17 Making other Biomolecules

Question 1

Tyrosine is the precursor of

Answer 1

1. Dopamine
2. Norepinephrine
3. Epinephrine

Question 2

Precursor of GABA

Answer 2

Glutamate

Question 3

Precursor of histamine

Answer 3

Histidine

Question 4

Precursor of serotonin

Answer 4

Tryptophan

Question 5

Describe the formation of heme

Answer 5

Glycine reacts with succinyl CoA to form delta-aminolevulinate
2 molecules of delta-aminolevulinate condense to form porphobilinogen
4 molecules of porphobilinogen combine to form protoporphyrin
Fe ion is inserted into protoporphyrin to form heme

Question 6

Causes and symptoms of porphyria/vampire disease

Answer 6

Mutations or misregulations of enzymes in the heme biosynthesis pathway -> accumulation of precursor uroporphyrinogen I in red blood cells, body fluids and liver

Urine becomes discolored
Teeth may show red fluorescence under UV
Skin is sensitive to UV
craving for heme

Question 7

Degredation of heme & excretion

Answer 7

Heme oxygenase converts heme to biliverdin
Biliverdin reductase converts biliverdin to bilirubin

Bilirubin is excredted through urine and bile
- degradation to urobilin -> major pigment of urine
- degradation to stercobilin by intestinal microbiota -> color of faeces

Question 8

Jaundice cause

Answer 8

• Impaired liver function (liver cancer, hepatitis)
• Blocked bile secretion (gallstones, pancreatic cancer)
• Insufficient glucouronyl bilirubin transferase to process bilirubin (in infants) -> treated with UV to cause photochemical breakdown of bilirubin

Question 9

Precursor of
- amino groups
- purines
- pyrimidines

Answer 9

• Glutamine
• Glycine
• Aspartate

Question 10

Where does biosynthesis of nucleotides occur?

Answer 10

mostly in the liver

Question 11

Origin of ring atoms in purines

Answer 11

N - aspartate, amide N of glutamine
C - formate, CO2
N-C=C - glycine

Question 12

How are purines and pyrimidines synthesized de novo?

Answer 12

Purines - built on ribose base
Pyrimidines - pyrimidine rings are first synthesized before being attached onto the ribose base

Question 13

Reaction steps of de novo synthesis of purinesPurine salvage pathways

Answer 13

• 5-phosphoribosyl 1-pyrophosphate (PRPP) react with glutamine
• Addition of 3 carbons from glycine
• Inosinate (IMP) with full purine ring formed
• AMP and GMP are synthesized from IMP
• AMP and GMP are phosphorylated to diphosphate then triphosphate (ATP and GTP)

Question 14

Purine can be salvaged from

Answer 14

free bases and nucleosides

Question 15

How does the pathway for AMP and GMP degradation merge?

Answer 15

Xanthine production -> oxidized into uric acid -> excreted by kidneys

Question 16

Cause, symptoms and treatments of gout

Answer 16

Cause - excess uric acid

Paintful joints due to deposits of sodium urate crystals

Treated with avoidance of purine-rich foods (seafood, liver) / allopurinol (xanthine oxidase inhibitor)

Question 17

Steps in de novo synthesis of pyrimidine nucleotides

Answer 17

• Aspartate and carbamoyl phosphate provide atoms for pyrimidine ring structure
• PRPP is attached to the pyrimidine ring (orotate)
• converted to uridylate (UMP)
• UMP phosphorylated to UTP -> converted to CTP

Question 18

How ribonucleotides are converted to deoxyribonucleotides?

Answer 18

• Ribose is reduced to deoxyribose (diphosphate level, NDP) (catalyzed by ribonucleotide reductase)
• Phosphorylated to triphosphate level (dNTP)
• dNTPs are used for DNA synthesis
• dUTP -> dTTP

Question 19

Degradation of pyrimidine bases

Answer 19

dephosphorylated -> nucleosides are cleaved to produce ribose-1-phosphate and free pyrimidine bases

• cytosine -> uracil -> CO2 + NH4 + beta-alanine
• thymine -> CO2 + NH4 + beta-aminosobutyrate