important during pregnancy: infants born to PKU mothers have microcephaly, mental retardation and low birth weight
- screen infants a couple days after birth
14
Q
MCC elevated BUN
A
dehydration
15
Q
Transaminases use what as a coenzyme?
A
- Vitamin B6 aka pyridoxine
16
Q
Seeing alanine in the blood is a sign of
A
starvation
17
Q
IF BUN is too high
A
kidney dysfunction
18
Q
if BUN is too low
A
liver dysfunction
19
Q
What causes Parkinson’s Diasease?
A
- loss of dopamine producing cells in the brain
- catecholamines (Dopamine) don’t enter the brain, brain synthesizes them locally
- L-dopa can cross the BBB
- Sx: bradykinesia
- mask-like facies
- pill rolling, RESTING tremor
- cogwheel rigidity
20
Q
Homogentisate Oxidate Deficiency
A
- causes alcaptonuria
- accumulation of homogentistic acid in the blood
- causes excretion in urine, which darkens when exposed to air
- Dark pigment also accumulates over years in cartilage (ochronosis)
- and may be seen in the sclera of the eye, in ear cartilage
- puts develop arthritis in adulthood, usually in 3rd decade
- Sx not present in all patients w enzyme deficiency
21
Q
Branched Chain Ketoacid Dehydrogenase Deficiency
A
- Maple Syrup Urine Disease
- BCKDH metabolizes branched-chain ketoacids produced from their cognate AA: valine, leucine and isoleucine)
- infants normal for the first few days of life
- progressive lethargy, weight loss
- alternating episodes of hypertonia and hypotonia
- urine develops a maple syrup odor
- ketosis, coma and death if not treated
- tx: restrict dietary valine, leucine and isoleucine
- Branched Chain Ketoacid Dehydrogenase is similar to alpha-ketoglutarate dehydrogenase (both are TLCFN enzymes) “Tender Loving Care For Nancy” Requires:
T: Thiamine
L: lipoic acid
C: CoA
F: FAD
N: NAD+
22
Q
Propionyl-CoA Carboxylase Deficiency
A
- accumulation of propionic acid, methyl citrate, hydroxypropionic acid
- valine, methionine, isoleucine and threonine are all metabolized through the propionic acid pathway (used for all odd-C FA)
- deficiency of either Propionyl-CoA Carboxylase or methylmalonyl-CoA Mutase results in
- neonatal ketosis from failure to metabolize ketoacids produced from these 4 AA
- the presence of methyl citrate and hydroxypropionate distinguish PCoA CD from MMCoA MD
23
Q
When a biochemical reaction requires a methyl group (methylation) what is the usual methyl donor?
A
S-adenosylmethionine (SAM)
24
Q
Alcaptonuria
A
- Homogentisate Oxidate Deficiency
- accumulation of homogentistic acid in the blood
- causes excretion in urine, which darkens when exposed to air
- Dark pigment also accumulates over years in cartilage (ochronosis)
- and may be seen in the sclera of the eye, in ear cartilage
- puts develop arthritis in adulthood, usually in 3rd decade
- Sx not present in all patients w enzyme deficiency
25
Maple Syrup Urine Disease
- Branched Chain Ketoacid Dehydrogenase Deficiency
- BCKDH metabolizes branched-chain ketoacids produced from their cognate AA: valine, leucine and isoleucine)
- infants normal for the first few days of life
- progressive lethargy, weight loss
- alternating episodes of hypertonia and hypotonia
- urine develops a maple syrup odor
- ketosis, coma and death if not treated
- tx: restrict dietary valine, leucine and isoleucine
- Branched Chain Ketoacid Dehydrogenase is similar to alpha-ketoglutarate dehydrogenase (both are TLCFN enzymes) "Tender Loving Care For Nancy" Requires:
T: Thiamine
L: lipoic acid
C: CoA
F: FAD
N: NAD+
26
How are valine, methionine, isoleucine and threonine are metabolized?
- valine, methionine, isoleucine and threonine are all metabolized through the propionic acid pathway (used for all odd-C FA)
- deficiency of either Propionyl-CoA Carboxylase or methylmalonyl-CoA Mutase results in
- neonatal ketosis from failure to metabolize ketoacids produced from these 4 AA
- Distinguish btw the 2 enzyme deficiencies by:
- presence of methyl citrate and hydroxypropionic acid in propionyl CoA carboxylase deficiency
- presence of methylmalonic aciduria (results from accumulation of methylmalonic acid) in methylmalonyl CoA mutase deficiency
27
Methylmalonyl-CoA Mutase Deficiency
- accumulation of methylmalonic acid
- valine, methionine, isoleucine and threonine are all metabolized through the propionic acid pathway (used for all odd-C FA)
- deficiency of either Propionyl-CoA Carboxylase or methylmalonyl-CoA Mutase results in
- neonatal ketosis from failure to metabolize ketoacids produced from these 4 AA
- results in methylmalonic aciduria
- presence of methylmalonic aciduria distinguishes MM-CoA MD from P-CoA CD
28
Homocystinuria
- can be caused (rarely) by cystathionine synthase deficiency (Vit B6 cofactor)
- methionine accumulates in blood (bc CS converts homocysteine to cystathionine; but when CS is missing, homocysteine is converted to methionine via homocysteine methyl transferase (VB12 cofactor))
- atherosclerosis (can cause stroke)
- DVT, thromboembolism
- dislocation of lens downward and inward* (ectopic lens)
- marfan-like habitus (may see abnormally long "spidery" fingers)
- mental retardation
- joint contractures
- excrete high-levels of homocysteine in urine (dx with cyanide-nitroprusside test)
- often have MI before 20 yo
- Tx: diet low in methionine and supplement folate and vitamin B6
- many patients with homocystinuria who have partial activity of CS respond well to pyridoxine administration
* in Marfan, subluxation of the lens is upward and outward
29
homocystinemia from vitamin deficiencies
- caused by:
- folate deficiency
- Vitamin B12 (required for homocysteine methyl transferase activity)
- Vitamin B6 (required for cystathionine synthase deficiency)
- assoc with increased risk of atherosclerosis, DVT, stroke
30
Branched Chain Ketoacid Dehydrogenase
- metabolizes branched-chain ketoacids produced from their cognate AA: valine, leucine and isoleucine)
- TLCFN enzyme. Requires:
T: Thiamine
L: lipoic acid
C: CoA
F: FAD
N: NAD+
- "Tender Loving Care For Nancy"
- similar to alpha-ketoglutarate dehydrogenase (also requires TLCFN)
31
Subluxation of lens in Marfans vs cystathionine synthase deficiency
- Marfan: subluxation up and down
| - cystathionine synthase deficiency: subluxation down and in
32
in homocystinuria what reactions are impaired
- can be caused (rarely) by cystathionine synthase deficiency (Vit B6 cofactor)
- bc CS converts homocysteine to cystathionine
- but when CS is missing
- homocysteine is converted to methionine via homocysteine methyl transferase (VB12 cofactor, THF cofactor)
- methionine accumulates in blood
- thus, Vitamins B6 and B12 deficiencies can also be causes of homocystinuria
- folate deficiency can also cause homocystinuria and THF is a C donor for homocysteine methyl transferase (methionine synthesis)
33
If a 1C unit in antoher oxidation state is required (i.e. methylene, methyl, formyl)
- tetrahydrofolate (THF) serves as its donor
| - ie cofactor in homocysteine methyl transferase activity, which converts homocysteine to methionine
34
branched chain ketoacids are produced from
- produced from their cognate AA
| - valine, leucine and isoleucine
35
Tetrahydrofolate
- formed from the vitamin folate through 2 reductions (same reaction occurs 2x: Folate to DHF and DHF to TFH)
- catalyzed by dihydrofolate (DFH) reductase
- Folate needed for:
1. homocysteine metabolism
2. DNA synthesis
- DEFICIENCY: megaloblastic anemia results from insufficient active THF to support cell division the BM
- methotrexate inhibits DHF reductase = used as antineoplastic drug
- if VB12 deficient, methyl THF is stuck in the storage form (reduced folate = inactive = storage).
- This can be overcome with folate.
- but if have megaloblastic anemia and you're NOT making DNA, you know it's caused by folate deficiency
36
Folate Deficiency
- megaloblastic, macrocytic (MCV greater than 100) anemia
- PMN nucleus m ore than 5 lobes
- homocysteinemia with risk for CV disease
- deficiency develops in 3-4 months
- risk factors:
- pregnancy* (fetus may develop neural tube defects)
- alcoholism
- severe malnutrition
* women of childbearing age should supplement with 400-800 mg of folic acid/day
- if VB12 deficient, methyl THF is stuck in the storage form (reduced folate = inactive = storage).
- This can be overcome with folate.
- but if have megaloblastic anemia and you're NOT making DNA, you know it's caused by folate deficiency
37
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38
Vitamin B12 deficiency
- Vitamin B12 aka cobalamin
- MCC = pernicious anemia (failure to absorb B12 in the absence of intrinsic factor from parietal cells)
- megaloblastic, macrocytic (MCV greater than 100) anemia
- PMN nucleus m ore than 5 lobes
- homocysteinemia with risk for CV disease
- deficiency develops in years (bc excess stored in body)
- methylmalnoic aciduria (MMA in urine)
- progressive peripheral neuropathy (not corrected by giving folate)
- risk factors:
- pernicious anemia (failure to absorb B12 in the absence of intrinsic factor from parietal cells)
- gastric resection (affects B12 absorption, so need supplements)
- chronic pancreatitis
- severe malnutrition
- vegan
- infection with D. Latum (parasite found in raw fish)
- VB12 absorption also decreases with age and in individuals with chronic pancreatitis
- can crease a 2dary deficiency of active THF
- if VB12 deficient, methyl THF is stuck in the storage form (reduced folate = inactive = storage).
- This can be overcome with folate, and corrects megaloblastic anemia
39
What enzymes require Vitamin B12?
- methylmalonyl CoA mutase
| - homocysteine methyltransferase
40
Tyrosine (AA) is used to produce
- thyroid hormones T3, T4
- melanin
- catecholamines
41
Tryptophan (AA) is used to produce
- serotnonin
| - NAD, NADP
42
Arginine (AA) is used to produce
NO
43
pernicious anemia
- pernicious anemia = failure to absorb B12 in the absence of intrinsic factor from parietal cells
- Vitamin B12 aka cobalamin
44
Glutamate (AA) is used to produce
gamma-aminobutyric acid (GABA)
45
Histidine (AA) is used to produce
histamine
46
Where does Heme synthesis occur
- occurs in almost all tissues bc heme proteins include not only Hb and myoglobin, but
- also the cytochromes (ETC, cytochrome p450, cytochrome b5)
- also the enzymes catalase, peroxidase
- and the soluble guanylate cyclase stimulated by NO
- pathway producing heme is controlled INDEPENDENTLY in different tissues
47
Rate-limiting step of heme synthesis in the liver
- ALA synthase (found in mitochondria)
- converts glycine + succinyl-CoA to ALA
- inhibited by heme in liver
48
Acute Intermittent Porphyria
- porphobilinogen deaminase (hydroxymethylbilane synthase) deficiency
- converts PBG (porphobilinogen) to hydroxymethylbilane
- late-onset
- AD, with variable expression
- abdominal pain (often resulting in multiple laparoscopies (scars on abdomen)
- anxiety, paranoia, depression
- paralysis
- motor, sensory or autonomic neuropathy
- weakness,
- excretion of ALA and PBG during episodes
- no photosensitivity
- in severe cases, dark port-wine color to urine on standing
- episodes may be induced by hormonal changes or drugs (i.e. barbiturates)
- barbiturates are metabolized by cytochrome p450,
- so barbiturates increase cp450 synthesis, decrease heme levels
- reduction in heme lessens the repression of ALA synthase, so more porphyrin precursors are made, which exacerbates the disease
49
Porphyria cutanea tarda
- MC porphyria
- caused by deficiency of uroporphyrinogen decarboxylase (converts uroporphyrinogen-III to corproporphyrinogen III)
- Hepatocytes unable to decarboxylate uroporphyrinogen in heme synthesis
- AD, late onset
- MC symptom = photosensitivity (chronic inflammation to overt blistering and shearing of sun-exposed skin)
- beta-carotene often administered to porphyria puts with photosensitivity to reduce the production of ROS
- hyperpigmentation
- exacerbated by alcohol
- red-brown to deep-red urine (uroporphyrin accumulates, spills out of liver, enters urine)
- may be origin of Dracula myth
50
Lead poisoning
- inactivates many enzymes including ALA dehydrate and ferrochellatase
- failure of ferrocheatase to insert Fe2+ into protoporphyrin IX to form heme
- results in the non enzymatic insertion of Zn2+ to form zinc-protoporphyrin (extremely fluorescent, easily detected)
- can produce microcytic sideroblasticanemia with ringed sideroblasts in the BM
- coarse basophilic stippling in RBC
- ringed sideroblasts in BM
- Sx: coarse basophilic stippling of erythrocytes
- headache, nausea, memory loss
- abdominal pain, diarrhea (lead colic)
- lead lines in gums (increased serum iron)
- lead deposits in abdomen and epiphyses of bone seen on Xray
- neuropathy (claw hand, wrist drop)
- increased urinary ALA
- increased free RBC protoporphyrin
- lead paint; pottery glaze; batteries
- Dx: blood lead level
51
Vitamin B6 deficiency
- Vitamin B6 aka pyridoxine
- microcytic anemia
- ringed sideroblasts in BM
- decreased protoporphyrin
- decreased ALA
- increased ferritin
- increased serum iron
- MCC = isoniazid for TB
52
Hemochromatosis
- AR, 1/200 incidence
- Mutation in hephaestin
- generally seen in men over 40 and older women
- daily intestinal absorption of 2-3mg of Fe compared to the normal 1mg
- Over 20-30y, results in levels of 20-30g of Fe in the body (compared to the normal 4g)
- hemosiderin deposits found in
- liver: cirrhosis
- pancreas: diabetes
- skin: dermatitis
- joints: arthritis
- heart: arrhythmias because Fe conducts e-
- Tx: phlebotomy (body can only lose Fe through bleeding or losing epithelial cells)
53
porphobilinogen deaminase (hydroxymethylbilane synthase) deficiency
- Acute Intermittent Porphyria
- converts PBG (porphobilinogen) to hydroxymethylbilane
- late-onset
- AD, with variable expression
- abdominal pain (often resulting in multiple laparoscopies (scars on abdomen)
- anxiety, paranoia, depression
- paralysis
- motor, sensory or autonomic neuropathy
- weakness,
- excretion of ALA and PBG during episodes
- no photosensitivity
- in severe cases, dark port-wine color to standing urine (i.e. pt notices if doesn't flush toilet immediately)
- episodes may be induced by hormonal changes or drugs (i.e. barbiturates)
- barbiturates are metabolized by cytochrome p450,
- so barbiturates increase cp450, decrease heme levels
- reduction in heme lessens the repression of ALA synthase, so more porphyrin precursors are made, which exacerbates the disease
54
Iron Deficiency
- the last enzyme in heme synthesis (Ferrochelatase) introduces G32+ into the heme ring
- failure of ferrocheatase to insert Fe2+ into protoporphyrin IX to form heme
- results in the non enzymatic insertion of Zn2+ to form zinc-protoporphyrin (extremely fluorescent, easily detected)
- deficiency of Fe produces microcytic, hypochromic anemia
- Ferrochelatase is inhibited by lead
- microcytic anemia
- increased protoporphyrin
- normal ALA
- decreased ferritin
- decreased serum iron
- dietary iron insufficient to compensate for normal loss
- Fe deficiency anemia = SE of Vitamin C deficiency
55
ferroxidase
- aka ceruloplasmin, a Cu2+ protein
| - oxidizes Fe2+ to Fe3+ for transport and storage
56
transferrin
carries Fe3+ in blood
57
ferritin
- oxidizes Fe2+ to Fe3+ for storage of normal acts of Fe3+ in tissues
- loss of Fe from the body accomplished by leading and shedding epithelial cells of mucosa and skin
- body has no mechanism for excreting iron, so controlling its absorption into mucosal cells is crucial
- NO OTHER NUTRIENT IS REGULATED IN THIS MANNER
58
hemosiderin
binds excess Fe3+ to prevent escape of free Fe3+ into the blood, where it's toxic
59
Bilirubin Metabolism
- subsequent to lysis of older RBC in spleen, heme released from Hb is converted to bilirubin in histiocytes
- bilirubin is not H20 soluble; transported in blood by albumin
- hepatocytes conjugate bilirubin w glucuronic acid, increasing its water solubility
- conjugated bilirubin is secreted in bile
- intestinal bacteria convert conjugated bilirubin into urobilinogen
- portion of urobilinogen further converted to bile pigments (stercobilin) and excreted in feces (makes feces red-brown)
- bile duct obstruction results in clay-colored stools
- some urobilinogen is converted to urobilin (yellow) and secreted in urine
60
jaundice
- yellow skin, whites of eyes (icterus)
- occurs when blood [bilirubin] greater than normal
- increase in unconjugated (indirect) bilirubin, conjugated (direct) bilirubin, or both
- kernicterus: accumulation of bilirubin (usually unconjugated aka indirect) in brain; may result in death
- when conjugated bilirubin increase, may be excreted in urine = urine deep yellow/red color
61
hemolytic crisis
- w severe hemolysis, more bilirubin released into blood than can be transported on albumin and conjugated in liver
- unconjugated and total bilirubin increase and may produce jaundice and kernicterus
- confirm by low Hb and elevated reticulocyte count
- Ex) hemolysis in G6PDH deficiency
- sickle cell crisis
- Rh disease of newborn
62
UDP-glucuronyl transferase deficiency
- when bilirubin conjugation is low bc of genetic/functional deficiency of glucuronyl transferase system
- unconjugated and total bilirubin increase, causes jaundice
- Ex) crigler-najjar syndromes
- gilbert syndrome
- physiologic jaundice of the newborn (enzymes may not be fully induced; esp in premies)
63
AST increases more than ALT in
alcoholic liver disease
64
Bile duct occlusion
- ie gallstone, primary biliary cirrhosis, pancreatic cancer
- prevents conjugated bilirubin from leaving liver
- conjugated bilirubin increases in blood and may also appear in urine
- light-colored stool
- jaundice
65
ALT increases more than AST in
viral hepatitis
