Chapter 17
1) Ammonia is the most assimilatable form of a nitrogen
2) N2 gas is the most abundant form of nitrogen.
3) N2 gas can be fixed to assimilatable forms by:
a. N2 gas reacts with UV light and lightning in the upper atmosphere to form
NO2 (Nitrite) and NO3 (Nitrate). Delivered to earth in rain. (15% of total
fixed nitrogen)
b. N2 gas can be converted to NH3 (ammonia) by a chemical method
(requires high temperature and pressure) (25% of total fixed nitrogen)
c. N2 gas can be converted to NH3 (ammonia) in a reaction catalyzed by the
enzyme nitrogenase. (60% of total fixed nitrogen)
4) Nitrogen cycle – know how different forms of nitrogen get to assimilatable form.
(i.e. Nitrate  Nitrite  ammonia  organic compounds.
5) Nitrogenase
a. know what types of organisms have this enzyme.
b. Nitrogenase has 2 subunits – Fe-protein (reductase) and MoFe-protein
(nitrogenase)
c. Requires 8 electrons to reduce N2 gas to ammonia.
d. 16 ATPs are needed.
e. Nitrogenase is labile to oxygen.
6) Assimilation of ammonia. Know the reactions catalyzed by the enzymes below.
Know structure of substrates and products. Know co-enzymes and co-substrates
involved.
a. glutamate dehydrogenase
b. glutamine synthetase
c. glutamate synthase
7) Know how glutamine synthetase is regulated in bacteria (Feedback inhibition,
Covalent modification (interconverts between inactive and active forms), Know
the allosteric inhibitors of this enzyme.
8) Know the alosteric inhibitors and activators of the mammalian glutamine
synthetase.
9) Know the general aminotransferase (transamination) reaction (i.e. donation of
ammonia from glutamate to alpha-keto acid acceptor). Know pyridoxal phosphate
is involved.
10) Know the essential and non-essential amino acids.
11) Know how dietary proteins are degraded to amino acids. Know in which tissue
type different processes occur.
12) Know that cellular proteins have different half lives. Know how this relates to
there function.
13) Know the two cellular protein degradation pathways (lysosomal and ubiquitin
mediated).
14) Ubiquitin mediate protein degradation
a. ubiquitin is a protein that is covalently linked via an amide linkage to
lysine residues on proteins.
b. Ubiquitinated proteins are tagged for destruction
c. Three proteins involved in ubiquitination. E1(ubiquitin activating protein),
E2 (unbiquitin conjugating protein), and E3 (ubiquitin-protein ligase).
Know the functions and reactions of these proteins.
d. Know the amino acid sequence signals (PEST sequences and gln, glu, asn,
asp on amino terminal end) on proteins that are recognized by the E3
protein for ubiquitination.
15) 26S proteosome
a. Proteolytic complex that recognizes ubiquitinated proteins and degrades
them to amino acids.
b. 2 major components:
i)
20S proteosome core – contains proteolytic function
ii)
19S cap – recognizes ubiquitinated proteins.
c. ATP dependent process. ATP hydrolysis required for protein binding to
proteosome.
d. Ubiquitin is cleaved from target protein and recycled to tag other proteins.
e. Target protein is unfolded as it enters the 20S proteosome core complex
16) Catabolism of amino acids (Fate of amino acids from degraded proteins)
a. Amino acids and proteins are not stored in animals as an energy source or
store for Nitrogen and carbon.
b. If not immediately reused in the synthesis of new protein, amino acids are
degraded
c. Two steps in amino acid catabolism
i)
Deamination
ii)
Toxic ammonia must be transported to liver where it enters
the urea cycle
iii)
catabolism of carbon skeleton
17) Ammonia is toxic, know the mechanisms that birds and reptiles, fish, and
mammals us to remove ammonia.
18) Know how ammonia is removed from amino acid backbones (deamination)Two
step process
a. transamination to a-ketogluturate
b. oxidative deamination (requires NAD+)
19) Urea Cycle
a. Know reactions, name of compounds, enzymes
b. Know which reactions occur in which subcellular organelle
c. Know where the nitrogens in urea came from (aspartate, ammonia)
d. Know where carbon atom in urea came from (bicarbonate)
e. Know that the urea cycle only occurs in the liver.
20) Glucose-Alanine Cycle
a. Know what it does.
b. Know the intermediates involved in this process.
c. Be able to draw a diagram depicting the glucose-alanine cycle if asked.