Phase 2 Metabolism Flashcards
phase 2 reactions are also known as ____ reactions
conjugation reactions
what is the most dominant conjugation pathway?
why?
glucuronic acid conjugation (glucuronidation)
this is because it uses glucose which is extremely abundant, and it has broad substrate specificity
glucuronic acid is very water soluble and would make all conjugates sufficiently water soluble for elimination
what is the starting material for glucuronidation?
alpha Glucose-1-Phosphate
the starting material for glucuronic acid conjugation is alpha glucose-1-phosphate.
what does this react with?
what does it produce?
a-G-1-P reacts with UTP (uridine triphosphate) to produce UDPG (urine diphosphate glucose) + PP (pyrophosphate)
true or false
activated carbon can often serve as a polar functional group to undergo glucuronidation
true
give the entire formula for the glucuronic acid conjugation pathway
a-G-1-P + UTP -> UDPG + PP
UDPG + 2NAD -> UDPGA + 2NADH (UDPG dehydrogenase)
UDPGA + ROH (substrate) -> B glucuronic acid + UDP (UDP glucuronyl transferase)
what is the conjugating enzyme in glucuronidation
UDP glucuronyl transferase
what is the linkage of UDPGA?
is this always the case?
ALWAYS alpha linkage
pointing ___ is alpha and pointing ___ is beta
down = alpha
up = beta
what is the activated conjugating group in glucuronidation?
what does it do?
UDPGA (uridine diphosphate glucuronic acid)
reacts with substrate. has substrate as part of its structure
____ is connected to ____ via alpha linkage
what molecule is this?
glucuronic acid is connected to diphosphate via alpha linkage
this is the activated conjugating group (UDPGA)
what enzyme produces UDPGA?
UDPG dehydrogenase
does the substrate attack UDPGA or vice versa? explain
substrate attacks UDPGA
ALWAYS FROM THE TOP TO GIVE BETA LINKAGE EVERY TIME
What cofactor is used in glucuronidation?
what is it used to for?
2NAD are used to oxidize glucose. UDPG -> UDPGA via UDPG dehydrogenase
2NAD reduced to 2NADH
what is the activated conjugating group in sulfation?
PAPS
activated sulfate.
3’-phosphoadenosine 5’-phospho sulfate
given the substrate as “ROH”,
what happens to it after sulfation?
becomes ROSO3
what supplies sulfate in sulfation?
PAPS
explain the substrate specificity of sulfation
typical substrate is phenol.
very narrow specificity
explain WHY the specificity of sulfation is very limited
for glucuronidation, glucose is constantly being supplied through carbs in our diet
however, sulfate isn’t usually stored in the body
explain in detail how PAPS is formed
inorganic sulfate (SO42-) combines with ATP to form APS (adenosine-5’-phosphosulfate) and PP (pyrophosphate). catalyzed by ATP-Sulfate Adenyl Transferase
APS combines with another ATP to form PAPS + ADP.
catalyzed by ATP Kinase
full reaction = 2ATP + SO42- ->
PAPS + ADP + PP
what does ATP kinase do in the phase 2 metabolic reactions?
in sulfation, ATP kinase adds phosphorus to the 3’ position of APS to form PAPS
give an example of amino acid conjugation
glutamine conjugation
in glutamine conjugation, what is used as the conjugating molecule?
glutamine
what can you say about the substrate specificity for amino acid conjugation?
rather limited.
limited to mainly aryl carboxylic acids and alkyl carboxylic acids
explain how amino acid conjugation is different from the previous 2 (glucuronidation and sulfation)
the conjugating molecule (glutamine) just sits and waits while the substrate is being activated
in previous cases the conjugated group is activated and the substrate waits
explain the mechanism of glutamine/amino acid conjugation
substrate + ATP -> substrate -O-AMP (AMP substrate ester)
AMP substrate ester reacts with CoA (has SH on its tail) to give ACYL COA + AMP that comes off
acyl coA reacts with the conjugating group (glutamine) which has been sitting and waiting which causes CoA-SH to come off
the biological activity of histamine is terminated by____
methylation
what are the typical substrates for GSH conjugation?
electron deficient (electrophilic), chemically reactive species that want to combine with a nucleophile
cysteine contains a THIOL (SH) functional group which is the MOST NUCLEOPHILIC GROUP at physiological pH
Explain the mechanism of GSH conjugation
substrate (Benzyl Chloride in this case) reacts with GSH in the presence of GLUTATHIONE TRANSFERASE
the benzylic carbon is electrophilic and thus GSH attacks it and kicks chlorine out (good leaving group) and the initial conjugate is formed.
gamma glutamic acid comes off of this initial metabolite in the presence of Glutathionase, to give a cysteine-glycine dipeptide
with Peptidase (cysteinylglycianse), glycine comes off and the remaining molecule is the parent benzylic molecule with cysteine attached.
N-acetyl transferase incorporates an acetyl group to this molecule to give
BENZYLMERACPTURIC ACID — an acetylated cysteinyl conjugate is the end metabolite
another name for glutathionase.
what does it do?
Y-glutamyl transpeptidase
removes Y-glutamic acid from the initial metabolite in GSH conjugation
another name for peptidase
what does it do?
cysteinylglycinase
removes glycine from the cysteine-glycine dipeptide in GSH conjugation
is nitro electron withdrawing or donating?
NO2
electron withdrawing
are the halogens electron withdrawing or donating
withdrawing
aromatic ring with an EWG – is it reactive enough for GSH conjugation?
no, but if you happen to have a halogen (Cl,Br, or I) with 2 extra EWGs, it is most likely reactive enough
could displace the bromine in GSH
What is the activated conjugating group in methylation?
SAM
S-adenosylmethionine
state the conjugation reaction for methylation
(only the conjugating reaction”
SAM + RZH (Z could be OH, NH, OR S) —> RZ-CH3 + S-adenosylhomocysteine
enzyme = methyl transferase
what is the substrate for methylation?
RZH
Z = OH/NH/S
so basically either alcohol, amine, or thiol
explain the ENTIRE PROCESS of methylation
step 1 – activate SAM (the conjugating group)
methionine is added to adenosine via METHIONINE ADENOSYL TRANSFERASE with the help of ATP. this produces SAM (S-adenosylmethionine)
SAM reacts with the substrate RZH to produce RZ-CH3 (substrate is methylated) via METHYL TRANSFERASE
SAM is now S-Adenosylhomocysteine
which conjugation is more popular in our biological system (the body) rather than in the metabolism of ingested drugs?
methylation
the methylation of norepinephrine especially at the meta OH terminates the biological activity of norepinephrine
explain the methylation of norepinephrine
SAM + norepinephrine —>
3-Methoxynorepinephrine
via COMT (catecholamine-o- methyl transferase) remember – biological activity especially terminated at meta position
if via PNMT (phenylethanolamine), the methyl group on SAM gets transferred to the NH on norepinephrine to produce epinephrine
explain the specificity of acetylation
VERY LIMITED SPECIFICITY
typical substrate is a primary amine
R-NH2 and the NH2 MUST be the end terminal
for __________phase 2 reactions, there is no enhancement of water solubility. The purpose is to terminate biological activitiy
methylation and acetylation and GSH conjugation
what is the conjugating group activated by in acetylation?
acetyl CoA
what is the catalyzing enzyme in acetylation?
acetyl transferase
give an example of a specific drug that can undergo acetylation
isoniazid - ends in NH2
what is the major metabolite of isoniazid?
acetyl isoniazide
-substrate for acetylation
is acetyl isoniazide bioactive?
NO
it undergoes acetylation which terminates biological activity WITHOUT ENHANCING WATER SOLUBILITY
What does UDPGA stand for?
how is it produced?
uridine disphosphate glucuronic acid
produced via UDPG dehydrogenase + 2NAD+
