Flashcards in Unit 6 - Pentose Phosphate Shunt and Gluconeogenesis Deck (20):
where are Shunt enzymes located? what are the 2 most important products?
all in the cytosol (like in glycolysis)
-NADPH and ribose-5-phosphate are most important
what are the 2 phases of the shunt? what are the distinguishing features and main products?
1. oxidative phase (first 3 steps, all irreversible)
-G6P + 2 NADP+ H2O --> 2 NADPH + 2 H+ + CO2
2. nonoxidative phase (next 5 steps, all reversible)
-Ru5P --> 2 F6P + 1 GAP
what is the overall gain and cost of the shunt? why is this so? (assume starting with 3 glucose)
gain 6 NADPH, cost 1 GAP; if starting with 18 C...
-glycolysis would make 6 GAP (18 C)
-oxidative phase gives 3 CO2 (3C) + 3 Ru5P (15 C) + 6 NADPH
--nonoxidative uses 3 Ru5P to make 1 R5P (5 C) + 2 Xu5P (10 C) to make 2 F6P (12 C) + 1 GAP (3 C)
compare and contrast NAD+ (nicotinamide adenine dinucleotide) and NADP+ ("" phosphate)
have same REDOX RXNs, but cannot be converted to each other
-C2 hydroxl O of ribose has H+ in NAD, and phosphoryl group in NADP+ so enzymes can distinguish
-pool of NADPH (primary reductant) is highly reduced state for detox and biosynthesis, but pool of NAD+ (primary oxidant) is in highly oxidized state to push glycolysis and TCA forward
where is NADPH used in the SI and liver?
used by cytochrome P450 (ER monooxygenases) to detox xenobiotics
what is the purpose of the pentose phosphate pathway in:
1. steroid synthesis
2. cholesterol and FA synthesis, plus detox xenobiotics
3. FA synthesis
4. steroid synthesis
5. FA synthesis
6. detox xenobiotics
7. detox ROS
what happens if you have G6P dehydrogenase deficiency?
most common human enzyme deficiency
-causes favism (hemolytic anemia, especially if take antimalarial drugs like quinine or chloroquine, or eat high oxidant foods)
-evolutionary advantage b/c it's better to pay metabolic cost of faster rate of turnover of RBCs than die of malaria
what is ribose used for? how is it obtained via the PPS?
info storage (RNA/DNA), E transfer (NTPs), REDOX (NADH/FAD), and enzyme catalysis (CoA)
-run nonoxidative phase in reverse so that F6P --> R5P (don't do oxidative phase, don't get NADPH)
what happens to the shunt if cells need both NADPH and ribose?
run oxidative phase of shunt, and convert all Ru5P to R5P
what percentage of metabolized glucose goes thru the PPS?
where is glucose 6 phosphatase?
liver and (to a small extent) kidney
why shouldn't you go cold-turkey into a low-carb diet?
the gluconeogenesis enzymes are only in moderate levels in the body, so you need to go slowly to build up higher levels (or else get hypoglycemic)
what are the 3 "bypasses" to circumvent irreversible glycolytic steps?
bypass I - pyruvate carboxylase + PEPCK (pyruvate --> PEP)
bypass II - FBPase (FBP --> F6P)
bypass III - glucose-6-phosphatase (G6P --> glucose)
what are the only AA's that cannot contribute to gluconeogenesis?
leucine and lysine
can ACoA from FA and AA contribute to gluconeogenesis?
where does gluconeogenesis begin, and why?
mitochondria, b/c that's where the citric acid cycle and pyruvate carboxylase is
-exception is glycerol, which gets converted to DHAP in cytosol
how does the malate/aspartate shuttle contribute to gluconeogenesis
no transport for OAA, but it can exit mitochondria by mal/asp shutting running in reverse
-cytosol: malate or aspartate --> OAA
-mitochondria: OAA --> malate or aspartate
-thru inner mitochondrial membrane: channels for both malate and aspartate
what is the cost of gluconeogenesis?
net loss of 4 ATP per round trip
-glucose --> pyruvate = gain 2 ATP
-pyruvate --> glucose = use 2 ATP + 2 GTP (bypass I), use 2 ATP at PGK = 6 ATP
what is the Cori cycle?
when ATP demands in muscle exceed capacity of oxidative phosphorylation for generating ATP; make lactate instead (also in RBC)
-lactate carried to liver, converted to glucose, released back into bloodstream
-continues after exercise so glycogen stores are replenished