Biochem Exam 3 Quiz 2 - Nitrogen Catabolism

Nitrogen Metabolism Big Picture

Sources of Amino Acids

Intracellular proteolysis: Misfolded proteins and stuff

Digestion of Dietary Proteins

De novo synthesis

Essential Amino Acids

Trp, Val, Phe, Met, Thr, Leu, His, Iso, Lys

Intracellular protein turnover

Degradation of our own proteins

Regulated by tightly controlled processses to maintain protein homeostasis (Proteostasis)

Ensure that functional proteins are maintained at correct concentrations

THREE PATHWAYS
Lysosome

proteasome

autophagic

Degradation pathways

Autohpagy: Cytosolic components in bulk; Engulfs them

Lysosomal breaks down proteins after endocytosis of protein

Ubiquitination

Ubiquitin is tagged onto a protein to target proteins for degradation.

Adds ubiquitin to target protein using a 3-step protein complex

REQUIRES ATP

polyubiquinated protein can enter proteasome which is acidic to break down only the protein

Digestion: Controlled proteolysis

Protease activation cascade

Degraded through sequential proteolysis: Stomach first with pepsin initially denaturing and cleaving. Then small intestine for bulk digestion

Enzymes released as zymogens (inactive form of the enzyme which are activated when they enter the right environment i.e. stomach)

Zymogens

Zymogens prevent self digestion

Pepsinogen becomes pepsin at low pH. Pepsin activates more pepsinoens in a cascade

Tryspinogen becomes trypsin in intestine which activates other zymogens

Absorption of proteins

AAs and Di and Tripeptides are absorbed in small intestine across intestinal epithelial cells (enterocytes)

AAs brought into the enterocytes via transporters then inside the cell peptides are broken down to AAs which enter portal circulation and go to the liver

Amino acid transporters

AAs brought in through secondary transport with sodium. Na/K pump keeps the gradient good enough for the cotransport.

Into the blood is passive diffusion

AA oxidation

AAs undergo oxidative degradation when protein turnover isn’t needed for new proteins

Ingested AAs exceed the bodys need for protein

Carbs are unavailable or not properly utilizes

Keto Acids and Amino Acids

Pyruvate

Oxaloacetate

Alpha ketoglutarate

Keto acid is replacement of amine in amino group with carbonyl

Transamination

Removal of amino group from AA

Necessary step in AA breakdown

promoted by enzymes called aminotransferases or transaminases

Basically alpha amine is swapped with alpha keto acid basically swapping both from AA to Keto acid and vice versa

NO NET DEAMINATION BECAUSE AMMONIA IS TOXIC

Pyeidoxal Phosphate (PLP)

Vitamin B 6 derivaive

Group that accepts NH2 onto aldehyde to make pyridoxamine phosphate

Then alpha keto glutamate takes the NH2 group to become glutamate

ALT and AST aminotransferases

Alanine and

Which AAs can’t undergo transamination

Secondary amines which cant undergo transamination: Proline and hydroxy Proline

Lysine and threonine because their R would interact and form a toxic nonmetabolite

Glutamate dehydrogenase and deamination

Happens in mitochondria in liver

Major route for oxidative deamination

Regenerates alpha KG and provides ammonia to make urea

PRODUCES NADH AND NADPH

REVERSE IS SAME ENZYME JUST OPPOSITE

Coupling Reaction (Transdeamination)

No net deamination

Basically, aminotransferase makes glutamate which is reverted back to alpha kg to contnue through glutamate dehydrogenase

Regulation of Glutamate dehydrogenase

High energy (ATP GTP NADH) promotes protein synthesis to inhibit GDH

LOW energy (ADP GDP NAD free AA) promotes deamination and promotes GDH WANTS TO ENHANCE TRANS DEAMINATION TO MAKE TCA INTERMEDIATES

Glutaminase

A mitochondrial enzyme that catalyzes the breakdown of glutamine to form glutamate

All throughout body

Ammonia formed is consumed by urea cycle

Asparaginase

Breaks down asparagine

How is aspartate brought into cytosol for urea cycle

Aspartate is formed by Alpha KG and sent out through transporters to the cytosol

Malate also made by OAA and turned to fumarate for urea cycle

Urea cycle overview

Link between TCA and urea cycle through aspartate arginosuccinate shunt

Glutamine turned to glutamate through glutamate synthase

How is urea cycle activated

CPS-I (caraboyl phosphate synthetase 1) stimulates conversion of ammonia and co2 to carbamoyl phosphate

CO2 provided by HCO3-

Step 1 urea cycle

Formation of citrulline from carbamoyl phosphate

CATALYZZED BY OTC (ornithine)

Step 2 of urea cycle

Citrulline transported from mitochondria to cytosol where argininosuccinate synthease catalyzes its combination with arginine

Step 3 of urea cycle

Argininosuccinate is cleaved by argininosuccinate lyase to produce arginine and fumarase (fumarate leaves)

Step 4 of urea cycle

Arginine converted to ornithine, creating Urea through using water. Ornithine released back to the mitochondria to continue

Links between urea cycle and TCA

Alpha KG can be converted to aspartate which can create fumarate through argininosuccinate

Fumarate goes back and is integrated into the TCA by becoming Malate through malate synthase?

Regulation of Urea cycle and TCA balance

High protein diet induces urea cycle to dispose of excess nitrogen

GLUCAGON STIMULATES BIOSYNTHESIS OF ALL UREA CYCLE ENZYMES

AAs such as ALA, Gly, Met, and Cys are effective in stimulating glucagon release and trigger synth of NAG (N acetylglutamate)

N acetylglutamte NAG

ARGININE ACTIVATES NAG SYNTHASE

NAG acts as allosteric activator of CPS-I which creates carbamoyl phosphate which is necessary for urea cycle (creation of carbamoyl phosphate)

Ammonia production

Occurs in all tissues through regular metabolism

Glutamate and glutamine provide main method of incorporation of NH4+

Ammonia assimilation is catalyzed by

GDH

Glutamine synthetase

carbamoyl phosphate synthetase I

Glycine Synthase

Hyperammonemia

Abouve 30 micromolar in adults, 80 micromol in infants, 55 micro in adolescent

Can be caused by bad liver as well as excessive protein turnover

Hyperammonemia causes

Urea cycle disorders

Deficiencies in NAGS can be treated with carbamoyl glutamate which is an analog of NAG

LIVER DISEASE ALSO

Aromic acid benzoate or phenylbutyrate can successfully assimilate excess ammonia

Double filter system for urea/ammonia cleanup

Liver portal triad towards central vein

Periportal zone (1) perivenous zone (3)

1: enzymes have high capacity to clean up ammonia through cycle

3: Mostly glutamine synthetase to remove ammonia through high affinity (lower capacity than zone 1)

Happens because vessels are fenestrated

Ornithine transcarbamoylase deficiency

Can’t combine Carbamoyl phosphate and ornitine to make citrulline for urea cycle.

Most common urea issue and can be treated by decrease protein intake

Causes hyperammonemia and buildup of carbamoyl phosphate in liver which can backup into cytosol which can cause a buildup of errodic acid (errodic acidria)

Arginase Deficiency (A UCD)

Can’t form ornitine which is bad

NOT AS BAD AS OTC DEFFICIENCY

GDH Reductive amination

Picks up ammonia, forming glutamate from alpha keto glutamate

NADPH is needed (GDH) is present in whole body

Glutamine synthetase

Cytosolic enzyme which assimilates ammonia to form glutamine from glutamate (USES ATP)

PRESENT IN ALL DISTANT ORGANS AND SHUTTLES STUFF TO LIVER

Glutamine synthetase Regulation

Glutamine end products inhibit glutamine synthetase

Glutamine importance

Transports ammonia to liver to be picked up.

VERY IMPORTANT FOR BRAIN

Ammonia scavenging in the brain

Converts alpha KG to glutamate to glutamine. Which must be turned back to alpha KG in liver

Disposition of the carbon skeleton

Glycogenic AAs can be converted to intermediats in TCA cycle and gluconeogenesis (Ala, Asp, Asn, Glu, Gln)

Ketogenic AAs converted to ketone bodies (Leu, Lys, Ala, Cys, Gly, Ser)

BOTH: Ile, Phe, Tyr, Trp, Thr