Urea Cycle+ Regulation

How does a high protein diet effect the urea cycle?

-Stimulates biosynthesis of all urea cycle enzymes

-amino acids stimulate glucagon release

-AA catabolism release ammonia and increase OA and a-KG for gluconeogenesis

How does high levels of glutamate and aspartame affect N-acetylglutamate (NAG)

Increase synthesis of NAG

What is the first committed step of the urea cycle and how is it regulated?

-CPS I

-Allosterically activated by N-acetyl glutamate

-When AA catabolism increases, glutamate and N-acetylglutamae increases

How is N-acetylglutamate made

-Acetyl CoA + glutamate are converted to NAG by N-acetylglutamate synthase

-Arginine is an activator of NAG Synthase

Which enzymes allow ammonia assimilation

• Glutamate Dehydrogenase

• Glutamine Synthetase

• Carbamoyl-Phosphate Synthetase-I

• Glycine Synthase

How do cells rid themselves of ammonia?

Reductive ammination + glutamine synthetase

What are normal levels of ammonia?

10 between 30 microM

Hyperammonemia in infants

80 micrometers on ammonia

Hyperammonemia in older children

55 micrometers of ammonia

Hyperammonemia in adults

30 micrometers of ammonia

name two Causes of Hyperammonemia

-Urea Cycle Disorders

-Deficiencies in NAGS

Describe Urea Cycle Disorder. What is the normal treatment

Deficiency in any urea cycle enzymes leading to impaired function and build up of ammonia. Treatment with aromatic acid benzoate or phenylbutyrate for excess ammonia

Describe deficiencies in NAGS. What is the treatment?

Affects CPS-1, can be treated with carbamoyl glutamate, which is an analog of NAG and can activate CPS1

Describe the Double Filter System of hepatocytes

Blood ends the liver at the outer edges and flows toward the central vein. Oxygen gets used up so you get a gradient where zone 1 is more oxygenated and zone 3 is least oxygenated, these zones differ in affinity for ammonia

Describe Zone 1 of the double filter system

Urea synthesis. Zone 1 hepatocytes have the urea cycle enzymes, but these enzymes have low affinity for ammonia (need high concentration to work well). Since Zone 1 gets blood first, straight from the gut/portal vein, ammonia concentration here is at its highest — perfect match for a low-affinity system. This handles the bulk of ammonia clearance.

Describe Zone 3 of the Double Filter System

Glutamine synthesis. By the time blood reaches Zone 3, most ammonia is gone, but not all. Zone 3 has glutamine synthetase, a high-affinity enzyme that can scavenge whatever low levels of ammonia slipped past the urea cycle. This is the "second filter" — a backup system for leftover NH₃ so none escapes into systemic circulation.

Differentiate between UCD and pyrimidine synthesis

UCD > problem with CPS 2 > build up of orotic acid and carbamoyl phosphate > hyperammonemia

build up of orotic acid > other clinical manifestations > problem with pyrimidine synthesis

What are the characteristics of Arginase deficiency? (UCD)

Ammonia concentration: Mild/moderate

Location: final step

Key metabolite: Arginine

Clinical pattern: Chronic, neurological

What are the characteristics of UCD disorder? (Location, ammonia conc., metabolite, clinical pattern)

Location: early, mitochondria

Ammonia: high concentrations

Key metabolite: orotate is very high (no conversion to citrulline)

clinical pattern: acute, neonatal

What are the two directions GDH (glutamate dehydrogenase) can catalyze, what cofactors does each use, and what is the biological purpose of each direction?

• Oxidative deamination (glutamate → α-ketoglutarate + NH₃): uses NAD⁺/NADP⁺ as oxidizing cofactor; releases ammonia from glutamate, feeding nitrogen disposal (urea cycle).

• Reductive amination (α-ketoglutarate + NH₃ → glutamate): uses NADH/NADPH as reducing cofactor; assimilates/traps free ammonia into glutamate, detoxifying it.
  GDH is notable for using either NAD⁺ or NADP⁺, unlike most enzymes.

Front: What are the two steps of the glutamine synthetase reaction, what is the net/sum reaction, and why is this reaction biologically important?

1. Glutamate + ATP → γ-glutamyl phosphate + ADP (activates glutamate)

2. γ-glutamyl phosphate + NH₄⁺ → glutamine + Pi + H⁺ (ammonia replaces phosphate)

Sum: glutamate + NH₄⁺ + ATP → glutamine + ADP + Pi + H⁺

Importance: Traps toxic free ammonia into glutamine, a safe nitrogen-carrier molecule, which shuttles ammonia through the blood to the liver. There, glutaminase releases the ammonia again, feeding it into CPS-I to start the urea cycle.

Why is glutamine so important

SOURCE OF NITROGEN IN NEARLY ALL BIOSYNTHETIC REACTIONS

In brain ammonia scavenging, what are the two products α-KG/glutamate can be converted into to remove nitrogen from the brain, which enzymes catalyze each, and why does glutamine carry more nitrogen out per molecule than alanine?

• GDH converts α-KG + NH₃ → glutamate (traps first ammonia)

• GS (glutamine synthetase) converts glutamate + NH₃ → glutamine (traps a second ammonia); glutamine is shipped out to the liver

• ALT converts glutamate + pyruvate ⇌ α-KG + alanine (transamination, no new ammonia trapped); alanine is also released from the brain, and α-KG is regenerated for reuse (TCA)

Glutamine carries 2 nitrogens (one from each amination step), while alanine carries only 1 (transferred from glutamate) — making glutamine a more nitrogen-dense export molecule. Increased ammonia drives more flux through both pathways.

What are glycogenic (gluconeogenic) amino acids and which ones are they?

Can be converted into TCA and gluconeogenesis materials

Alanine (Pyruvate)

Aspartate (Oxaloacetate)

Asparagine (Oxaloacetate)

Glutamate (a-ketoglutarate)

Glutamine (a-ketoglutarate)

What are ketogenic amino acids are which ones are they?

Amino acids that can be converted to ketone bodies

Leucine and Lysine

Alanine, Cysteine, Glycine, and Serine are potentially ketogenic

Which amino acids are both ketogenic and glycogenic

Isoleucine, phenylalanine, tyrosine, tryptophan, threonine