1. Protein Catabolism
?Can you give me some examples of what chemicals you think youve used, or how you think chemistry may have impacted your life?

2. Overview of Protein Catabolism
C/B
75-80% reused
30-40 g/day lost (5-7 g N assuming 16% N)
Proteases are zymogens
Excess amino acids are not stored
Normal protein turnover 1-2% of body protein per day
~75-80% amino acids reused; lose g/day
Not excreted as amino acids or ammonia

3. Methods of Nitrogen Excretion
Uricotelic (birds, reptiles)
Ammonotelic (fish)
Ureotelic (mammals)

4. Two Amino Acids are Used to Transport Nitrogen
Transaminase
Nonspecific for one pair, specific for the other pair
Pyr-Ala
aKG-Glu
are the two most common
Gln synthetase particularly important to the brain

5. Transamination Mechanism

6. Urea Cycle Importance means of excreting nitrogen in nontoxic form
Location
Mitochondrial matrix and cytoplasm of liver and kidney
Reactions
Energetics
Regulation

7. Intracellular Ammonium Generation
Glutaminase mitochondria only
Liver quantitatively more important
Oxidative deamination of Glu Only
Glu DH can use NAD or NADP

8. Carbamoyl Phosphate Synthetase I
Liver Mitochondrial enzyme
RDS of urea cycle
Activated by (requires) NAcGlu

9. Ornithine Transcarbamoylase
Mitochondrial enzyme
Transported to Cp after synthesis

10. Arginosuccinate Synthetase
Cytoplasmic enzyme

11. Arginosuccinase Enzyme restricted to liver and kidney
Fumarate converted back to Asp

12. Arginase Primarily liver enzyme
Ornithine transported back into mitochondrion by ornithine-citrulline antiporter

13. Regulation GluNAc as described earlier occurs by changing ATP affinity
10-20–fold change in enzyme levels based on “nitrogen balance” i.e. how much protein is consumed relative to needs

14. Energetics False Claim #27 of Vegetarians
“consuming protein uses energy”
4 ATP equivalents consumed in the urea cycle
However, the carbon backbone is now available for energy use
Carnivores obtain large amount of energy from amino acid carbon, herbivores ~10-15%

15. Carbon Backbone Catabolism
Glucogenic amino acids vs. ketogenic amino acids
Glucogenic are converted to metabolite of glycolysis (e.g., pyruvate) and can be converted into glucose
Ketogenic form molecules such as acetoacetate, which can be converted to fat
Most amino acids are glucogenic and ketogenic
Leu is the sole amino acid which is ketogenic

16. Ala

17. Gln, Glu

18. Asn, Asp

19. Thr, Ser, Gly

20. Pro

21. Met

22. Arg

23. Phe

24. Phenylketonuria (PKU) is a Congenital Absence of Phe Hydroxylase

25. Leu

26. Maple Syrup Urine Disease
The result of branched chain amino acid dehydrogenase deficiency
Oxidation products of keto compounds accumulate

27. Histidine

28. Cysteine Has Two Catabolic Pathways