Nitrogen IA %% (+- Flashcards
Nitrogen cycle
Note: nitrogen fixing bacteria are diazotrophs
Nitrogen fixation
- Nitrogen fixation requires enzyme nitrogenase (diazotrophs) and a LOT of energy
- Nitrogenase needs anaerobic conditions to function
Nitrogenase O2 inactivation
- Nitrogen fixing bacteria live anaerobically
- Some uncouple mitochondria – increases electron flow and “burns” off O2 in cell
- Some cyanobacteria (blue-green bacteria) form heterocysts (a differentiated cyanobacterial cell that carries out nitrogen fixation.) whose “cell wall” prevents O2 entry
- Leguminous plants produce leghemoglobin which binds to O2 and keeps the concentration low enough to allow nitrogenase to work
Nitrogen diagram
4 high concentration amino acids
- alanine
- aspartate
- glutamine
- glutamate
Plasma aminotransferases
- Aminotransferases are intracellular enzymes; presence in plasma indicates cell damage
- Aspartate aminotransferase (AST) - catalyses:
aspartate ⇒oxaloacetate
- Alanine aminotransferase (ALT)– catalyses:
alanine ⇒ pyruvate
Oxidative catabolism of amino acids condtions
–Leftover amino acids from normal protein turnover are degraded
–Dietary amino acids that exceed body’s protein synthesis needs are degraded
–Proteins in the body are broken down to supply amino acids for catabolism when carbohydrates are in short supply (starvation, diabetes mellitus),
Dietary proteins
- Pepsin cuts protein into peptides in the stomach
- Trypsin and chymotrypsin cut proteins and larger peptides into smaller peptides in the small intestine
- Aminopeptidase and carboxypeptidases A and B degrade peptides into amino acids in the small intestine
Amino acid digestion
- Digestion of dietary proteins in the intestine and degradation of proteins within cells provide a steady supply of amino acids
- Stomach – acidic environment + enzymes ⇒ free amino acids and di- or tri-peptides
- Intestine – membrane bound enzymes (aminopeptidases) degrade proteins further
Cellular proteins destruction
-Used to degrade:
- Misfolded proteins
- Foreign proteins
- Unwanted proteins
-Same end point as dietary proteins:
•Individual amino acids
Protein catabolism overview
Transamination
- no loss or gain of nitrogen (robbing Peter to pay Paul)
- Readily reversible
- One of the 2 substrate pairs is often glutamate
- Reaction is reversible so transaminases participate in amino acid synthesis AND degradation
Fates of Nitrogen in organisms
- Plants conserve almost all of their nitrogen
- Many aquatic vertebrates release ammonia to their environment
–Passive diffusion from epithelial cells
–Active transport via gills
•Many terrestrial vertebrates and sharks excrete nitrogen in the form of urea
–Urea is far less toxic than ammonia
–Urea has very high solubility
•Some animals, such as birds and reptiles excrete nitrogen as uric acid
–Uric acid is rather insoluble
–Excretion as paste allows to conserve water
•Humans and great apes excrete both urea (from amino acids) and uric acid (from purines)
Ammonia
- Ammonia is Transported in the Bloodstream Safely as Glutamine
- Excess glutamine is processed in intestines, kidneys and liver
Alanine formation
- Vigorously working muscles operate nearly anaerobically and rely on glycolysis for energy
- Glycolysis yields pyruvate that muscles cannot metabolize aerobically; if not eliminated lactic acid will build up
- This pyruvate can be converted to alanine for transport into liver
- Glutamate can Donate Ammonia to Pyruvate to Make Alanine
Glu + NH3 + Pyr ⇒ Ala
Glucose-alinine cycle
- Proteins are broken down in exercising muscle if required
- Transported to the liver as alanine (or glutamine)
- Carbon skeleton ⇒ pyruvate
- Nitrogen excreted as ammonia (ammonium ion) and converted to urea by the urea cycle
The Glutamate Dehydrogenase Reaction
- 2 electron oxidation of glutamate followed by hydrolysis
- Net process is oxidative deamination of glutamate
- Occurs in mitochondrial matrix in mammals
- Can use either NAD+ or NADP+ as electron acceptor
Summary diagram
Diagram
Amino acid catabolism diagram
Diagram
Diagram
Phenylketonuria (PKU)
•Absence/deficiency of Phenylalanine hydroxylase (PAH):
Phenylalanine ⇒ Tyrosine
- Autosomal recessive disorder
- Associated with increased phenylalanine (Phe) levels (toxic)
- Untreated individuals exhibit signs of impaired brain development
- Treatable condition – Reduced protein diet supplemented with Tyrosine
- Incidence in UK – 1 in 10,000 live births
- Neonatal screening programme (carried out on day 5)
PKU clinical features
- Normal at birth with near normal blood Phe levels
- Phe levels rise rapidly once feeding is established
- Days 3-4 may present with irritability and feeding difficulties
- If untreated, delayed mental development and neurological features are evident by 6 months of age
- Musty odour
