Biochemistry Chapter 21

why can’t glucose be stored

high concentrations of glucose disrupt the osmotic balance of the cell, causing cell damage or death

glycogen

a significantly less osmotically active and highly branched polymer that can be rapidly broken down to yield glucose molecules when energy is needed – controlled release maintains blood-glucose concentration between meals – good source of energy for sudden, strenuous activity as it can be metabolized in the absence of O2

what are the primary storage sites for glycogen

liver and muscle tissue

how does glycogen appear in the cytoplasm

as granules consisting of multiple glycogen molecules

why is it useful to make glycogen over glucose

allows the body to conserve glucose for later use, regulates blood sugar levels, prevents potential osmotic issues that can arise from high glucose concentrations

what do individual glycogen molecules have

~12 layers of glucose molecules. – can be as large as 40 nm. – contain ~55,000 glucose residues and a single glycogenin protein at the core.

what are most of the glucose residues in glycogen linked by

alpha 1,4 glycosidic bonds

what are branches in glucose residues in glycogen created by at every 12 residues

alpha 1,6 glycosidic bonds

what are the three steps of glycogen degradation

metabolized by glycolysis

converted into free glucose in the liver for release into the bloodstream

processed by the pentose phosphate pathway to yield NADPH and ribose derivatives

what is the ultimate production of glycogen degredation

glucose 6-phosphate

when does glycogen synthesis occur

when glucose is abundant and glycogen is depleted

what are some pathways glucose 6-phosphate can be used in

glycolysis (pyruvate to lactate or CO2+H2O), (liver) glucose released into blood for use by other tissues, PPP (ribose + NADPH)

glycogen phosphorylase (phosphorylase)

cleaves glycogen by the addition of orthophosphate (Pi)

phosphorolysis

cleavage of a bond by the addition of orthophosphate

phosphorylase mechanism

catalyzes the sequential removal of glucosyl residues from the nonreducing ends

orthophosphate mechanism

splits the glycosidic linkage between C-1 of the terminal residue and C-4 of the adjacent one. – the α configuration at C-1 is retained – The active site excludes water to save the ATP required to phosphorylate free glucose. – Glucose 1-phosphate can be converted to glucose 6- phosphate by phosphoglucomutase

what bonds can glycogen phosphorylase cleave

ONLY alpha 1,4 glycosidic bonds

transferase

hifts a small block of three glucosyl residues from one outer branch to another – exposes a single glucose residue joined by an α-1,6- glycosidic bond

α-1,6-glucosidase

hydrolyzes the α-1,6-glycosidic bond

debranching enzyme

bifunctional enzyme in eukaryotes that contains transferase and α-1,6-glucosidase activities

hexokinase

phosphorylates glucose if the glucose will enter glycolysis or the pentose phosphate pathway

what are the three distinct catalytic activities glycogen remodeling requires

Glycogen phosphorylase

Glycogen debranching enzyme P

Phosphoglucomutase

what does glucose 6-phohsphatase do so that free glucose can leave the liver

removes the phosphate on glucose

glucose 6-phosphatase

hydrolytically cleaves the phosphoester linkage of glucose 6-phosphate in liver, yielding glucose and orthophosphate. absent from most other tissues. – Muscle tissues retain glucose 6-phosphate for ATP generation. – glucose is not a major fuel for the liver

where is free glucose used after it is released into the blood by the liver

used by the brain, red blood cells, and other tissues

where is glucose 6-phosphatase located

on the lumenal side of the smooth ER membrane

where is G-6P transported

into the ER

where are glucose and orthophosphate shuttled

back into the cytoplasm

what is glycogen phosphorylase regulated by

allosteric effectors that signal the energy state of the cell. – reversible phosphorylation, which is responsive to hormones

what are the two isozymic forms of glycogen phosphorylase

a liver one and a skeletal muscle one

what are the two forms dimeric phosphorylase exists in

a usually active phosphorylated a form. – a usually inactive unphosphorylated b form

what do both dimeric phosphorylase forms exist in

equilibrium between an active relaxed (R) and less active tense (T) state

what does eq for phosphorylase a favor

R state

what does eq for phosphorylase b favor

T state

what is liver phosphorylase allosterically inhibited by

glucose

how do we transform phosphorylase a from R state to T state

add 2 glucose

what is the default form of muscle phosphorylase

b form

when is phosphorylase active

during muscle contraction

what is muscle phosphorylase regulated by

intracellular energy change

how does AMP activate muscle phosphorylase b

binding to a nucleotide-binding site and stabilizing the R state

what is muscle phosphorylase b activated by

AMP

what allosterically inhibits muscle phosphorylase and stabilizes the T state

ATP and G-6P

why do ATP and G-6P allosterically inhibit muscle phosphorylase and stabilize the T state

ATP competes with AMP, G-6P binds at the ATP binding site and stabilizes phosphorylase b

how do we transform phosphorylase b (muscle) from R to T state

add 2 ATP or 2 G-6P

what does phosphorylation promote

the conversion of phosphorylase b to phosphorylase a

glucagon

peptide hormone released in response to low blood glucose levels

epinephrine (adrenaline)

hormone released during strong emotions (fear, excitement, exercise)

phosphorylase kinase

regulatory enzyme that catalyzes the phosphorylation of a single Ser residue in each subunit of phosphorylase to yield phosphorylase a – catalyzed in response to glucagon or epinephrine – both liver and muscle phosphorylase can be covalently modified

how does phosphorylation make phosphorylase kinase more active

moves a peptide loop out of the active site of the b form

phosphorylase kinase subunit composition

alpha beta gamma delta x 4

phosphorylase kinase alpha and beta subunits

phosphorylation targets

phosphorylase kinase gamma subunit

active site

phosphorylase kinase delta subunit

Ca2+ binding protein calmodulin

what is phosphorylase kinase activated by

phosphorylation and Ca ions

when is activation of phosphorylase kinase initiated

when Ca2+ binds to the delta subunit

when does maximal activation of phosphorylase kinase occur

with the phosphorylation of the beta and alpha subunits by protein kinase A

what partly activates phosphorylase kinase

Ca2+ from nerve impulse, muscle contraction, certain hormones

what fully activates phosphorylase kinase

PKA and certain hormones

what happens after phosphorylase kinase is fully active

can convert to phosphorylase a or b

when fasting, where is glycogen broken down

in the liver - both glycogen and lactate can be broken down to glucose

when exercising, where is glycogen broken down

in muscle cells - to glucose 6-phosphate, pyruvate, lactate, blood pathway. glucagon from pancreas and epinephrine from adrenal medulla stimulates

where does fasted glucose go after creation

into blood, to muscle cells where it will be further broken down to lactate back into blood

what does glycogen synthesis require (simple)

several enzymes and uridine diphosphate glucose

UDP glucose

uridine diphosphate glucose. activated glucose donor for glycogen

what does UDP-glucose pyrophosphorylase synthesize

UDP-glucose

what else does UDP-glucose pyrophosphorylase synthesize

pyrophosphate (PPi)

what type of reaction is UDP-glucose pyrophosphorylase (reverse or irreverse)

readily reversible, but hydrolysis of PPi drives synthesis of UDP glucose (so we don’t want to)

what does glycogen synthase catalyze

the transfer of glucose from UDP-glucose to a growing chain

glycogen synthase

key regulatory enzyme in glycogen synthesis that adds new glucosyl units to the nonreducing terminal residues of glycogen – forms an α-1,4- glycosidic linkage

why does glycogen synthase require a primer

because it can only add to a chain containing 4+ residues

glycogenin

a dimer of two identical subunits, each of which catalyzes the formation of α-1,4-glucose polymers until a primer of 10–20 glucosyl units is formed

what happens once the primer forms by glycogenin

glycogen synthase takes over

what does every glycogen molecule have covalently attached at its core

a glycogenin monomer

what does a branching enzyme form

alpha 1,6 linkages

what can glycogen synthase synthesize

ONLY alpha 1,4 linkages

how does branching enzyme generate branches

by cleaving an α-1,4-linkage, transferring a block of ~7 residues, and reattaching the block with an α-1,6 linkage

rules for branching enzyme

The block must include the nonreducing terminus. – The block must come from a chain at least 11 residues long. – The new branch must be at least four residues away from existing branches

what does branching enzyme do

removes approximately seven residues from the nonreducing end and reattaches them with a α-1,6 Linkage

what is the key regulatory enzyme in glycogen synthesis

glycogen synthase

what are the two forms glycogen synthase exists in

an active nonphosphorylated a form – a usually inactive phosphorylated b form

what is glycogen synthase phosphorylated by

an active nglycogen synthase kinase, which is under the control of insulin. – PKAonphosphorylated a form – a usually inactive phosphorylated b form

what is a powerful activator of glycogen synthase b and why

G-6P, because it stabilizes the R state of the enzyme relative to the T state

what are glycogen breakdown and synthesis reciprocally controlled by

hormones

what is glycogen synthesis inhibited by and glycogen breakdown stimulated by

the same glucagon and epinephrine signaling pathways

what does PKA phosphorylate

phosphorylase kinase

what happens when PKA phosphorylates phosphorylase kinase

activates phosphorylase kinase and initiates glycogen breakdown

what phosphorylates glycogen synthase

glycogen synthase kinase and PKA, decreasing enzyme activity and inhibiting glycogen synthesis

what happens after glycogen synthase is phosphorylated

decrease in enzyme activity and inhibiting of glycogen synthesis

what does protein phosphatase 1 reverse

the regulatory effects of kinases on glycogen metabolism

PP1

protein phosphatase 1

protein phosphatase 1 (PP1)

dephosphorylates proteins to decrease the rate of glycogen breakdown. – inactivates phosphorylase a – inactivates phosphorylase kinase – converts glycogen synthase b to the more active glycogen synthase a

what is glycogen synthesis regulated by

protein phosphatase 1

what is PP1 regulated by

hormonally controlled cascades

How do glucagon and epinephrine affect PP1 activity

generally inhibit PP1 activity, leading to decreased glycogen synthesis and increased glycogen breakdown in liver and muscle (activation of PKA)

what stimulates glycogen synthesis (hormone)

insulin

how does insulin stimulate glycogen synthesis (general)

inactivates glycogen synthase kinase

what happens when blood glucose concentration is high

insulin inactivates glycogen synthase kinase – stimulates synthesis of glycogen – the inactive kinase cannot maintain glycogen synthase in its phosphorylated, inactive state

what does PP1 dephosphorylate

glycogen synthase, activating it and restoring glycogen reserves

when does PP1 dephosphorylate glycogen synthase

when insulin inactivates glycogen synthase kinase, stimulating glycogen synthesis