1. Branched pathways require sophisticated regulation; 1) Feedback inhibiton and activation by Threonine deaminase
Regulation of Threonine deaminase: Thr is converted into alpha-ketobutyrate in the committed step leading to synthesis of Ile.
Inhibited by Ile, activated by Val. The product of a parallel pathway

2. Similarity of regulatory domain suggests that feedback-inhibition processes may have evolved by the linkage of specific regulatory domains to the catalytic domains of biosynthetic enzymes.
Both have 4 alpha helices and 8 beta strand in similar location. This a.a. binding regulatory domain is present in other enzymes as well.

3. Domain structure of 3 aspartokinase: Each catalyzes the committed step in biosynthesis of Met (top); Thr(middle); Lys (bottom).
They have a common catalytic domain, but differ in regulatory domains.

4. Branched pathways require sophisticated regulation; 2) enzyme multiplicity
The committed step can be catalyzed by 2+ enzymes with different regulatory properties.

5. 3) Cumulative feedback inhibition:
In cumulative inhibition, each inhibitor can reduce the activity of the enzyme, even when other inhibitors are bound at saturating levels.

6. The Activity of Glutamine Synthetase is Modulated by an Enzymatic Cascade
Reversible covalent modification by AMP unit.
This adenylated enzyme is more susceptible to cumulative feedback inhibition than is the deadenylated form.
A specific Tyr in each subunit in Glm synthetase is modified by adenylation.

7. Adenylation of Tyr is catalyzed by a complex of AT and a regulatory protein, PA
Deadenylated when complexed with PD

8. Higher level in regulatory cascade of Glm synthetase: interconvertible PA and PD . PA is converted into PD by uridylation, reversed by hydrolysis.
The enzymes catalyzing these reactions are regulated by concentrations of metabolic intermediates.

9. enzymatic cascade: Amplifies signals, potential for allosteric control is markedly increased when each enzyme in the cascade is an independent target for regulation.

10. Amino Acids are Precursors of Many Biomolecules
Purines.
Pyrimidines.
Sphingosine.
Histamine.
Thyroxine.
Epinephrine.
Melanin.
Serotonin.
Nicotinamide ring.

11. Selected biomolecules derived from amino acids: The atom contributed by amino acids are shown in blue.

12. Glutathione, a gamma–Glutamyl Peptide, Serves as a Sulfhydryl Buffer and an Antioxidant
Glutathione: tripeptide consists of Cys flanked Gly and Glu that is linked to Cys by an isopeptide bond between Glu’s carboxyl group and Cys’s amino group

13. 2GSH + RO-OH ====== GSSG + H2O + ROH
Glutathione, present at high levels (~5mM) in animal cells, serves as a sulfhydryl buffer.
It cycles between GSH and oxidized GSSG
GSH/GOOG in most cells: >500
Glutathione plays a key role in detoxification by reacting with hydrogen peroxide and organic peroxides, the harmful byproducts of aerobic life.
2GSH + RO-OH ====== GSSG + H2O + ROH
Glutathione reductase, a flavoprotein that uses NADPH as e source

14. Glutathione peroxidase: role in peroxide detoxification, contains selenocysteine in active site

15. catalytic cycle of Glutathione peroxidase
The selenolate form of this residue reduces peroxide substrate to an alcohol and in turn oxidized to selenenic acid. 2nd glutathione regenerates the active form of enzyme by attacking selenosulfide to form oxidized glutathione.

16. Nitric Oxide (NO), a Short-Lived Signal Molecule, is Formed from Arginine
Nitric oxide (NO) by Nitric oxide synthase (NOS).
NO is well suited to serving as a transient signal molecule within cells and between adjacent cells.
Formation of Nitric oxide (NO): NO is generated by the oxidation of Arg.

17. Porphyrins in Mammals are Synthesized from Glycine and Succinyl Coenzyme A
The first step in the biosynthesis of porphyrins in mammals is the condensation of glycine and succinyl CoA to form delta-aminolevulinate.
catalyzed by delta-aminolevulinate synthase.

18. Heme labeling: The origin of atom in heme revealed by results of isotopic labeling.

19. Heme biosynthesis: The pathway for the formation of heme starts with 8 of delta-aminolevulinate synthase.

20. Heme degradation: The formation of heme-degradation products, biliverdin and bilirubin is responsible for the color of bruises.
UDP-glucuronate, derived from the oxidation of UDP-glucose, is the activated intermediate in the synthesis of bilirubin diglucuronide.
Bilirubin is a very effective antioxidant; The end product of a degradative pathway may be selected in evolution to exert a beneficial action.