Breakdown & Excretion of N-Compounds

2. deamination amino acids TCA cycle NH3 pyrimidines alanine + NH3 + CO2 (cytosine, thymidine, uracil) purines uric acid allantoin/allantoic acid (adenine, guanine) urea NH3 + CO2 Breakdown & Excretion of N-Compounds

3. Excretory Products • NH3 (NH4+) - ammonotelic • diffuses rapidly & used for ion exchange • toxic • mainly in aquatic spp. • Urea - ureotelic - from purine degradation or • synthesized via Krebs ornithine cycle. • less toxic & used as organic osmolyte • highly soluble - requires water for excretion • Uric Acid - uricotelic • water conservative - sparingly soluble • crystallizes - excreted as paste

4. Ammonia-Excreting Animals (Ammonotelic) • Toxic – only small quantities stored in body • Requires considerable water to excrete • Gills typical site for excretion in aquatic animals • Isopods, some land snails, crabs---can excrete NH3 through volatization into air

5. Acid-Base Balance Intracellular (and therefore extracellular) pH must be regulated within narrow limits. In humans, arterial blood pH at core temp. = 7.4 Convulsions and death result from pH > 7.7 alkalosis - inc. in blood pH Coma and death result from pH < 6.8 acidosis - dec. in blood pH

6. H+ H+ + _ _ H+ + _ H+ _ _ _ + _ H+ + H+ Deleterious effects of changes in pH results from titration of charges on proteins: Alters the 2°, 3° and 4° structures of protein

7. Urea-Excreting Animals (Ureotelic) • More soluble, less toxic than NH3 • Better deal…get rid of 2 nitrogen molecules O H2N—C—NH2 • Formed by ornithine-urea cycle

9. Uric Acid-Secreting Animals (Uricotelic) • Major nitrogenous end-product in most terrestrial arthropods, reptiles, birds • Advantage – 4 nitrogen molecules, but requires energy to synthesize • Advantage – does not contribute plasma osmotic pressure

10. More on next slide