1st phase of the PPP
oxidative generation of NADPH (2 NADP+ yields 2 NAPDH)
2nd phase of the PPP
nonoxidative interconversion of a variety of phosphorylated 3-, 4-, 5-, 6-, and 7-C sugars
PPP reactions take place in the
cytoplasm
nonoxidative PPP phase has three reactions. The net result of these reactions is
the conversion of three pentoses into two hexoses and one triose ( 3 C5 <--> 2 C6 + C3 )
Transaldolase (in PPP) converts a 3-C and 7-C sugar into
a 6-C and 4-C sugar
Transketolase (in PPP) converts a 4-C and a 5-C sugar into
a 6-C sugar and a 3-C sugar
the PPP and Glycolysis are linked by
transketolase and transaldolase
the rate of the oxidative phase of the PPP is controlled by the level of
NADP+
Which reaction is the rate-limiting step of the PPP
the first reaction (the dehydrogenation of glucose 6-phosphate by glucose 6-phosphate dehydrogenase)
4 modes of PPP
1. ribose 5-phosphate needs exceed the needs for NADPH
2. the NADPH and ribose 5-phosphate needs are balanced
3. more NADPH is needed than ribose 5-phosphate
4. NADPH and ATP are both required
PPP is required for
rapid cell growth
the Calvin cycle and the PPP are
mirror images
Glycogen is in the ___ of all tissues
cytoplasm
the largest stores of glycogen are in the
liver and skeletal muscle
3 steps of glycogen degradation
1. release of glucose 1-phosphate from glycogen
2. remodeling of glycogen to allow continued degradation
3. conversion of glucose 1-phosphate into glucose
3 possible fates of glucose 6-phosphate
1. processing by the glycolytic pathway
2. conversion into free glucose for release into the blood
3. processing by the PPP
glycogen synthesis requires
an activated form of glucose, UDP-glucose
glycogen breakdown: ___ degrades glycogen from the non-reducing ends of the molecule; ___ is converted to glucose 6-phosphate by ___ - no ATP is expended
glycogen phosphorylase; glucose 1-phosphate, phosphoglucomutase
PLP (Pyridoxal Phosphate Cofactor) forms a ___ base with a ___ residue at the ___ site of the ___; the ___ substrate promotes cleavage of an a-___-linkage in glycogen by ___ a proton to the ___ glucose; a ___ intermediate forms; the ___ and ___ substrate
Schiff, lysine, active, phosphorylase; phosphate, 1,4, donating, departing; carbocation; carbocation, phosphate, glucose 1-phosphate
glycogen phosphorylase cannot cleave near branch points and can only cleave
a-1,4-glycosidic bonds
a transferase shifts a small oligosaccharide near the branch point to a nearby chain, thereby making the glucose moieties accessible to the
phosphorylase
what does a debranching enzyme like a-1,6-glucosidase do?
cleaves the a-1,6 bond at the branch point, releasing a free glucose
a ___ at the active site of phosphoglucomutase is ____; phosphoglucomutase forms a ___ intermediate by ___ its bound phosphoryl group to ___; the phosphoryl group is restored to the enzyme with the formation of ___
serine, phosphorylated; 1,6-biphosphate, donating, glucose 1-phosphate; glucose 6-phosphate
glucose 6-phosphatase generates free glucose from
glucose 6-phosphate in liver
phosphorylase regulation:
the key regulatory enzyme for glycogen degradation is
glycogen phosphorylase
phosphorylase regulation:
phosphorylase exists in 2 forms
a less active b form where a serine residue is phosphorylated and a more active a form
(both the a form and the b form display an equilibrium between R and T states; in the b form, the T state is favored, whereas in the a form, the R state is favored)
a ___ at the active site of phosphoglucomutase is ____
...
glycogen breakdown: glycogen phosphorylase ___ glycogen from the ___ ends of the molecule
degrades, non-reducing
what does the liver maintain
adequate blood glucose levels
the default state of liver phosphorylase is
the a form in the R state
liver phosphorylase is prepared to
generate blood glucose unless signaled otherwise
liver phosphorylase and muscle phosphorylase are
isozymes
what is a negative regulator of liver phosphorylase?
glucose
(facilitating the transition from the R state to the T state)
the monomer that is used to extend the glycogen chain in synthesis
UDP-glucose
UDP-glucose (uridine diphosphate glucose)
the glucose donor in glycogen synthesis
how is UDP-glucose synthesized?
UDP-glucose pyrophosphorylase
(in a reaction that produces a pyrophosphate (PPi))
glycogen synthase
transfers a glucose moiety from UDP-glucose to the C-4 terminal residue of a glycogen chain to form an a-1,4-glycosidic bond
primer
a polysaccharide of glucose residues
glycogenin
a dimer of two identical subunits
a branching enzyme generates branches by
cleaving an a-1,4-linkage, moving a block of approximately 7 glucoses, or synthesizing an a-1,6 linkage
glycogen synthase is the key regulatory enzyme in
glycogen synthesis
oxidation of glucose 6-phosphate derived from glycogen yields
about 31 molecules of ATP
Glycogen is an efficient storage form of
glucose