Chapter 24 Flashcards
Used to generate only L - AAs
PLP transaminations
Source of all AA nitrogen
NH3
AA C backbone comes from
CAC, glycolysis, pentose phosphate
net nitrogen fixation rxn
16H20 + 8H+ + N2 + 8e- + 16 ATP»_space; 2NH3 + 16ADP + 16 Pi + H2
nitrogen fixation enzyme? Two subunits?
nitrogenase complex. Consists of reductase and nitrogenase.
reductase function
provides e- with reducing power. e- transfer to nitrogenase is coupled to ATP hydrolysis.
nitrogenase function
reduces N2 to NH3
nitrogenase complex become inactive by ?
O2
O2 in root nodules binds to ? to lower O2 concentration to keep nitrogenase complex active
leghemoglobin
8 high potential electrons for N2 reductiuon come from ?
ferredoxin
ATP required per electron transfer from reductase to nitrogenase
2 ATP
importance of ATP hydrolysis in reductase
lowers activation energy to make the rxn KINETICALLY feasible
reductase and nitrogenase are both this type of protein
Fe-S
electrons are transferred to this part of the nitrogenase
P-cluster
ATP binds to this part of reductase
P-loop
site of nitrogen fixation within nitrogenase
FeMo cofactor
FeMo cofactor is coordinated to ?
homocitrate
Acts as N donor for AA’s
glutamate
Acts as N donor for AA side chains
glutamine
glutamate dehydrogenase reaction catalyzed
a-KG + NH4+ + NADPH (or NADH) + H+»_space; L-glutamate + H2O + NADP+ (or NAD+)
glutamine synthase reaction catalyzed
L-glutamate + ATP + NH3»_space; glutamine + ADP + Pi
glutamate synthase reaction catalyzed
a-KG + glutamine + NADPH»_space; 2glutamate + NADP+
glutamate synthase function
to synthesize glutamate when NH4+ is scarce. Energetically expensive therefore only highly active at low NH4 concentrations. Low Km for NH4+.
glutamte dehydrogenase has a low/high Km, therefore it is only affective at low/high NH4+ concentration
high, high
Alanine is made from ?
pyruvate
serine is made from ? and is used to make ? and ?
3PG. Cysteine and glycine.
glutamate is used to make ? (3)
glutamine, proline, arginine
asparagine is made from ? which is made from ?
aspartate, OAA
histidine is made from ?
R5P
Tyrosine is made from ?
PEP and E4P
? AA’s must be consumed in diet
essential
negative nitrogen balance
caused by deficiency in as little as one AA. More protein is degraded than synthesized. More N is excreted than ingested.
complicated AA that is synthesized by humans
Arginine (synthesized in ten steps or in three steps from ornithine of the urea cycle)
PLP transamination dependent AA’s formed from alpha-keto acids OAA, pyruvate, and a-KG respectively
aspartate, alanine, glutamate
model aminotransferase
aspartate aminotransferase
conserved sequence among all amontransferaces
lysine residue and arginine residue
function of lysine residue in aminotransferase
forms schiff base with PLP cofactor
function of arginine residue in aminotransferase
interacts w/ a-carboxylate group of the ketoacid
How is only L-AA formed
in the protonation step of the quinonoid intermediate to form an external aldimine, the arginine residue orients the substrate so that the lysine residue adds a proton to the bottom face of the quinonoid substrate.
mechanism for synthesis of asparagine from asparate
aspartate is adenylated by ATP to form acyl-adenylate intermediate and PPi. NH3 (glutamine in mammals) is added to form asparagine and AMP.
Why do mammals use glutamine instead of NH3 in asparagine synthesis?
cell is not exposed to toxic NH4+
intermediates between glutamate and Arginine
glutamic gamma-semialdehyde, ornithine
intermediates between glutamate and Proline
glutamic gamma-semialdehyde, delta-pyrolline 5-carboxylate
intermediates between 3PG and serine
3-phosphohydroxy pyruvate, 3-phosphoserine
reaction catalyzed by serine hydroxymethyltransferase
serine + tetrahydrofolate»_space; glycine
significance of intermediate in synthesis of proline
nonenzymatic formation of schiff base
Formation of cysteine from serine uses S from ?
methionine/homocysteine
significance of THF
used in glycine synthesis. cannot synthesize in its entirety. must intake as folic acid in our diet. amphibolic. a one-carbon group carrier.
sulfa drugs
antibiotics that inhibit synthesis of THF
THF as an activated one-carbon carrier
group is bonded to its N10 or N5 or both. methyl group in most reduced form. intermediate form carrier methylene. most oxidized form carries formyl, formimo, or methenyl.
fully oxidized one carbon unit ?, is carried by ?
CO2, biotin
activated methyl donor is usually ? because THF transfer potential is too low
SAM
SAM is synthesized by ?
adenylation of methionine with ATP (uses all three Pi)
SAM loses a methyl group to form ?(2)
adenosine and homocysteine
methinine synthase catalyzed formation of methionine from ?. Uses ? as a catalyst.
homocysteine. B12
cysteine is synthesized from ? (2)
homocysteine and serine
two PLP enzymes used in cysteine synthesis
cystathione B-synthase, cystathionase
high homocysteine levels correlate with ?. Caused by mutations in ?.
vascular disease. cystathione b-synthase
betaine-homocysteine methyltransferase reaction catalyzed
betaine + homocysteine»_space; dimethylglycine + methionine
key intermediates in aromatic AA synthesis
shikimate and chorismate
shikimate is formed from ? (2)
PEP and E4P
chorismate is made from ?
shikimate
glyphosate significance
uncompetitive inhibitor of of chorismate synthesis
chorismate is the precursor for ? (2)
phenylalanine and tyrosine
key substrates in tryptophan synthesis
chorismate, glutamine, PRPP, Serine
tryptophan synthetase structure
a2B2. alpha subunit generates and channels indole. beta uses PLP to activate serine for attachment to indole.
inhibition in serine synthesis
3-PGDH is inhibited allosterically by serine. 4 serines inactivate completely.
a-ketobutyrate is deactivated by high ? conc and activated by ?
isoleucine, valine
3PGDH is structurally similar to ?
threonine deaminase. 4 alpha helices and 8 beta strands.
in E. coli there are three isozymes of ? with different regulatory properties
aspartokinase
glutamine synthase shows ? feedback inhibition from ? (5)
cumulative. Trp, His, carbamoyl-P, CTP, AMP
glutamine synthetase shows ? modification by attachment of ? making it less active
reversible covalent, AMP
adenylation of glutamine synthetase is controlled by ?
uridylylation
uridylylation
interconversion of Pa and Pd. Pa adenylates glutamine synthetase.
organisms capable of nitrogen fixation
bacteria and archaea
transamination swaps ? and ?
carbonyl oxygen and amino group
intermediate in synthesis of phenylalanine, tryptophan, and tyrosine
chorismate