BCH 400 Amino Acid Degradation and Urea Cycle

Chapter 30

What makes ammonia a problem in amino acid breakdown?

It's toxic

What pathway converts toxic ammonia into urea?

Urea cycle

What is the major site of amino acid degradation in mammals?

Liver

Are amino acids stored like fats and carbohydrates?

No

What is protein turnover?

Constant protein degradation and resynthesis

Name the two main fates of amino acids from protein.

Recycle or oxidize for energy

When are body proteins broken down for energy?

Starvation (carbohydrates scarce)

What happens to dietary amino acids beyond synthesis needs?

Oxidized for energy

Which animals rely heavily on amino acids for energy?

Carnivores

What is the first step in degrading all amino acids?

Removal of amino nitrogen

Name the two ways amino nitrogen is removed.

Transamination, direct deamination

What happens to carbon skeletons after nitrogen removal?

Enter central metabolism

What enzymes transfer α-amino groups to α-ketoacids?

Aminotransferases (transaminases)

What cofactor do aminotransferases require?

PLP (pyridoxal phosphate)

PLP is derived from which vitamin?

Vitamin B6

What is the usual amino group acceptor in transamination?

α-Ketoglutarate

What does α-ketoglutarate become after accepting an amino group?

Glutamate

Alanine + α-ketoglutarate ⇌ ?

Pyruvate + glutamate

Aspartate + α-ketoglutarate ⇌ ?

Oxaloacetate + glutamate

What does ALT stand for?

Alanine aminotransferase

What does AST stand for?

Aspartate aminotransferase

Can transaminases also synthesize amino acids?

Yes (reversible)

Keto counterpart of alanine?

Pyruvate

Keto counterpart of aspartate?

Oxaloacetate

Keto counterpart of glutamate?

α-Ketoglutarate

Keto counterpart of leucine?

α-Ketoisocaproate

Keto counterpart of phenylalanine?

Phenylpyruvate

Keto counterpart of tyrosine?

p-Hydroxyphenylpyruvate

What does transferring a second amino group yield?

Glutamine

What does high blood ALT/AST indicate?

Liver damage

Why do aminotransferases leak into the blood?

Damaged liver cell membranes

Name three causes of liver damage that raise ALT/AST.

Hepatitis, alcohol, acetaminophen

Normal blood ALT activity?

5–30 units/L

Normal blood AST activity?

40–125 units/L

Kinetic mechanism of aminotransferases?

Ping-pong (double displacement)

Which enzyme releases NH4+ from glutamate?

Glutamate dehydrogenase (GDH)

What reaction type does GDH catalyze?

Oxidative deamination

Name the products of the GDH reaction.

α-KG, NH4+, NAD(P)H

What are GDH's electron acceptors?

NAD+ or NADP+

Why is GDH unusual among dehydrogenases?

Uses NAD+ or NADP+

Where in the cell is GDH located?

Mitochondria

Why is GDH's mitochondrial location useful?

Sequesters toxic ammonia

In which tissue is GDH found (per textbook)?

Liver

What is GDH's approximate Keq in liver?

Close to 1

What drives the GDH reaction forward?

Rapid NH4+ removal

Allosteric inhibitor of mammalian GDH?

GTP

Allosteric activator of mammalian GDH?

ADP

What mechanism do GTP and ADP use to regulate GDH?

Abortive complex formation

Effect of GTP on abortive complexes?

Promotes them

Effect of ADP on abortive complexes?

Destabilizes them

Net result of transaminases plus GDH?

Amino acid → α-ketoacid + NH4+

Which two amino acids are directly deaminated?

Serine, threonine

Which enzyme directly deaminates serine?

Serine dehydratase

Which enzyme directly deaminates threonine?

Threonine dehydratase

Another name for threonine dehydratase?

Threonine deaminase

Cofactor of serine/threonine dehydratases?

PLP

Serine deamination products?

Pyruvate + NH4+

Threonine deamination products?

α-Ketobutyrate + NH4+

α-Ketobutyrate is converted to what?

Propionyl CoA

Which amino acids can the liver NOT deaminate?

Leucine, valine, isoleucine

Which tissue mainly burns branched-chain amino acids?

Muscle

Why can't muscle make urea?

Lacks urea cycle enzymes

Two main nitrogen transport forms from muscle?

Alanine, glutamine

What cycle ships muscle nitrogen to liver as alanine?

Glucose-alanine cycle

What does muscle pyruvate become after receiving nitrogen?

Alanine

What happens to alanine in the liver?

Transaminated to pyruvate

Fate of liver pyruvate from alanine?

Gluconeogenesis (glucose)

Which older cycle resembles the glucose-alanine cycle?

Cori cycle

Why is glucose-alanine cycle more efficient than Cori?

Pyruvate not reduced to lactate

Which enzyme makes glutamine from glutamate?

Glutamine synthetase

Glutamine synthetase reaction?

Glutamate + NH4+ + ATP → glutamine

Energy source for glutamine synthetase?

ATP

Intermediate in the glutamine synthetase mechanism?

γ-Glutamyl phosphate

Why convert ammonia to glutamine?

Safe transport in blood

Which enzyme releases NH4+ from glutamine?

Glutaminase

Where is liver glutaminase located?

Mitochondria

Where is excess glutamine processed?

Intestines, kidneys, liver

Three forms of nitrogen entering the liver?

Alanine, glutamine, dietary amino acids

Which two enzymes generate free NH4+ in liver?

Glutaminase, glutamate dehydrogenase

Net result of liver nitrogen processing?

High mitochondrial NH4+

Where does the urea cycle occur?

Liver (mitochondria, cytoplasm)

Term for organisms that excrete urea?

Ureotelic

Term for organisms that excrete NH4+ directly?

Ammoniotelic

Term for organisms that excrete uric acid?

Uricotelic

Example of ammoniotelic organisms?

Aquatic animals

Example of uricotelic organisms?

Birds

Why do uricotelic animals excrete uric acid?

Requires little water

Which animal becomes ureotelic during drought?

Lungfish

At what two points does nitrogen enter the urea cycle?

Carbamoyl phosphate, aspartate

Source of urea's first nitrogen?

Free NH4+

Source of urea's second nitrogen?

Aspartate

Source of urea's carbon?

HCO3− (from CO2)

What are the immediate sources of urea's two nitrogens?

Carbamoyl phosphate, aspartate

What is the first reaction of the urea cycle?

Carbamoyl phosphate synthesis

Which enzyme synthesizes carbamoyl phosphate?

Carbamoyl phosphate synthetase I

What are CPS I's substrates?

NH4+, HCO3−, 2 ATP

How many ATP does CPS I consume?

Two

Why is carbamoyl phosphate synthesis irreversible?

Consumes two ATP

First intermediate in CPS I reaction?

Carboxyphosphate

Second intermediate in CPS I reaction?

Carbamic acid