Chapter 21 Flashcards
The efficient breakdown of glycogen requires requires 4 enzyme activities. Name them
- to degrade glycogen
- to remodel glycogen (2)
- convert the product of glycogen into a form suitable for further metabolism
What does glycogen phosphorylase do?
cleaves the substrate by the addition of Pi to yield G1P
What do phosphorylases do?
catalyze the sequential removal of glucosyl residues from the nonreducing ends of the glycogen molecule (the ends with a free OH group on carbon 4)
Why is a debranching enzyme necessary in the breakdown of glycogen?
the alpha1,6 bonds at branch points are not susceptible to cleavage by phosphorylase
How do the debranching enzymes work?
the transferase shifts a block of three glucosyl residues from one outer branch to the other
a1,6-glucosidase hydrolyzes the a1,6 glycosidic linkage exposed by the transferase
a free glucose molecule is released and then phosphorylated by hexokinase
T/F
Phosphoglucomutase has kinase activity.
True
How is G1P converted to G6P?
Phosphoglucomutase transfers a phosphoryl group to G1P forming G16BP
the enzyme then from the phosphoryl group from C1 creating G6P
What are the two forms of phosphorylase?
a: mostly in R state (has Pi)
b: mostly in T state (no Pi)
What form of phosphorylase resides in the muscle?
Since the muscle stores G, the b form of phosphorylase is in the muscle
What are the activators of phosphporylase in the muscle?
AMP - put b in R state
ATP - put b in T state
G6P - put b in T state
What form of phosphorylase resides in the liver?
since the liver uses G, the a form of phosphorylase resides in the liver
What is the only activator of phosphorylase in the liver?
glucose (G) - puts a in the T state
What enzyme phosphorylates phosphorylase?
glycogen phosphorylase kinase
ATP + GP”b” —————-> ADP + GP”a”-Pi
GPK
Who takes Pi off phosphorylase?
protein phosphatase 1 (PP1)
What does glycogen synthase need to build a glycogen granule?
It needs 4 glucose molecules to add more glucose molecules. However, a primer is needed to put 4 glucose molecules together to “start” glycogen granule
How is a glycogen granule “started?”
glycogenin attaches glucose to Tyr in its active site and acts as a primer to build 4 glucose molecules
What type of linkage does glycogen synthase form?
a-1,4
Why does glucose need a-1,6 branches?
to increase solubility/directly store glucose
How does the branching enzyme work?
has glucosidase activity; cut a-1,4 (after 7 G)
transfers 7 G molecules down to make new a-1,6 linkage
What does it mean when the enzymes are reciprocally regulated?
1 works while the other doesn’t
synthase works ——–phosphorylase doesn’t
What are the two forms of the synthase?
a —- active (R state)
b —- inactive (T state, w/Pi)
What is one of the major functions of the liver?
to maintain a nearly constant level of glucose in the blood
How does glucose leave the liver?
the liver contains a hydrolytic enzyme, glucose-6-phosphotase that enables glucose to leave
What hormones signal the need for glycogen breakdown?
epinephrine and glucagon
How do hormones trigger the breakdown of glycogen?
they initiate a cyclic AMP signal-transduction cascade
What is the first step of cAMP?
the signal molecules epinephrine and glucagon bind to specific transmembrane receptors in the plasma membrane of target cells
these binding events activate Gs protein
a specific external signal has been transmitted into the cell through structural changes, first in the receptor and then in the G protein
What is the second step of cAMP?
The GTP bound subunit of Gs activates the transmembrane protein adenylate cyclase. This enzyme catalyzes the formation of the second messenger cyclic AMP from ATP.
What is the third step of cAMP?
the elevated cytoplasmic level of cyclic AMP activates protein kinase A. The binding of cyclic AMP to inhibitory regulatory subunits triggers their dissociation from the catalytic subunits. The free catalytic subunits are now active
What is fourth step of cAMP?
protein kinase A phosphorylates phosphorylase kinase, which subsequently activates glycogen phosphorylase
