Lecture 2: Central Metabolism, PPP, and ED pathway - Exam 4 Flashcards
Who has the Pentose Phosphate Pathway? Why is PPP required?
It is highly conserved and found in all three domains.
PPP doesn’t produce reducing agents for ETC, but produces NADPH for biosynthetic reductions and aromatic AA precursors. It produces sugars required to produce DNA and RNA. No ATP is used or produced in the PPP.
Who uses the Enter-Duodoroff pathway (ED)? What is it primarily used for?
ED is unique to prokaryotes and widespread among G- bacteria.
It is primarily used for growth on gluconic acids or by strict aerobes incapable of carrying out Phase 1 of glycolysis.
What is the PPP?
An alternative branch off glycolysis that produces the pentose sugars that make up DNA and RNA, and for the 4-carbon molecules needed for aromatic amino acids.
What are the two phases of the PPP?
Oxidative phase and non-oxidative phase.
What are the two reducing agents? What do each of them do?
NADH and NADPH are both important reducing agents.
NADH: donates e- to ETC
NADPH: donates e- for biosynthesis (anabolic reactions).
What is the ratio of NAD+ to NADH?
What is the ratio of NADP+ to NADPH?
The ratio of NAD+ to NADH is high (more NAD+ than NADH)
The ratio of NADP+ to NADPH is low (more NADPH than NADP+)
What is NAD+ needed for?
What is NADPH needed for?
NAD+ is needed as an oxidizing agent (e- acceptor) for all sorts of catabolic reactions.
NADPH is needed as a reducing agent to donate electrons in anabolic reactions.
NADPH has a similar structure to what other molecule?
NADH, which is a high energy electron shuttle.
Describe NADPH. What is it often used in? What role does it play? Why is its structure important?
NADPH has an added phosphate group that is used in the cell to donate electrons, just like NADH.
NADPH is used in reactions that build molecules (e.g. Calvin cycle) and occurs in a high concentration in the cell, so that it is readily available for these types of reactions.
NADPH plays a vital role in the glutathione detoxification of reactive oxygen species that protects cells from oxidative stress.
What are the four main reasons that the PPP is important?
- Production of precursor (ribose-5-phosphate) for nucleic acids.
- Production of erythrose-4-phosphate (precursor of aromatic amino acids.)
- Production of NADPH, a major reducing molecule for anabolic reactions. (NADPH can also be generated during photosynthesis ore reverse electron flow in some bacteria, but the PPP is a major pathway)
- Catabolism of pentoses or nucleic acids as a carbon source (as with hemicellulose degradation for biofuels).
Describe the oxidative phase.
From Glucose-6-phosphate to Ribulose-5-phosphate.
-During this phase, NADPH is generated.
-There are three irreversible reactions in the oxidative phase:
First reaction by the glucose-6-phosphate dehydrogenase is the rate-limiting enzyme of the PPP. Two oxidations lead to synthesis of two NADPH, which is needed for many anabolic reactions.
Glucose-6-phosphate dehydrogenase is the rate-limiting enzyme of the PPP. What does it catalyze?
The first reaction, Glucose-6-phosphate to 6-phosphogluconolactone.
The regulation of glucose-6-phosphate dehydrogenase has consequences. What are they? What is this enzyme activated by?
The regulation of glucose-6-phosphate dehydrogenase has downstream consequences for the activity of the rest of the PPP.
It is activated by its substrate, glucose-6-phosphate, and low levels of NADPH.
What happens in the non-oxidative phase of PPP? What are the starting molecules? How many reactions are reversible?
The ribulose-5-phosphate is converted to ribose-5-phosphate, which may be needed for nucleotide biosynthesis.
If not, the excess ribose-5-phosphate can be converted into other molecules that can be used by the cell:
as a precursor for amino acids (4-carbon molecule) or used in glycolysis (3-carbon or 6-carbon molecules) for energy and/or other metabolic precursors.
All of the reactions are reversible (4).
The starting molecules are ribose-5-phosphate or xylulose-5-phosphate.
Fructose-6-phosphate and glyceraldehyde-3-phosphate go to glycolysis.
What determines if ribulose-5-phosphate (from the oxidative phase) is converted to to ribose-5-phosphate or xylulose-5-phosphate?
It is all depending on what the cell needs. It is very modular. If the cell needs nucleotides, then it will convert it to ribose-5-phosphate. If it needs glyceraldehyde-3-phosphate and fructose-6-phosphate to go to glycolysis, then it will convert it to xylulose-5-phosphate.
