Lecture 25: Amino Acids II

PLP (B6 derivative)

Interconverts alanine an pyruvate by transfering the amino group to a-KG forming glutamate

Interconverts aspartate and OAA by transfering the amino group to a-KG forming glutamate

transfers teh amino group from valine, isoleucine, and leucine to a-KG forming glutamate

-Much higher in liver than other tissues
-Elevated in blood of pts with liver disease
-Extremely high with viral hepatitis, ischemic liver injury, toxin/drug induced liver injury, prolonged hypotension, acute heart failure

-found at similar concentrations in most cells
-elevated in pts with liver/kidney/ CV disease
-extemely high with prolonged hypotension, MI

-normally serum contains low levels
-upon tissue injury the enzymes are released into the vascular system
-levels of AT's, LDH and CK are diagnostic markers for tissue damage

dehydratases and lyases
(threonine dehydrtase, serine dehydratase, histidine ammonia lyase)

alanine, glutamine, serine

1. Ammonia is transfered onto a-KG from AAs via ALT, AST or other AT's to form glutamate. Glutamate is the converted into glutamine by glutamine synthetase
2. Free ammonia is added to a-KG by glutamate dehydrogenase. The Glutamate is then converted to glutamine by glutamine synthetase

Free ammonia is added to a-KG by glutamate dehydrogenase to form glutamate. ALT then catalyzes the transfer of ammonia from glutamate to pyruvate forming alanine. Alanine transports carbon and ammonia to the liver.

1. Aminotransferases in the liver cytoplasm transfer ammonia from AAs to a-KG forming glutatmate which delivers the ammonia to the mitochondrion.
2. Free ammonia can enter the mitochondrion directly or be used for glutamine synthesis.

glutamine, glutamate, free ammonia

1. Ammonia is released from glutamate by glutamate dehydrogenase
2. Ammonia is released from glutamine by converting it to glutamate using glutaminase, then glutamate dehydrogenase releases the ammonia

Allosteric control by the cellular energy level. Stimulated by high ADP, inhibited by high GTP
(high ADP, high GTP indicates a catabolic state where protein is being broken down and the ammonia must be delt with)

a-KG goes to the TCA cycle for conversion to OAA and synthesis of aspartate

The ammonia reacts with HCO3- to form carbamoyl phosphate. This is catalyzed by carbamoyl phosphate synthetase 1.

occurs in the matrix. Two ATP molecules are required for energy. N-acetylglutamate is an essential activator

N-acetylglutamate is an essential activator for ccarbamoyl phosphate synthetase 1. It is synthetized from acetyl CoA and glutamate by acetylglutamate synthetase

carbamoyl phosphate

in the matrix, contrast to the rest of the cycle reactions that occur in the cytosol

Carbamoyl phosphate condensed with ornithine to form citrulline. The reaction is catalyzed by ornithine transcarbamoylase

cytosol

Citrulline is transported from the matrix to the cytosol where is is converted to a citrullyl-AMP intermediate and condenses with aspartate to make argininosuccinate. This is catalyzed by argininosuccinate synthetase

Citrulline is converted to argininosuccinate via a citrullyl-AMP intermediate. This reaction requreis ATP and Aspartate

Argininosuccinate is cleaved into fumarate and arginine. This is catalyzed by argninosuccinate lyase. Note that this is a direct cleavage rxn that does NOT require water.

Arginine is cleaved by arginase to form urea and ornithine. This enzyme requires a water moelcue.

-most is transported from the liver via the blood to the kidney for excretion in the urine
-some diffuses into the intestine and is cleaved into CO2 and ammonia by bacterial urease. Most of the ammonia is readsorbed but a small amount is excreted in the feces

NH4+ + HCO3- + 3 ATP + H2O+ Asparate=> Urea + 2 ADP + AMP +2Pi + PPi + fumarate

short term-availability of N-acetylglutamate to activate carbamoyl phosphate synthetase I
long term-level of urea cycle enzymes
Note that the cycle is unregulated during starvation and in people on high protein diest

Fumarate is converted to malate by cytosolic fumerase. Malate then crosses into the matrix and is used in the TCA cycle.

OAA from the TCA cycle is used to synthesize asparate via AST which is needed for the argininosuccinate synthetase reaction

Ornithine transcarbamoylase, carbamoyl phosphate synthetase argininosuccinate synthetase deficiencies are the most severe because the products synthesized are toxic and cannot be directly excreted in the urine.

Carbamoyl phosphate synthetase deficiency results in very high levels of ammonia because free ammonia cannot be efficiently metabolized. In addition, the urea cycle is inoperable because carbamoyl phosphate is not made to “fuel” the cycle.

Ornithine transcarbamoylase deficiency results in high levels of carbamoyl phosphate and free ammonia that cannot be detoxified by the urea cycle. Because of the high levels of carbamoyl phosphate in the mitochondria, a large portion will cross the mitochondria membranes and mix with the cytosolic pool of carbamoyl phosphate produced by carbamoyl phosphate synthetase II. Carbamoyl phosphate then is metabolized to orotate in the pyrimidine synthesis pathway. Excess orotate not needed to synthesize pyrimidines is excreted in the urine.

ornithine transcarbamoylase

-Argininosuccinate can be excreted into the urine so there is no concern of toxicity
-Arginine is give to fuel the urea cycle so that ornithine can be regenerated so that CP does not build up

Reduce levels of ammonia by reacting with glycine and glutamine and forming hippurate and phneylacetylglutamine which can be excreted in the urine

X linked
note that all other deficiencies of urea synthesis are autosomal recessive

1. liver transplant
2. waste product medications
3. low protein diet