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Other metabolic pathways
Pentose phosphate pathway (phosphogluconate pathway)
Produces NADPH and ribose 5-phosphate
Glc 6-phosphate + 2NADP+ + H2O  ribose 5-phos + CO2 + 2NADPH + 2H+
NADPH - used in fatty acid and cholesterol synthesis (found most in
mammary gland, adrenal cortex, liver and adipose tissue)
Ribose 5-phosphate - used to synthesize nucleic acids (occurs at high
rates in growing and regenerating tissues and in tumors)
Other metabolic pathways
Amino Acid Metabolism - Breakdown
Needed to produce amino groups because we cannot use N2 in air for
most biological processes
Other metabolic pathways
Amino Acid Metabolism - Breakdown
Injested protein
Other metabolic pathways
Amino Acid Metabolism - Breakdown
Other metabolic pathways
Nitrogen Excretion & Urea Cycle
Ammonia is toxic, if not used for synthesis of new AAs or other nitrogenous
products - excreted!
Ammonium deposited in mitochondria of hepatocytes is converted to urea in the
urea cycle
Other metabolic pathways
Urea Cycle - regulation
1. By flux of nitrogen through cycle - depends on diet
lots protein in diet = carbon skeletons used for fuel, lots of urea
starvation = breakdown muscle protein for energy, lots of urea
All enzymes (CPS-I and 4 in cycle) synthesized at higher rates in
starving animals and animals on high protein diets
2. Carbomyl phosphate synthetase I allosterically activated by product
from another pathway (N-acetylglutamate)
Other metabolic pathways
Urea Cycle - genetic defects
Patient cannot tolerate protein-rich diet, need small amounts of protein
Humans cannot live on a protein-free diet though
We are incapable of synthesizing half of the 20 AA, these are essential
AAs that must be provided in diet
must be
provided in
diet
Bacteria & plants can synthesize all 20 AAs
Other metabolic pathways
Amino Acid Metabolism - Breakdown pathways
Other metabolic pathways
Amino Acid Metabolism - Breakdown pathways
Coenzymes are important to AA catabolism
Tetrahydrofolate - transfers 1-carbon units
S-Adenosylmethionine - transfers methyl groups
Other metabolic pathways
Amino Acid Metabolism - Breakdown pathways
Glucogenic AAs (red) - degraded to pyruvate, -ketoglutarate,
succinyl-CoA, fumarate, oxaloacetate
Go on to be converted into glc & glycogen
Ketogenic AAs (blue) - degraded to acetoacetyl-CoA, acetyl-CoA
Go on to yield ketone bodies in liver
Some AAs are both - Trp, Phe, Tyr, Ile
Other metabolic pathways
Amino Acid Metabolism - Breakdown pathways
Genetic defects affecting AA catabolism
Other metabolic pathways
Amino Acid Metabolism Synthesis of all 20 AAs in bacteria or plants
Other metabolic pathways
Amino Acid Metabolism
Gly is a precursor of porphyrins
Porphyrins are part of heme, cytochromes
Gly, Met, Arg - synthesis of creatine
creatine = energy buffer in skeletal muscle
Gly, Glu, Cys - synthesis of glutathione
Glutathione = redox buffer
Tyr - synthesis of catecholamines (correlate with changes in BP)
dopamine (Parkinson’s), epinephrine, norepinephrine
Glu - synthesis of neurotransmitter
GABA (epilepsy)
His - synthesis of histamine
vasodialator, allergic response, stimulates acid secretion in stomach
Tagamet is an antagonist of histamine
Met, Ornithine - synthesis of spermine (+4) & spermidine (+3)
packaging of DNA