Medical Biochemistry Review #2 By Jason Elmer firstname.lastname@example.org Obi Ekwenna email@example.com YOUR EXAM Lectures 14-24 ~44 questions (4 questions per lecture) Take a calculator to the exam Exam on Monday October 4 th . DO THE STUDY QUESTIONS; if nothing else read the answers!!!!!!!!!!
~44 questions (4 questions per lecture)
Take a calculator to the exam
Exam on Monday October 4th.
DO THE STUDY QUESTIONS; if nothing else read the answers!!!!!!!!!!
Of course TLEs are highly recommended!
It is impossible to memorize every possible bit of biochemistry trivia. ‘They’ simply know way too much about metabolism for a single person to be able to regurgitate it all.
Do not sit and stare at the handouts
Do not try to read 50 review books. (Make your own review book instead!)
Do focus on identifying key concepts
Do actively draw and redraw pathways and connections
Do learn to identify relevant information
What is the purpose of a pathway?
What are the starting and ending molecules?
Where is the pathway (in the cell, in a tissue, in an organ system)?
How does the pathway connect to other pathways?
What metabolic conditions turn the pathway on and off?
What are the control points for regulating the pathway?
reactants, products and enzyme name of each regulatory step
additional regulatory molecules involved (vitamins, cofactors)
make sure you know every step that makes or uses ATP
What structural features are important for the function and interaction of specific regulatory molecules in a pathway?
What biochemical techniques are used to study these pathways?
What specific drugs or diseases associated with the pathway?
Citric Acid Cycle (Krebs Cycle)
Electron Transport Chain
Oxidation of glucose is known as Glycolysis.
Anaerobic Lactic Acid
Occurs in the Cytosol
Glucose + 2 ADP + 2 NAD+ + 2 Pi 2 Pyruvate + 2 ATP + 2 NADH + 2 H+
NADH generated during glycolysis is used to fuel mitochondrial ATP synthesis via oxidative phosphorylation. Does not pass through mitochondrial membrane
2 ATP generatedglycerol phosphate shuttle
3 ATP generatedmalate-aspartate shuttle
If used to transport the electrons from cytoplasm NADH into the mitochondria.
Found in the cytosol of most tissues
Low specificity: it’s a “hoe” for hexoses
Low Km: high affinity for glucose
Inhibited by Glucose-6-phosphate
Glucokinase: Found in the Liver and pancreatic b cells
Also a ‘hexokinase’
High specificity for glucose
inhibited by fructose-6-phosphate
Hexokinase, PFK-1 and PK all proceed with a relatively large free energy decrease. These non-equilibrium reactions of glycolysis would be ideal candidates for regulation of the flux through glycolysis.
Hexokinase is not key because of G6P is generated by glycogenolysis
PK reaction is reversed in Gluconeogenesis
Therefore rate limiting step in glycolysis is the reaction catalyzed by PFK-1.
PFK-1 is a tetrameric enzyme that exist in two conformational states termed R and T that are in equilibrium.
ATP is both a substrate and an allosteric inhibitor of PFK-1. F6P is the other substrate for PFK-1 and it also binds preferentially to the R state enzyme. ATP binds the T state.
The most important allosteric regulator of both glycolysis and gluconeogenesis is fructose 2,6-bisphosphate, F2,6BP, which is not an intermediate in glycolysis or in gluconeogenesis.
Also important to note that Insulin/Glucagon ratio i.e. fed/starve state, regulate Pyruvate Kinase activity. The last enzyme in the pathway.
Glucagon: high in starvation, b/cos blood glucose levels are low, therefore it favors gluconeogenesis in Liver.
Insulin: on the contrary favors glycolysis.
Key points about the Shuttle System:
Malate-Asparate shuttle is the primary system
By default Glycerol shuttle is secondary
Two enzymes are involved in this shuttle:
1.cytosolic version of the enzyme glycerol-3-phosphate dehydrogenase (glycerol-3-PDH) which has as one substrate, NADH.
2.mitochondrial form of the enzyme which has as one of its' substrates, FAD+. Since the electrons from mitochondrial FADH2 feed into the oxidative phosphorylation pathway at coenzyme Q (as opposed to NADH-ubiquinone oxidoreductase [complex I]) only 2 moles of ATP will be generated from glycolysis. G3PDH is glyceraldehyde-3-phoshate dehydrogenase.
The electrons are "carried" into the mitochondria in the form of malate. Cytoplasmic malate dehydrogenase (MDH) reduces oxaloacetate (OAA) to malate while oxidizing NADH to NAD+
Cytoplasmic malate dehydrogenase (MDH) reduces oxaloacetate (OAA) to malate while oxidizing NADH to NAD+.
Malate then enters the mitochondria where the reverse reaction is carried out by mitochondrial MDH
mitochondrial OAA goes to the cytoplasm to maintain this cycle ; must be transaminated to aspartate (Asp) with the amino group being donated by glutamate (Glu). The Asp then leaves the mitochodria and enters the cytoplasm. The deamination of glutamate generates a-ketoglutarate (a-KG) which leaves the mitochondria for the cytoplasm.
When the energy level of the cell rises, the rate of mitochondrial oxidation of NADH to NAD+ declines and therefore, the shuttle slows.
In the nonphosphorylated form the enzyme is known as PFK-2 and serves to catalyze the synthesis of F2,6BP by phosphorylating fructose 6-phosphate.
The result is that the activity of PFK-1 is greatly stimulated and the activity of F-1,6-BPase is greatly inhibited. More glycolysis!
When the bifunctional enzyme is phosphorylated it no longer exhibits kinase activity, but a new active site hydrolyzes F2,6BP to F6P and inorganic phosphate.
This enzyme is regulated by ProteinKinase A, which is a cyclic AMP dependent enzyme. cAMP is generated depending on the hormonal changes in the body. Eg. With Glucagon, high cAMP thus PKA is active thus less glycolysis.
In addition to these Pyruvate Kinase is activated by F1,6BP and inhibited by ATP.
Substrates for Gluconeogenesis: Lactate, pyruvate, glycerol, propionny-CoA and certain Amino Acids but never FAT!!!
The glucose-alanine cycle is used primarily as a mechanism for skeletal muscle to eliminate nitrogen while replenishing its energy supply. Glucose oxidation produces pyruvate which can undergo transamination to alanine. This reaction is catalyzed by glutamate-pyruvate transaminase, GPT (also called alanine transaminase, ALT in Figure).
See regulation of Glycolysis via F2,6 P
Do not forget Hormonal regulations: Insulin and Glucagon
Other things to keep in mind
Pyruvate carboxylase is present in mitochondria, requires Biotin as a cofactor to convert Pyruvate OAA
MDH present in mitochondria, OAA to malate, then MDH present in cytosol converts malate back to OAA
OAA is then converted to PEP, as shown in the previous slide.
Pyruvate Carboxylase: inhibited by ADP and activated Acetyl CoA
PEP Carboxykinase in the cytosol is inhibited by ADP
The cycle is located in the mitochondria
All cells have a mitochondria except RBCs
This is the Final common pathway of oxidative metabolism
Acetyl coenzyme A condenses with OAA to begin the cycle. Catabolism of CHO, Fats and Proteins provide the acetyl CoA
The bulk of ATP used by many cells to maintain homeostasis is produced by the oxidation of pyruvate in the TCA cycle
During this oxidation process, reduced NADH and reduced FADH2 are generated. The NADH and FADH2 are principally used to drive the processes of oxidative phosphorylation, which are responsible for converting the reducing potential of NADH and FADH2 to the high energy phosphate in ATP
The PDH complex requires 5 different coenzymes: CoA, NAD+, FAD+, lipoic acid and thiamine pyrophosphate (TPP) . Three of the coenzymes of the complex are tightly bound to enzymes of the complex (TPP, lipoic acid and FAD+) and two are employed as carriers of the products of PDH complex activity (CoA and NAD+).pyruvate + CoA + NAD+ CO2 + acetyl-CoA + NADH + H+
The TCA cycle showing enzymes, substrates and products. The abbreviated enzymes are: IDH = isocitrate dehydrogenase and a-KGDH = a-ketoglutarate dehydrogenase. The GTP generated during the succinate thiokinase (succinyl-CoA synthetase) reaction is equivalent to a mole of ATP by virtue of the presence of nucleoside diphosphokinase. The 3 moles of NADH and 1 mole of FADH2 generated during each round of the cycle feed into the oxidative phosphorylation pathway. Each mole of NADH leads to 3 moles of ATP and each mole of FADH2 leads to 2 moles of ATP.
acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O ----> 2CO2 + 3NADH + FADH2 + GTP + 2H+ + HSCoA
The GTP generated by Succinyl CoA SYNTHETASE IS VIA SUBSTRATE LEVEL PHOSPORYLATION.
Regulation of TCA:Regulation of the TCA cycle like that of glycolysis, occurs at both the level of entry of substrates into the cycle as well as at the key reactions of the cycle. Fuel enters the TCA cycle primarily as acetyl-CoA. The generation of acetyl-CoA from carbohydrates is a major control point of the cycle. This is the reaction catalyzed by the PDH complex
PDH complex is inhibited by acetyl-CoA, ATP, and NADH
PDH activated by non-acetylated CoA (CoASH) and NAD+.
The pyruvate dehydrogenase activities of the PDH complex are regulated by their state of phosphorylation. This modification is carried out by a specific kinase (PDH kinase) and the phosphates are removed by a specific phosphatase (PDH phosphatase).
The phosphorylation of PDH inhibits its activity which leads to decreased oxidation of pyruvate.
PDH kinase is activated by NADH and acetyl-CoA and inhibited by pyruvate, ADP, CoASH, Ca2+ and Mg2+. The PDH phosphatase, in contrast, is activated by Mg2+ and Ca2+
Citrate Synthase: inhibited by ATP and citrate
Isocitrate Dehydrogenase: Isocitrate, AMP, ADP activates, ATP and NADH inhibits
A-ketoglutarate dehydrogenase: succinoyl CoA and NADH inhibits
Important again to remember this is an oxidation-reduction reaction thus our friend Nerst is back:
DeltaG' = -nFDE'
ATP synthesized in the matrix is transported out of the matrix via an ATP/ADP translocase (an antiport) also coupled to proton motive force.
Rotenone: e- transport inhibitor Complex I
Under resting conditions, with a high cell energy charge, the demand for new synthesis of ATP is limited and, although the Proton Motive Force is high, flow of protons back into the mitochondria through ATP synthetase is minimal. When energy demands are increased, such as during vigorous muscle activity, cytosolic ADP rises and is exchanged with intramitochondrial ATP via the transmembrane adenine nucleotide carrier ADP/ATP translocase. Increased intramitochondrial concentrations of ADP cause the Proton Motive Force to become discharged as protons pour through ATP synthetase, regenerating the ATP pool.
The rate of electron transport is dependent on the PMF
ANY BLOCKADE AT ANY POINT IN THE ELECTRON TRANSPORT CHAIN STOPS ATP SYNTHESIS!!!!!!!!!SOME MORE STUFF
a. One half of the ATP-ADP cycle involves the coupling the energy derived from the hydrolysis of the high energy phosphate bonds of ATP to endergonic reactions so that they will occur spontaneously.
b. The work that requires energy derived from ATP hydrolysis includes the transport of electrons down the electron transport chain.
c. One half of the ATP-ADP cycle involves the generation of ATP that starts with the formation of reduced coenzymes like NADH and FADH2and the ultimate transfer of their electrons to oxygen
d. An important part of oxidative phosphorylation and ATP biosynthesis is the generation of an electrochemical gradient across the inner membrane of the mitochondria.
Many catabolic reactions, like the TCA cycle and fatty acid oxidation, provide the reduced coenzymes for the start of oxidative phosphorylation and ATP biosynthesis
a. AMP concentrations are increased as ADP concentrations fall
b. Low [ADP] accelerates the Krebs (TCA) cycle reaction rates, thereby providing more NADH to activate electron transport
c. The transmembrane proton gradient is dissipated with low [ADP]
d. The ATP/ADP antiport system is not functional when mitochondrial [ADP] is low
e. Proton translocation across the inner mitochondrial membrane is decreased when ATP-synthase lacks bound ADP and Pi, secondarily retarding electron transport
A. Complex I (NADH dehydrogenase)
B. Complex II (succinate-Q reductase)
C. Complex III (cytochrome b-c1)
D. Complex IV (cytochrome oxidase)
E. Complex V (F1F0 ATPase)
1. FMN, NADH dehydrogenase, ubiquinone, cytochrome c, cytochrome oxidase, F1F0-ATPase
2. Complex I, Complex III, ubiquinone, cytochrome a1-a3, cytochrome c, Complex IV, Complex V
3. FAD(2H)/succinate dehydrogenase, Coenzyme Q, cytochrome b-cl, cytochrome c, cytochrome a1-a3, F1F0-ATPase
4. NADH dehydrogenase, CoQ, cytochrome b-cl, cytochrome c, cytochrome oxidase, ATP synthase
5, NADH dehydrogenase, CoQ, cytochrome c, cytochrome oxidase, cytochrome b-cl, F1F0-ATPase
a. Both #1 and #2
b. Both #3 and #4
c. Only #4
d. Only #3
e. None of the above
a. Electron transport and oxygen consumption are inhibited
b. Electron transport and phosphorylation of ADP remain tightly coupled
c. The inner mitochondrial membrane remains impermeable to protons
d. Protons pass through the membrane-bound Fo fragment, but they do not sustain any ATP formation
e. The F1 fragment forms ATP at an accelerated rate until ADP is depleted or the proton gradient is dissipated
a. conversion of pyruvate to acetyl CoA via pyruvate dehydrogenase and glutamate to a-ketoglutarate via transaminases
b. conversion of cc-ketoglutarate to glutamate and GABA
c. production of ketone bodies (acetoacetate and P-hydroxybutyrate)
d. conversion of pyruvate to oxaloacetate via pyruvate carboxylase, biotin, bicarbonate ion, and ATP
e. both (A) and (D)
the flavin bound to NADH dehydrogenase.
has a Km considerably greater than the normal blood glucose concentration..
is found in muscle.
is inhibited by glucose 6-phosphate.
is also known as the GLUT-2 protein.
has glucose 6-phosphatase activity as well as kinase activity.
What Pathway is affected, which enzyme could it be? How would you confirm your speculation?
AcDH acetyladehyde dehydrogenase
Acetaldehyde forms adducts with Proteins, nucleic acids, and other compounds results in hangover.
NADH/NAD+ imbalance causes Liver to over work.
Diversion of gluconeogenesis by Lactic Acid dehydrogenase decreases ability of Liver to deliver glucose to the blood.
In addition, there is increased synthesis of FAT. Acetate + CoA gives you acetyl-CoA which is a precursor for Fatty acid sythesis. You already have enough NADH to go to work. So let the FATTYLIVER BEGIN! HepatoMEGALLY! Lets go!
FMN and FAD are both synthesized from riboflavin, which contains the electron-accepting ring structure of FAD
Severe Riboflavin deficiency decreases the ability of mitochondria to generate ATP via oxidative phosphorylation
In general, impairment of Complex I (NADH Dehydrogenase) induces formation of mitochondria with structural abnormalities.
Iron Deficiency Anemia
Characterized by decreased levels of Hb and other heme containing proteins in blood.
Iron-containing cytochromes and Fe-S centers of ETC are decreased as well.
Fatigue partly due to impaired ETC for ATP generation
ETC inhibitors at specific sites
Rotenone and Amytal block Complex I
Antimycin blocks cytochrome b1 in Complex III
Cyanide blocks cytochrome a/a3 in Complex IV. Prevents reduction of e- from reduced cytochrome c.
CO binds to reduced iron of cytochrome oxidase
CN- causes a rapid and extensive inhibition of ETC at the cytochrome oxidase step. Prevents O2 from serving as the final e- acceptor.
Mitochondrial respiration and energy production cease, resulting in cell death
Occurs from tissue asphyxiation, most notably in the Nervous System
Treatment: nitrites administered to convert oxyHb to MetheHb, which can then compete with cytochrome a,a3 for the CN-, forming a complex.CLINICAL CORRELATIONS
Uncoupling of ETC with Ox-Phos
Proton gradient from ETC coupled to ATP production from Oxidative Phosphorylation. If uncoupled and proton gradient dissipated, ATP and ADP concentrations lose their ability to regulate the rate of e- transport.
Uncouplers:proton ionophores, which rapidly transport H+ from cytosolic to matrix side of inner mitochondiral membrane
DNP – picks up H+ on cyto side, drops H+ on matrix side
Oligomycin: inhibits F1F0-ATPase…ATP synthesis stops.
Respiration and transport are blocked
Addition of an uncoupler (DNP) induces initiation of O2 consumption…ETC continues but w/o ATP synthesis since the pathways are uncoupled.
Brown Adipose Tissue and Thermogenesis
Large deposits of brown fat around vital organs (in human infants)…specialized for ‘non-shivering thermogenesis.’
Cold or excessive food intake stimulates NE release
Then Thermogenin, proton conductance uncoupler, is activated, pumping H+ back into mitochondria…dissipating the gradient.
ETC is induced, increasing rate of NADH and FADH2 oxidation, which generates more heat = biological heating pad
Thyroid hormone influences bioenergetics via actions on mitochondrial ox phos.
In Hyperthyroidism, energy derived from ox. Phos is significantly less than normal.
Thryoid causes ‘uncoupling’ of Ox Phos.
Results in increased heat production – patients complain of feeling hot and sweaty.
Salicylate (aspirin) poisoning
At high concentrations, salicylate can partially uncouple mitochondrial Ox Phos.
Decreased ATP [ ] and increased cytosolic AMP induce glycolysis
Results in increased blood pyruvate and lactate and metabolic acidosis and fever
Myoclonic Epileptic Ragged Red Fiber Disease (MERRF)
Debilitating, progressive spontaneous muscle jerking
Mitochondrial myopathy with enlarged, abnormal mitochondria
Neurosensory hearing loss, dementia, hypoventilation, mild cardiomyopathy
Maternal inheritance (sex linked)
Impaired energy metabolism….lactic acidosis
Hemolysis caused by Reactive Oxygen Species (ROS)
G6PD deficiency in pentose phosphate pathway
Causes increased production of radicals from GSH, since can’t produce sufficient NADPH to re-reduce glutathione….result in hemolysis
Heinz Bodies in RBCs
Due to G6PD deficiency
RBCs need the enzyme to re-reduce glutathione with NADPH to protect against oxidative stress
ROS peroxidation of membrane lipids lyses the RBC membrane
G6PD Mediterranean disease most severe G6PD deficiency
Lecture 21 – Monosaccharides and interconversion of sugars
Deficiency of Galactosyl-1-P uridylyltransferase
Accumulation of G-1-P in tissues and inhibition of glycogen metabolism, which require UDP-sugars
Higher level of galactose in blood and urine
More serious form
Unable to convert galactose to galactose-1-P
Dysfunction of glycogen synthase
Due to hyperglycemia…produces insulin resistance
Due to production of hexosamines that inhibit hexokinase, protein phosphatase 1, and glycogen synthase.
Lecture 23 – Glycogen Degradation
Von Gierke’s Disease
Defective G-6-Phosphatase enzyme
Increased amount of glycogen, normal structure
Affects liver and kidney
Massive enlargement of the liver. Severe hypoglycemia, ketosis, hyperuricemia, hyperlipemia.
Lecture 24 – Glucose/Glycogen Regulation
Type I – Insulin-dependent diabetes mellitus
Continuous glucagon expression causes ketogenesis, lipolysis, and gluconeogenesis.
Hyperchylomicronemia occurs (liver TG syn and VLDL transport faster than adipose LPL breakdown of TG)
Risk of ketoacidosis
Type II – Noninsulin-dependent Diabetes Mellitus
Peripheral tissues insulin resistant
Glucose accumulates in blood due to poor uptake by peripheral tissues, particularly muscles
Hypertriacylglycerolemia, which results from increase of VLDL without hyperchylomicronemia. New FA and VLDL synthesized in liver instead of increased delivery of fatty acids from adipose tissue.
What is the PPP and why is it important?
What is the PPP and why is it important?
The pentose phosphate pathway is primarily an anabolic pathway that utilizes the 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents
The reactions of the PPP operate exclusively in the cytoplasm. From this perspective it is understandable that fatty acid synthesis (as opposed to oxidation) takes place in the cytoplasm
The oxidation steps, utilizing glucose-6-phosphate (G6P) as the substrate, occur at the beginning of the pathway and are the reactions that generate NADPH
Reactions catalyzed by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase generate one mole of NADPH each for every mole of glucose-6-phosphate (G6P) that enters the PPP
Non-oxidative reactions are to convert dietary 5 carbon sugars into both 6 (fructose-6-phosphate) and 3 (glyceraldehyde-3-phosphate) carbon sugars which can then be utilized by the pathways of glycolysis
The primary enzymes involved in the non-oxidative steps of the PPP are transaldolase and transketolase
Transketolase functions to transfer 2 carbon groups from substrates of the PPP, thus rearranging the carbon atoms that enter this pathway. Like other enzymes that transfer 2 carbon groups, transketolase requires thiamine pyrophosphate (TPP) as a co-factor in the transfer reaction
Transaldolase transfers 3 carbon groups and thus is also involved in a rearrangement of the carbon skeletons of the substrates of the PPP. The transaldolase reaction involves Schiff base formation between the substrate and a lysine residue in the enzyme
Oxidation of G6P, a 6 carbon sugar, into a 5 carbon sugar
Generation of NADPH
3 carbon sugar generated is glyceraldehyde-3-phsphate which can be shunted to glycolysis and oxidized to pyruvate OR it can be utilized by the gluconeogenic enzymes to generate more 6 carbon sugars (fructose-6-phosphate or glucose-6-phosphate)
Predominant pathways of carbohydrate metabolism in the red blood cell (RBC) are glycolysis, the PPP and 2,3-bisphosphogylcerate (2,3-BPG)
Glycolysis provides ATP for membrane ion pumps and NADH for re-oxidation of methemoglobin
The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione (Glutathione can reduce disulfides nonenzymatically)
Oxidative stress generates peroxides that in turn can be reduced by glutathione to generate water
Inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H2O2, that in turn results in a weakening of the cell wall and concomitant hemolysis
Glutathione removes peroxides via the action of glutathione peroxidase. The PPP in erythrocytes is essentially the only pathway for these cells to produce NADPH
Glycogen chains & branches are longer than shown
Glucose is stored as glycogen predominantly in liver and muscle cells.
Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the a(14) glycosidic linkages of glycogen, releasing glucose-1-phosphate as reaction product.
glycogen(n residues) + Pi
glycogen (n–1 residues) + glucose-1-phosphate
Most people don’t know…
The relative activity of the un-modified phosphorylase enzyme (phosphorylase-b) is sufficient to generate enough glucose-1-phosphate for entry into glycolysis for the production of sufficient ATP to maintain the normal resting activity of the cell; This is true in both liver and muscle cells
The hormones glucagonandepinephrine activate G-protein coupled receptors to trigger cAMP cascades.
The phosphorylated enzyme is less sensitive to allosteric inhibitors.
Thus, even if cellular ATP & glucose-6-phosphate are high, Phosphorylase will be active.
The glucose-1-phosphate produced from glycogen in liver may be converted to free glucose for release to the blood.
With this hormone-activated regulation, the needs of the organism take precedence over needs of the cell.
This identical cascade of events occurs in skeletal muscle cells
However, in these cells the induction of the cascade is the result of epinephrine binding to receptors on the surface of muscle cells
(Ca2+ ion-mediated pathway to phosphorylase kinase activation is through activation of a-adrenergic receptors by epinephrine)
Epinephrine is released from the adrenal glands in response to neural signals indicating an immediate need for enhanced glucose utilization in muscle, the so called fight or flight response
Muscle cells lack glucagon receptors. The presence of glucagon receptors on muscle cells would be futile anyway since the role of glucagon release is to increase blood glucose concentrations and muscle glycogen stores cannot contribute to blood glucose levels…why?
Phosphorylation of the enzyme, via a cAMP cascade induced by epinephrine, results in further activation
These regulatory processes ensure release of phosphorylated glucose from glycogen, for entry into Glycolysis to provide ATP needed for muscle contraction.
A class of drugs developed for treating the hyperglycemia of diabetes (chloroindole-carboxamides), inhibit liver Phosphorylase allosterically.
These inhibitors bind at the dimer interface, stabilizing the inactive (tense) conformation.
Question: Why would an inhibitor of Glycogen Phosphorylase be a suitable treatment for diabetes?
Liver Glucose-6-phosphatase catalyzes the following, essential to the liver's role in maintaining blood glucose:
glucose-6-phosphate + H2O glucose + Pi
Most other tissues lack this enzyme…why??
Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis
As glucose residues are added to glycogen, UDP-glucose is the substrate and UDP is released as a reaction product.
Spontaneous hydrolysis of the ~P bond in PPi (P~P) drives the overall reaction
Cleavage of PPi is the only energy cost for glycogen synthesis (one ~P bond per glucose residue).
Glycogenin initiates glycogen synthesis.
Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.
UDP is released as a product.
This is repeated until a short linear glucose polymer with a(1®4) glycosidic linkages is built up on Glycogenin
glycogen(n residues) + UDP-glucose
glycogen(n +1 residues) + UDP
A separate branching enzyme transfers a segment from the end of a glycogen chain to the C6 hydroxyl of a glucose residue of glycogen to yield a branch with an a(1®6) linkage.
If both pathways were active simultaneously in a cell, there would be a "futile cycle" with cleavage of one ~P bond per cycle (in forming UDP-glucose)
To prevent such a futile cycle, Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, by allosteric effectors and by phosphorylation.
Thus Glycogen Synthase is active when high blood glucose leads to elevated intracellular glucose-6-P
It is useful to a cell to store glucose as glycogen when the input to Glycolysis (glucose-6-P), and the main product of Glycolysis (ATP), are adequate.
The conformation of Glycogen Synthase induced by the allosteric activator glucose-6-phosphate is susceptible to dephosphorylation by Protein Phosphatase (PP1)
Glycogen Synthase is phosphorylated by Protein Kinase A as well as by Phosphorylase Kinase.
Phosphorylation of Glycogen Synthase promotes the "b" (less active) conformation.
The cAMP cascade thus inhibits glycogen synthesis.
Instead of being converted to glycogen, glucose-1-P in liver may be converted to glucose-6-P, and dephosphorylated for release to the blood.
Insulin, produced in response to high blood glucose, triggers a separate signal cascade that leads to activation of Phosphoprotein Phosphatase
This phosphatase catalyzes removal of regulatory phosphate residues from Phosphorylase, Phosphorylase Kinase, & Glycogen Synthase enzymes
Thus insulin antagonizes effects of the cAMP cascade induced by glucagon & epinephrine
Some enzymes whose deficiency leads to glycogen accumulation are part of the inter-connected pathways shown here