CH 15: nitrogen metabolism: Degradation BIG PICTURE INFO

What is nitrogen degradation?

ecycling organic nitrogen into a variety of metabolites before conversion back to inorganic forms (ammonia, nitrate, nitrite).

Which organisms convert organic nitrogen into ammonia, nitrate, and nitrite?

Soil- and water-inhabiting organisms degrade the nitrogen of dead organisms into ammonia and oxidized nitrogen compounds.

What is denitrification?

Conversion of nitrate back to atmospheric nitrogen by specific organisms.

How is ammonia handled in aquatic animals vs. mammals/reptiles/birds?

Aquatic animals excrete soluble ammonia directly; mammals convert ammonia to urea; birds/reptiles/insects convert it to uric acid.

What is the first step in amino acid catabolism?

Removal of the amino group (deamination).

What happens to amino groups removed during amino acid catabolism?

They are disposed of via urea synthesis.

What happens to carbon skeletons after deamination?

They form Acetyl-CoA, acetoacetyl-CoA, pyruvate, fumarate, α-ketoglutarate, succinyl-CoA, or oxaloacetate.

What does “ketogenic” mean in amino acid metabolism?

Amino acids degraded to Acetyl-CoA or acetoacetyl-CoA that can form ketone bodies or fatty acids.

What does “glucogenic” mean?

Amino acids whose carbon skeletons produce pyruvate or TCA intermediates used in gluconeogenesis.

What are the two types of deamination reactions?

Transamination and oxidative deamination.

What is the purpose of deamination reactions?

To reduce excess amino acids and provide amino groups for urea synthesis.

Biochemistry Chapter 15 Slides

Which amino acid carries excess muscle amino groups to the liver?

Alanine (via the alanine cycle).

How is glutamine formed?

From glutamate + NH₄⁺ + ATP catalyzed by glutamine synthetase.

What enzyme hydrolyzes glutamine to release NH₄⁺?

Glutaminase

What enzyme oxidatively deaminates glutamate?

Glutamate dehydrogenase (produces α-ketoglutarate + NADH + NH₄⁺)

Name additional sources of ammonia besides glutamate dehydrogenase.

L-amino acid oxidases, serine/threonine dehydratases, bacterial urease.

What is the major nitrogen disposal pathway in ureotelic organisms?

The urea cycle (Krebs–Henseleit cycle)

Where does urea synthesis occur?

In hepatocytes (mitochondria + cytoplasm).

What reaction initiates the urea cycle?

NH₄⁺ + HCO₃⁻ → carbamoyl phosphate by carbamoyl phosphate synthetase I (requires 2 ATP).

What does ornithine react with to form citrulline?

Carbamoyl phosphate via ornithine transcarbamoylase

Where is citrulline transported after its formation?

From the mitochondrial matrix to the cytoplasm

What are the final products of the urea cycle?

Urea + regenerated ornithine.

How does urea leave the body?

Released into blood → filtered by kidneys → excreted in urine.

How is the urea cycle regulated?

By substrate availability and by altering enzyme levels in response to dietary protein and hormones (glucagon, glucocorticoids).

How are amino acids grouped for carbon skeleton catabolism?

By end products: acetyl-CoA, acetoacetyl-CoA, pyruvate, and TCA cycle intermediates.

Why must neurotransmitters be degraded rapidly?

To maintain precision in synaptic information transfer.

Which enzyme inactivates catecholamines (dopamine, norepinephrine, epinephrine) by oxidation?

Monoamine oxidase (MAO).

What other enzyme methylates catecholamines for inactivation?

Catechol-O-methyltransferase (COMT).

What are the steps of serotonin degradation?

Oxidation by MAO → oxidation by aldehyde dehydrogenase → 5-hydroxyindole-3-acetate.