Nitrogen metabolism

18

Degradation of amino acids has 2 ways

Either via amino groups or carbon skeletons

NH3→ toxic- has to be excreted in a harmless form

Important precrusor in biosynthesis

When are amino acids degraded?

• When normal protein turnover are not needed for new protein synthesis

• Ingested amino acids exceed the body’s needs for protein synthesis

• Cellular proteins are used as fuel because carbohydrates are not available or during starvation or diabetes mellitus

Excretion of amino groups

Fish- excrete amino nitrogen as ammonia

Terrestrial animals - urea

Birds and reptiles- uric acid

Amino acids converted into the citric acid cycle

• Glutamate and glutamine - to alfa-ketoglutarate

• Alanine→ pyruvate

• Aspartate → OAA

• Can all be used in citric acid cycle

Uptake of amino acids from diet

mechanic degradation of food
- proteases - in stomach and intestine
- uptake over intestinal mucosa -> blood-> liver

Pepsinogen converted to Pepsin=protease-> degrade proteins→ smaller peptides

Low pH-> why? Not healthy for the tissue-> but
pepsin needs to be activated (optimal pH=2

Helps unfold globular proteins and bonds more accessible to enzymatic hydrolysis

When food enters the small intestine→ low pH trigger hormonal release of secretin in the blood→ stimulates pancreas to release bicarbonate→ neutralizes the gastric HCl in small intestine

Degradation of aa in the liver- transamination

• Removal of the alfa amino group from an aa to alfa keto glutarate→ transaminases/aminotransferases

• Transamination- alfa amino group is transferred to the alfa carbon atom of alfa keto glu→ amino group acceptor

• Requires PLP=pyridoxal phosphate → coenzyme of pyridoxine

• Glutamate =FINAL product=nitrogen carrier

Degradation of aa in the liver- oxidative deamination

• Removal of the amino group of glutamate releasing free ammonia NH3

• Glutamate from transamination in cytosol is transferred to mitochondria in liver (need to be converted to free ammonia)

• Cofactor NAD+/NADP+

• Reaction catalyzed by L-glutamate dehydrogenase

• Releases ammonia from glutamate and forms alfa ketoglutarate

Combined action of aminotransferase and glutamate dehydogenase is called

Transdeamination

Alfa keto glutarate is formed and can be used in citric acid cycle

Glutamine transports ammonia

• NH3 + glutamate → glutamine by the action of glutamine synthase

• 2 step process requiring ATP

• Detoxifies ammonia

• Excess glutamine→ breaks down to glutamate and ammonium NH4+→ by glutaminase

• Then ammonia → urea in liver

Alanine transports

• Muscles produce pyruvate, lactate and ammonia during anaerobic contracting muscles→ must go to liver

• Lactate + pyruvate→ incoorperated into glucose→ muscles

• Ammonia is converted to urea for excretion

• amino groups from muscle to liver

• Gluconeogenesis in liver

• Picture: glucose-alanine cycle

Urea cycle- excretion of N

• occur in both mitohondria and cytosol

• Urea is produced from ammonia in 5 steps

• In liver

• Uses 2 nitrogen sources: carbomoyl phosphate + aspartate

4 steps of urea cycle:

1. Ornithine + carbamoyl phosphate → citrulline (in mitochondria)

2. Citrulline → argininosuccinate (in cytosol; second nitrogen added from aspartate)

3. Argininosuccinate → arginine + fumarate

4. Arginine → urea + ornithine (ornithine re-enters mitochondria to continue the cycle)

Citric acid cycle and ureas cycle is linked

• Fumarate produced in argininosuccinase reaction→ intermediate for the citric acid cycle

Intermediates that can enter citric acid cycle

• degradation of aa carbon skeletons (alfa-keto acids)- to intermediates of the citric acid cycle→ gluconeogenesis + ketogenesis

• 6 major intermediates: pyruvate, acetyl CoA, alfa ketoglutarate, succinyl CoA, fumarate, OAA

• ketogenic=aa degraded to form ketone bodies

• glucogenic aa=degraded to form pyruvate, alfaketoglutarate, succinyl CoA, fumarate, OAA→ glucose and glycogen

Degradation of nucleotides into

• Degradation of nucleotides

  • purines→ uric acid

  • pyrimidines→ succinyl COA

Synthesis of aa and nucleotides

• By glutamine synthase

• Regulated allosteric and by adenylyation- nitrogen compounds can inhibit Gln synthase

• Reaction: Glu + ATP + NH4+→ Gln + ADP + Pi + H+

Protein P2- activates adenylyation of glutamine synthase

It activates AT that adds an adenylyl group (–AMP) to glutamine synthetase. This process is called adenylylation.

This modification regulates the activity of glutamine synthetase, which controls how cells use ammonia to synthesize amino acids

AA derived form intermediates in three pathways

All 20 standard amino acids in humans are synthesized from key intermediates of central metabolic pathways

Citric acid cycle- provides intermediates like alfaketoglutarate, OAA, fumarate

Glycolysis→ supplies intermediates such as 3-phosphoglycerate, pyruvate, phosphoenolpyryvate

Pentose phosphate pathway→ produces ribose-5-phosphate and more

Intermediates of the three pathways

*) aKG
pyruvate
3-phoshpoglycerate
PEP
Erythrose 4 -phosphate (PPP)
OAA
Ribose phosphate (PPP)

SYnthesis of nucleotides- 2 pathways

) salvage pathway -
using available riboses
and bases-recycles free bases or nucleosides derived from the breakdown of DNA and RNA.

*) de novo pathway->
synthesized from ribo 5
phosphate + CO2-R + NH3

From scratch-using small precursor molecules such as amino acids, carbon dioxide, and ribose-5-phosphate.