L24: Folate, B12, and Non-Essential Amino Acid Metabolism

Question 1

LO1: Name three carriers of one-carbon units, and compare the oxidation state of the carbon carried by each

Answer 1

THF (Folate)=active form of folic acid
-carries one-carbon units in a variety of oxidation states depending on attached group (most reduced=methyl, most oxidized=methenyl)

SAM (preferred activated methyl carrier)
-carriers one-carbon units as methyl groups with most reduced oxidation state

BIOTIN
-carries carboxylate groups in the most oxidized state

Question 2

LO2: List three biosynthetic pathways that require one-carbon units

Answer 2

• amino acid metabolism
• purine metabolism
• pyrimidine metabolism

Question 3

LO3: Describe the role of S-adenosylmethionine (SAM) in metabolism

Answer 3

SAM is the preferred activated methyl carrier in most methylation reactions (incl. but not limited to synthesis of creatine, phosphatidylcholine, methylated RNA, methylated DNA, epinephrine, carnitine)

1. Creatine synthesis: body’s greatest use of SAM
2. Carnitine synthesis: SAM used to add methyl group to lysine side chain within proteins—-followed by proteolysis to free trimethyllysine—->additional reactions to form carnitine

Question 4

LO4: Describe two types of reactions that are important in the synthesis of non-essential amino acids, and give an example of each

Answer 4

TRANSAMINATIONS

• transamination of an amino group to an alpha-ketoacid
• includes pyruvate to alanine, OAA to aspartate, and alpha-ketoglutarate to glutamate

AMIDATION

• glutamine synthethase catalyzes conversion of glutamate+ammonia+ATP—->glutamine + ADP + Pi
• asparagine synthetase catalyzes reaction of aspartate+glutamine+ATP—->asparagine+glutamate+ADP+Pi

Question 5

LO5: Name the intermediates in glycolysis and the citric acid cycle that are precursors for non-essential amino acids

Answer 5

9 of the 11 non-essential AAs are synthesized from intermediates generated in glycolysis or the TCA cycle

GLYCOLYTIC INTERMEDIATES
glucose—->phosphoglycerate—–>pyruvate
-phosphoglcerate—->serine (1)—->glycine (2)
-pyruvate—->alanine (3)

TCA CYCLE
pyruvate—>alpha-ketoglutarate—->OAA
-alpha-ketoglutarate—>glutamate (4)—->proline (5) or arginine (6)
-alpha-ketoglutarate—->glutamate—+N—>glutamine (7)
-OAA—>aspartate (8)—–+N—->asparagine (9)

Question 6

LO6: Name two non-essential amino acids that are synthesized from essential amino acids

Answer 6

1. Tyrosine (formed from phenylalanine)

2. Cysteine (formed from methionine)

Question 7

LO7: Compare and contrast classical PKU vs. atypical PKU vs. maternal PKU

Answer 7

CLASSICAL PKU

• deficiency in phenylalanine hydroxlyase, which synthesis tyrosine from phenyalanine
• tyrosine becomes essential AA since it can’t be synthesized, and products derived from tyrosine (catecholoamines, melanin. fumarate, acetoacetate) are deficient
• phenyalanine accumulates and forms phenylketones; competes with exogenous tyrosine (and tryptophan) for uptake across BBB

ATYPICAL PKU

• deficiency in enzymes needed for biopterin biosynthesis or regeneration of BH4
• deficiency in THB or BH4 leads to less reducing power for the phenylalanine hydroxylase to use

MATERNAL PKU
-damage to fetus by maternal phenylketones, due to mother’s PKU status

Question 8

LO8: Describe the consequences of a folic acid deficiency

Answer 8

• folic acid (activated form=THF) is the sole one-carbon carrier for most carbon units (except methyl, which can also be carried by SAM)
• deficiency leads to build up of formaldehyde as THF can’t carry free one-carbon groups
• would also result in deficient levels of products synthesized using one-carbon units carried by THF (most importantly, creatine and carnitine)

Question 9

LO9: Describe the two reactions in humans that require Vit B12, and how a deficiency in B12 can lead to a secondary folate deficiency

Answer 9

BCAA metabolism: B12 required for conversion of l-methylmalonyl-CoA to succinyl-CoA by methylmalonyl-CoA mutase

Remethylation of SAM: methionine synthase, which regenerates methionine from homocysteine and vice versa, forms part of the SAM biosynthesis and regeneration cycle

Secondary folate deficiency: results from B12 deficiency because methionine synthase needs B12 as a cofactor to regenerate SAM, and without it folate remains trapped as 5-(CH3)methyl-THF instead of THF
-no usable folate

Question 10

L10: Describe the pathway for endogenous synthesis of arginine and name the organs that participate

Answer 10

• arginine is essential in infancy/childhood until body can synthesize enough of it on its own (though if intestinal or renal function is impaired it may become essential again)
• while synthesized in the urea cycle by the liver, arginase is highly expressed in the liver so it is cleaved to urea/ornithine right away–>no net synthesis

INTESTINAL-RENAL-AXIS: source of arginine

• CPS-I and OTCase are located in the intestinal cells where the first two reactions of arginine biosynthesis occur (leading to citrulline biosynthesis using ornithine; ornithine derived from glutamine catabolism)
• citrulline is released into circulation, extracted by the kidney, and acted upon by ASS and ASL in the proximal tubules
• kidney efficiently converts citrulline to arginine (and fumarate), which is then released into blood

Question 11

L11: Name the AA whose plasma concentration is an indicator of a functional small intestinal mass

Answer 11

-citrullin is released from the enterocyte into the circulation, and its concentration in plasma is an indicator of functional small intestinal mass

Question 12

L12: Name the enzyme deficiency that can cause a patient’s urine to turn black, give its disease name, and describe the catabolic pathway for this enzyme

Answer 12

DEFICIENCY IN HOMOGENTISTIC ACID OXIDASE
-normally catalyzes third step in catabolism of tyrosine to fumarate and acetoacetate

ALKAPTONURIA

SYMPTOMS

• homogentistic acid accumulates and spills out into urine
• the acid oxidizes readily and turns the urine black; black pigments are also found deposited on the intervertebral disks