Branched Chain/Sulfur -Containing Amino Acid Metabolism Flashcards
What are the three branched chain amino acids
leucine, isoleucine and valine.
What are the wo common enzymes that begin the common catablism of branched chain AA
BCAA Aminotransferase and BC Alpha-Keto Acid Dehydrogenase
Function of BCAA aminotransferase and where is it active
BCAA aminotransferase is a transaminase enzyme that coverts BCAA into branched chain α-keto acids (BCAKA)
BCAA aminotransferase is highly active in skeletal muscle but not liver
What is BCAKA dehydrogenase
BC Alpha-Keto Acid Dehydrogenase
BCAKA dehydrogenase is located in the mitochondrial inner membrane and is analogous to the pyruvate dehydrogenase complex
BCAKA dehydrogenase is a large mitochondrial multi-enzyme complex.
The complex is composed of a decarboxylase (E1) with two α and β subunits, a transferase (E2), and a dehydrogenase (E3).
BCAKA dehydrogenase catalyzes the oxidative decarboxylation of BC α-keto acids forming NADH, CO2, and a corresponding hydrocarbon product linked to CoA.
Where is BCAKA high
Branched chain keto acids (BCKA) formed by transamination are transported from skeletal muscle to the liver where the activity of BCAKA dehydrogenase is high
Thus, catabolism is initiated in skeletal muscle but completed in the liver.
BCAKA dehydrogenase is most active in the muscle during starvation.
Clinical features of classic maple sugar urine disease
The clinical features of MSUD are largely dependent on the amount of residual BCAKA dehydrogenase activity present in a patient. For classical MSUD, patients have from 0-3% of the normal amounts of BCAKA dehydrogenase activity.
Clinical features for classic MSUD include:
neonatal onset
maple syrup odor
poor feeding
Irritability, lethargy
opisthotonus (severe hyperextension, spasticity), focal dystonia
“Fencing,” “bicycling”
Other forms of MSUD with up to 30% of normal BCAKA dehydrogenase activity show milder and more variable expression of the clinical features observed in classic MSUD. Metabolic stress caused by illness or poor feeding, which induce catabolic states can lead to neurologic deterioration in patients with milder forms of MSUD.
Genetics and epidemiology of MSUD
At least six nuclear genes encode components of the BCAKA dehydrogenase complex;
3 of these genes account for almost all cases of MSUD.
MSUD is relatively rare in the general population, affecting ~1 in 200,000 individuals.
Mennonites have an increased incidence, ~1 in 200, due to a founder mutation that causes an amino acid substitution in one of the subunits of BCAKA dehydrogenase.
Treatment for MSUD
Low protein diet with restriction of leucine (Leu), isoleucine (Ile), and valine (Val). Adding medical formula, supplements (BCAAs are essesntial AAs)
Thiamine supplements
The first step in the oxidative decarboxylation of the BCAA catalyzed by BCAKA dehydrogenase involves thiamine pyrophosphate as a coenzyme.
Some patients with residual enzyme activity respond to thiamine supplements, restoring activity to levels that ameliorate symptoms
Monitoring for MSUD
Plasma levels of BCAA need to be monitored on a regular basis to assess the delicate balance of providing sufficient essential amino acids to support normal growth or development without having excess BCAA or BCAKA accumulate to toxic levels.
Urine levels of BCAKAs if high can indicate a catabolic state and insufficient metabolic control indicating a need for change in treatment plan
Final breakdown products of BCAAs
Valine is ultimately broken down to propionyl-CoA which is glucogenic
Isoleucine gives rise to acetyl-CoA and propionyl-CoA and is therefore, ketogenic and glucogenic, respectively.
Leucine follows a path through hydroxymethylglutaryl- CoA which is cleaved to acetyl-CoA and acetoacetate. Leucine is strictly ketogenic.
The branched chain amino acids have large hydrocarbon side chains. SImilar to fatty acids
What is Propionyl-CoA Carboxylase
Works in the breakdown of valine anad isoleucine
converts propionyl-CoA to D-methylmalonyl-CoA (Fig. 2)
catalyzes a carboxylase reaction that employs biotin as a coenzyme
is a mitochondrial enzyme with 12 subunits, 6 alpha chains and 6 beta chains.
What are the sources of proprionic acid
VOMIT
These include the amino acids:
valine
isoleucine
methionine
threonine.
Another source of propionic acid, though in lesser amounts than from the catabolism of the amino acids shown, is the breakdown of:
odd chain fatty acids.
Clinical features of Propionic Acidemia
Severe forms of propionic acidemia present in the newborn period.
Symptoms include:
poor food intake and vomiting
lethargy
hypotonia
seizures
coma.
Labs for proprionic acidemia
Wide anion gap (metabolic acidosis)
Elevated propionic acid and glycine in blood
Ammonemia
How does proprionic acidemia lead to high ammonia levels
propionic acid inhibits N-acetylglutamate synthase, reducing levels of N-acetylglutamate, which reduces the activity of carbamoyl phosphate synthetase I, which in turn, shuts down the urea cycle, leading to hyperammonemia.
Genetics of proprionic acidemia
Primary cause - deficiency of propionyl-CoA carboxylase
Autosomal recessive disorder linked to mutations in genes encoding alpha and beta subunits.
Secondary causes
Deficiency of biotin uptake and utilization
Classified as multiple carboxylase deficiency as other carboxylase enzymes using biotin as a coenzyme are also affected
Deficiency of L-methyl malonyl-CoA mutase can lead to increased levels of both methylmalonic acid and propionic acid.
Treatment for proprionic acidemia
Coenzyme supplementation - Value of biotin therapy variable, Patients with multiple carboxylase deficiency are often biotin responsive, Patients with elevated propionic acid due to a deficiency of L-methyl malonyl-CoA mutase may respond to vitamin supplementation with B12.
Dietary protein restriction
Carnitine supplements - propionic acid bound to propionyl-CoA can be transferred to carnitine and excreted in urine
Antibiotics - Antibiotics are administered to reduce propionic acid synthesis from intestinal bacteria,
What is L-Methylmalonyl-CoA Mutase
Converts L-methylmalonyl-CoA to succinyl-CoA (Fig. 2)
The methylmalonic acid mutase reaction requires vitamin B12
Clinical features of Methylmalonic Acidemia
The clinical features of methylmalonic acidemia overlap somewhat with propionic acidemia, in part because propionic acids levels rise in patients with a deficiency of L-methylmalonyl-CoA mutase because the pathway tends to back up all the way to propionyl-CoA.
Notable features of L-methylmalonyl-CoA mutase
one of two enzymes in the human body that use vitamin B12 as a co-enzyme.
Adenosyl-cobalamin is the active co-enzyme utilized by L-methylmalonyl-CoA mutase.
Labs for Methylmalonic Acidemia
Wide anion gap (metabolic acidosis)
Elevated methylmalonic and propionic acids in the blood.
Glycine and ammonia can also be elevated.
Genetics behind Methylmalonic Acidemia
Primary cause
Deficiency of L-methylmalonyl-CoA mutase (single gene)
Secondary causes
Deficiency of enzymes needed to convert dietary B12 to adenosyl-cobalamin (multiple genes).
Patients with defects in B12 utilization may also present with megalobalstic anemia and other neurological symptoms relating to B12 deficiency.
Deficiency of a protein cofactor needed for loading adenosyl-B12 to L-methylmalonyl mutase (single gene).
Treatment for Methylmalonic Acidemia
Co-enzyme supplementation- Some patients will respond to various forms of B12 supplementation
Dietary protein restriction- The primary therapy for individuals who are not responsive to coenzyme therapy is dietary protein restriction (0.5-1.5 g/kg/d), since valine, isoleucine, methionine and threonine represent the primary sources of propionyl-CoA and methylmalonyl-CoA in the body.
Carnitine supplements
How is glycine synthesized
Glycine is derived from serine, which in turn, is derived from the glycolytic intermediate 3-phosphoglycerate making both serine and glycine non-essential amino acids.
Glycine is derived from serine in a reaction catalyzed by serine transhydroxymethylase. Reversible
Function of thymidilate synthase
Thymidylate synthase uses N5,N10-methylene-THF as the one-carbon donor in synthesizing TMP from dUMP
Fates of glycine
Conversion back to serine (the direction of this reaction is dictated by the needs of a cell at any particular instant in time).
Another fate for glycine is its degradation via a pathway initiated by D-amino acid oxidase, which forms glyoxylate and then gets converted to oxalate.
Oxalate is sparingly soluble and can precipitate in the kidney and is a source of kidney stones.
Breakdown into CO2
What is the glycine cleavage complex
The final fate of glycine is degradation through the action of the glycine cleavage complex which breaks glycine down to CO2.
Other products of the glycine cleavage reaction include ammonia, NADH, and N5 ,N10 -methylene-tetrahydrofolate.
What is Nonketotic hyperglycinemia (glycine encephalopathy) and SX
Nonketotic hyperglycinemia is caused by defects in enzymes that comprise the glycine cleavage complex.
Glycine is a neurotransmitter that when it accumulates to high levels can become toxic.Affected infants experience:
a progressive lack of energy (lethargy)
feeding difficulties
weak muscle tone (hypotonia)
abnormal jerking movements
life-threatening problems with breathing.
Most children who survive these early signs and symptoms develop profound intellectual disability and seizures that are difficult to treat.
Relationship between methionine and cysteine
cysteine is derived from the catabolism of the essential amino acid methionine.
Steps in the breakdown of methionine
The breakdown of methionine begins with the formation of S-adenosylmethionine (AdoMet or SAM) This reaction is catalyzed by methionine adenosyltransferase.
The substrates for this reaction are methionine and ATP.
All of the phosphates of ATP are lost in the formation of the product, AdoMet.
AdoMet is used as a methyl group donor in a wide range of biochemical reactions. The product of these methyltransferase reactions is S-adenosylhomocysteine (Fig. 4B).
S-adenosylhomocysteine is subsequently cleaved to adenosine and homocysteine.
Role of Cystathionine Synthase
Cystathionine synthase catalyzes the reaction where homocysteine condenses with serine to form cystathionine.
Cystathionine synthase is a pyridoxal phosphate-dependent enzyme that is feedback inhibited by cysteine.
Steps for breakdwon of cystathionine
The next reaction in the pathway is catalyzed by cystathionase which converts cystathionine to cysteine.
Cystathionase is also a pyridoxal phosphate-dependent enzyme.
In the transulfuration reaction catalyzed by cystathionase, the sulfur of homocysteine is transferred to the carbon skeleton of serine forming cysteine.
The other product of the cystathionase reaction is α-ketobutyrate, which goes on to form succinyl-CoA
Cysteine can be further metabolized to pyruvate and sulfate.
Both pyruvate and succinyl-CoA can give rise to gluconeogenic substrates, making both methionine and cysteine glucogenic amino acids.
Cause of homocystinuria
Classic homocysteinuria is caused by a cystathionine synthase deficiency.
Clinical features of homocystinuria
Main features (CNS):
developmental delay and cognitive impairment
psychiatric disturbances (personality disorders, behavior disorders, depression, obsessive compulsive disorder)
seizures
Vascular system - thromboembolic events are a major cause of morbidity and most frequent cause of mortality;
can involve any vessel
increased risk in pregnancy, postpartum, and postoperative periods
cumulative risk 25% by 15 years of age
The cardiovascular events observed in patients with homocysteinuria established the link between elevated homocysteine levels and cardiovascular disease discussed in HSHD1.
Eyes - myopia followed by ectopia lentis typically occurs after 1 year of age with majority of untreated individuals affected with ectopia lentis by 8 years of age
Skeletal system - tall, slender habitus; high arched palate, pes cavus, pectus excavatum or carinatum, genu valgum, scoliosis; when older, can have abnormal vertebrae (biconcave)
Osteoporosis - in untreated patients, affects at least 50% by their teens
Treatment of homocytinuria
Low protein, low methionine diet
Medical formula - methionine is absent but contains other amino acids and nutrients needed for proper growth
Medications
What is the remethylation pathway
The remethylation pathway gets its name because it is where homocysteine can be methylated back to methionine.
not a reversal of the pathway by which methionine is converted to homocysteine.
What is the alernative remethylation pathway
second remethylation pathway using betaine as a methyl group donor.
What can deficiency in methionine synthase cause
Since the remethylation pathway uses homocysteine as a substrate, a defect in methionine synthase can lead to elevated levels of homocysteine and thereby be another cause of homocysteinuria
Role of Methionine Synthase in folate and B12
the remethylation pathway as the critical intersection point between B12 and folic acid metabolism. The reaction catalyzed by methionine synthase is one of two B12-dependent reactions in the human body
Methyl-cobalamin is the methyl group donor for the methionine synthase reaction.
Methionine synthase creates methyl-cobalamin at its active site by transferring a methyl group from N5-methyl-tetrahydrofolate to cobalamin.
The transfer of the methyl group from N5-methyl-THF to cobalamin by methionine synthase is the only reaction of intermediary metabolism that consumes N5-methyl-THF.
What are the various sources of one carbon derivatives of tetrahydrofolate
You see one carbon units coming from serine, glycine, histidine, and tryptophan.
These one carbons units are used in purine synthesis and thymidylate synthesis
What is the folate trap
The derivatives N5,N10-methylene-THF, N5,N10-methenyl-THF, and N10-formyl-THF are all in equilibrium with one another.
The exception to the equilibration of THF derivatives is N5-methyl-THF, as the reaction catalyzed by methyltetrahydrofolate reductase is irreversible.
The only way to get N5-methyl-THF back into the pool of folate metabolites is through the methionine synthase reaction.
If the methionine synthase reaction is inhibited, folate pools drain into the production of the now dead end intermediate N5-methyl-THF, depleting all other THF
Mechanism behind folate trap
One way that methionine synthase is inhibited is insufficient amounts of B12.
Thus, a B12 deficiency also manifests as a folic acid deficiency.
This intersection of B12 and folic acid metabolism is known as the folate trap, where low levels of B12, trap folic acid as the dead end intermediate N5-methyl-THF.
Medications for treatment of homocystinuria
Cystathionine synthase, the enzyme affected in classic homocysteinuria, requires pyridoxal phosphate as a coenzyme. Consequently, some patients are responsive to pyridoxine supplements; decreasing levels of methionine and homocysteine and showing an improvement in clinical parameters including a significant reduction in thromboembolic events and decreased frequency of ectopia lentis.
Folic acid and B12 supplements - the goal here is to stimulate the remethylation pathway as a strategy to lower homocysteine levels.
Betaine (AKA trimethylglycine) serves as a methyl group donor for an alternative remethylation pathway, thereby converting homocysteine to methionine and reducing homocysteine levels.
Outcomes of homocystinuria
In infants:
dietary treatment improves developmental outcome (IQ), may delay or prevent ectopia lentis, may prevent seizures, can prevent osteoporosis, and can prevent thromboembolism
In older children and adults:
dietary treatment and medications can decrease Hcy levels and decrease risks for osteoporosis and TE
5-10% of the general population has mild homocysteinemia.
Vitamin supplementation with B12 , folate, and pyridoxine has been shown to decrease plasma homocysteine levels and therefore may provide some benefit in decreasing cardiovascular disease in people with elevated homocysteine.
