Protein II Flashcards
What is transamination?
The process to synthesize most non-essential amino acids
Transfer of alpha-amino group from an amino acid to an alpha-keto acid and is reversible
What are the enzymes associate with transamination and what cofactor is important for that enzyme?
Aminotransferases - specific to amino acid it transaminates
Need cofactor pyridoxal 5’-phosphate derived from Vitamin B6
What is the most common acceptor of the alpha amino group in transamination?
Alpha-Ketoglutarate
What is the most common donor of the alpha-amino group in transamination?
Glutamate
How is glutamate synthesized?
Glutamate - non-essential aa so can be obtained from diet
Can be synthesized by transamination of alpha-ketoglutarate
Also can be synthesized by deamidation of glutamine with ammonium released
Enzyme involved is glutaminase
How is Glutamine synthesized?
Glutamine can be generated from glutamate in a reaction that requires ATP and ammonium ions. Catalyzed by glutamine synthetase
Glutaminase in liver, kidney, or intestines can convert glutamine back to glutamate and the released ammonium ions can be incorporated in urea cycle or excreted by kidney
How is Alanine synthesized?
Synthesized from pyruvate by alanine aminotransferase (ALT) with cofactor PLP
Reaction is transamination and the precursors are pyruvate and glutamate (donates alpha-amino group)
What is important about the synthesis of alanine?
Plays an important role in connecting muscle and liver metabolism during fasting.
During fasting, muscle glycolysis produces pyruvate, which is converted to alanine by ALT
In the liver, pyruvate is used for gluconeogenesis and the glucose enters muscle and is used in glycolysis.
Called the glucose-alanine cycle
How is aspartate and asparagine synthesized?
Aspartate synthesized from oxaloacetate by aspartate aminotransferase (AST) with cofactor PLP. Precursors are oxaloacetate and glutamate
Asparagine is synthesized from aspartate by asparagine synthetase. Requires ATP and precursor are aspartate and glycine
What is unique about the production of aspartate and asparagine?
Asparate aminotransferase utilizes the NH3 of glutamate to form the amino group of aspartate
Asparagine synthetase utilizes the amide nitrogen of glutamine to form the amide of asparagine
What is the tissue distribution and elevated serum levels for aspartate aminotransferase (AST) and alanine aminotransferase (ALT)?
AST:
Tissues - liver, heart, skeletal muscle, kidney, brain
Serum levels - liver disease, kidney damage, acute myocardial infarction
ALT:
Tissues - high levels in liver, expression in other tissues not clinically relevant
Serum levels - liver damage
How is glycine synthesized?
Glycine - non-essential so can be found in diet
Synthesized by serine hydroxymethytransferase (SHMT) that converts serine to glycine.
Attached to cofactor PLP and tetrahydrofolate (THF) - methylene group acceptor
Precursor is serine (non-essential)
What does the reaction from serine to glycine also produce?
Glycine
H2O
N5N10-methyleneTHF
What is tetrahydrofolate (THF)?
Derivative of folate (vitamin B9)
Serves as acceptor of one carbon groups for the one carbon pool
One carbon pool supports synthesis of purine rings and thymidine (DNA synthesis)
While bound to THF, one carbon group can be oxidized or reduced
What is the most oxidized form of THF?
N10-formyl H4 folate
What is the most reduced form of THF?
N5-MethylH4 Folate
Not readily oxidized so tends to accumulate in the body
What is the form of THF important to DNA synthesis?
N5, N10MethyleneH4 Folate
How is cysteine synthesized?
Steps:
1. Begins with methionine and ATP, forming S-adenosylmethionine (SAM) via methionine adenosyltransferase
- SAM donates methyl group to THF and is converted to S-adenosylhomocysteine
- S-adenosylhomocysteine cleaved to yield adenosine and homocysteine
- Homocysteine reacts with serine to form cystathionine with cofactor PLP and enzyme cystathionine Beta-synthase
- Cleavage of cystathionine by cystathionine-gamma-lyase (PLP dependent) yields cysteine, ammonia, and alpha-ketobutyrate
Pt presents with elevated levels of homocysteine in the urine. What is the condition?
Homcystinuria
Genetic defect is cystathionine Beta-synthase so homocysteine builds up
Cystathioninuria
Rare autosomal recessive disorder in gene encoding cystathionine gamma-lyase.
Results in elevated cystathionine in blood and urine.
How is tyrosine synthesized?
Hydroxylation of essential amino acid phenylalanine which is catalyzed by phenylalanine hydroxylase in the liver.
Irreversible reaction that requires molecular oxygen , NADH, and tetrahydrobiopterin (THBtn)
THBtn is oxidized to dihydrobiopterin (DHBtn) during reaction and is reduced back to THBtn by dihydrobiopterin reductase and NADH
Phenylketonuria
Causes:
Genetic defect in phenyalanine hydroxylase (most common)
Genetic defect in DHBtn reductase
Diagnostic metabolites: phenylpyruvate and phenylalanine (can be toxic when accumulated)
Symptoms: Intellectual disability, recurrent seizures, and hypopigmentation
What is the mechanism of phenylketonuria (PKU)?
Accumulation of Phe inhibits transport of other amino acids across blood/brain barrier and interferes with neurotransmitter synthesis
Can impact myelin, dopamine (tyrosine deficiency in some cases)
Phe is competitive inhibitor of tyrosinase thus interfering with melanin (skin pigment)
What is the treatment for PKU?
Newborn screening for PKU
Low dietary Phe
Supplementation with tyrosine
Aspartame. How is it connected to PKU?
Artificial sweetener that is dipeptide of aspartate and methyl ester of phenyalanine
Upon degradation -> Asp, Phe, and methanol released
Patient with PKU, Phe produced from aspartame is toxic
