Study Guide: Biochemistry 411
37 Cards in this Set
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Phosphofructokinase I
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Phosphorylates Fructose-6-Pi to Frc-1,6-BisPi:
Catalyzes first committed step of glycolysis, key enzyme to regulate the flux of intermediates through glycolysis.
Regulated allosterically (T-R states)
Negative Inhibitors: ATP, citrate
Positive Effectors: AMP, fructose-2,6-bisph…
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Pyruvate Carboxylase
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Carboxylates pyruvate to form oxaloacetate,
ATP required
Biotin cofactor, covalently attached to a lysine residue
Bicarbonate is activated by ATP, it is the source of CO2. Activated CO2 ("carboxyphosphate intermediate") is carried by Biotin to 2nd active site.
Allosterica…
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Location of Pyruvate Carboxylase
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only found in the mitochondrial matrix; requires pyruvate translocase protien to symport a proton and pyruvate into the matrix (despite thermodynamic conditions)
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Regulation of Pyruvate Carboxylase
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Requires AcetylCoA--indicates cellular energy is high enough to carry out gluconeogenesis. If energy requirements are not met, pyruvate is directed into the TCA. AcetylCoA is necessary for the FIRST half of the reaction, the nucleophilic attack of pyruvate on carboxybiotin is not affected…
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Phosphoenolpyruvate Carboxykinase
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Catalyzes the phosphorylation and decarboxylation of oxaloacetate to produce PEP.
The decarboxylation reaction is very exergonic and drives the reaction of PEP synthesis.
**USES GTP FOR ENERGY**
(ATP equivalent)
Can be in the mitochondria or in the cytosol
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Compartmentalization of PEP Carboxykinase
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Can be either in cytosol or mitochondria. If in mitochondria, the oxaloacetate is converted to PEP directly there. PEP is then transported into the cytosol for the remainder of the glycolytic/gluconeogenic enzymes.
If in the cytosol, oxaloacetate must be REDUCED to malate by malat…
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Fructose-1,6-bisphosphatase
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Hydrolyzes fructose-1,6-bp to fructose-6-Pi,
First committed step of gluconeogenesis
Allosterically regulated by:
Negative Effectors: AMP, Fructose-2,6-bisphosphate
Positive Effector: Citrate
Key enzyme for regulating the flux of intermediates through gluconeogene…
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Glucose-6-Phosphatase
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Hydrolyzes the phosphate group off of Glucose-6-Pi to produce glucose.
Membrane bound protein of the LUMEN of the ER in LIVER AND KIDNEYS only.
this is the enzyme that localizes gluconeogenesis to the hepatic tissues.
Regulated by substrate level control (like glucokina…
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Location of Glucose-6-Phosphatase
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Lumen of the ER in liver and kindey cells
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Regulation of Glucose-6-Phosphatase
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High Km for substrate; under substrate level control (linear relationship with concentration of substrate)
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Regulation of Fructose-2,6-bisphosphatase
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Allosterically regulated:
Negative Effectors: AMP and Fructose-2,6-Bisphosphate
(reverse of PFK I)
Positive Effector: Citrate
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What is the only enzyme under direct hormonal control in the gluconeogenic and glycolytic cycles?
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Pyruvate Kinase:
PK is reversibly phosphorylated by the cAMP-dependent phosphorylation cascade. Glucagon causes phosphorylation, thus deactivating PK, and turning ON GLUCONEOGENESIS. Insulin causes the hydrolysis of the Pi group, thus activating the PK and turning on glycolysis.
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Cori Cycle
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Describes the equillibrium that exists between the muscles and liver in the production and degradation/use of lactate.
Muscles (when vigorously exercising under anaerobic conditions) produce lactate (to regenerate NAD+), lactate is released into the blood stream and carried to the…
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Three irreversible steps/enzymes of GLYCOLYSIS?
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Hexokinase (glucokinase in hepatic tissues)
Phosphofructokinase I
Pyruvate Kinase
All are allosterically regulated so they can be reciprocally regulated by gluconeogenic effectors/enzymes.
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Irreversible (exergonic) enzymes/steps of GLUCONEOGENESIS?
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Glucose-6-Phosphatase
Fructose-1,6-Bisphosphatase
PEP Carboxykinase and Pyruvate Carboxylase
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Regulation of Pyruvate Kinase
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Allosteric Regulators:
Positive Regulators: AMP, Fructose-1,6-bisphosphate (feed forward)
Negative Effector: ATP, Alanine
cAMP-dependent phosphorylation (direct hormonal control)
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Glycogen (general)
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storage polysaccaride in animals--glucose linked with a(1-4) glycosidic linkages
found in liver and muscle tissues
liver--to maintain blood glucose levels
muscle--quick energy reserve for muscle action
highly branched structure with many nonreducing ends--branches have a(…
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Importance of Branching in Glycogen
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Necessary for quick mobilization of glucose, many nonreducing ends allow for quick hydrolysis of glucose residues
Branching increases solubility
Lack of "branching enzyme" (can't create a(1-6) branch points) results in long, linear saccharide (like amylose) that alters the…
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Glycogen Phosphorylase
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catalyzes the sequential phosphorolysis of glucose residues from a nonreducing end of a glycogen molecule
releases Glucose-1-Pi from glucagon
oxonium ion intermediate, glycosidic linkage is cleaved by inorganic phosphate in such a way that the stereochemistry at the C1 car…
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Regulation of Glycogen Phosphorylase
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Allosterically regulated:
Positive Effectors: AMP (low cell energy = need to make ATP so we need glucose for glycolysis) (curve shifts to the LEFT, lower Km, higher affinity, Vmax not affected)
Negative Effectors: ATP, Glucose-6-Pi (curve shifts to the RIGHT, higher Km, Vm…
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Cofactor required for Glycogen Phosphorylast?
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A pyridoxyl-5'-phosphate cofactor is required
it is attached via a SCHIFF BASE (forms an electron sink) to a lysine residue
functions with inorganic phosphate in the gen acid/base catalysis
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Debranching Enzyme
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Removes the branches (glycogen phosphorylase loses activity within 4 residues of a branch point)
TWO ACTIVITIES:
1. a(1,4) transglycosylase (transferase)--move blocks of 3 glucose residues from the branch onto the nonreducing end of the next chain
2. a(1,6)-glucosidase--clea…
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Phosphoglucomutase
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Isomerizes Glucose-1-Pi (released by glycogen phosphorylase) to Glucose-6-Pi, which can be utilized by glycolytic enzymes
Requires a PHOSPHORYLATED SERINE residue in the active site (similar to phosphoglycerate mutase)
Intermediate = Glucose-1,6-bisphosphate
Also requir…
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Phosphoglucokinase
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Phosphorylates glucose-1-Pi to glucose-1,6-bisphosphate
binds to a dephosphorylated Serine in phosphoglucomutase (and subsequently phosphorylates it to restore enzymatic activity)
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UDP-glucose pyrophosphorylase
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catalyzes the acylation of Glucose-1-Pi with a UTP to create UDP-glucose
(Glc-1-Pi + UTP --> PPi + UDP-Glucose)
UDP glucose "carries" activated glucose molecules
Inorganic Pyrophosphatase is required to make this reaction irriversible, it hydrolyzes the released py…
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What makes the action of UDP-glucose pyrophosphorylase an irreversible reaction?
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the hydrolysis of released PPi to 2Pi is the thermodynamic determinant of the rxn
catalyzed by inorganic pyrophosphatase
pushes the reaction towards products because it rapidly removes product as soon as it is created
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Glycogen synthase
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adds new glucosyl residues to nonreducing ends of glycogen, adds residues to a chain of 4 or more residues
SN1 mechanism (retains stereochemistry)
requires a primer protien--glycogenin
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What is the substrate for glycogen synthase?
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UDP glucose is the substrate, UDP leaves (very good LG) in an SN1 mechanism
The remaining oxonium ion is attacked by the nonreducing end of a glycogen molecule, as long as there are open C4 groups, glucoses are continually added
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Glycogenin
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Primer protein for glycogen synthase
contains an oligosaccharide of a(1,4) glucose residues attached to a phenolic oxygen of a TYROSINE residue
glycogenin autocatalytically adds up to seven glucose residues after tyrosine glucosyltransferase attaches a glucose molecule to …
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Branching Enzyme
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synthesizes a(1,6) glycosidic linkages (glycogen synthase only synthesizes a(1,4)linkages.
Branching is crucial because it increases the solubility and number of reducing ends of a glycogen molecule. this increases the rate of biosynthesis and degredation.
Enzyme takes a block …
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Which hormones activate Glycogen Phosphorylase?
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Glucagon and Epinephrine activate Glycogen phosphorylase by phosphorylating in (via cAMP pathway)
"A" state = PHOSPHORYLATED (locks in the R state)
Phosphorylation is by PHOPHORYLASE KINASE (covalent modification of Ser-14 residue
Dephosphorylation (caused by INSULIN) i…
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What enzyme phosphorylates Glycogen Phosphorylase?
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Phosphorylase Kinase
(phosphorylates Ser residue--locks into R state (a))
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What enzyme dephosphorylates Glycogen Phosphorylase?
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Phosphoprotein phosphatase 1
dephosphorylates the Ser residue, decreasing activity (b-state), insulin sparks this dephosphorylation
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Regulation of Glycogen Synthase
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Allosterically activated by GLUCOSE-6-Pi --Key allosteric effector
Also reversible phosphorylation:
DEPHOSPHORYLATED = locked in R (a) state, this is the more active state of the synthase
PHOSPHORYLATED = less active, T (b) state of the enzyme, very high Km (phosphorylated b…
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What enzyme phosphorylates Glycogen Synthase? What is the effect of this phosphorylation?
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Phosphorylase kinase 1 phosphorylates Glycogen synthase.
This DEACTIVATES the enzyme, locking in 'b' or "T" state.
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What enzyme DEPHOSPHORYLATES Glycogen Synthase? What is the effect of this dephosphorylation?
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Phosphoprotein phosphatase 1 dephosphorylates glycogen synthase.
This locks enzyme in the active R (a) form.
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What is the "key" allosteric effector in Glycogen metabolism?
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Glucose-6-Phosphate
Allosteric effector of Glycogen phosphorylase (-)
and Glycogen synthase (+)
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Gluconeogenesis - Flashcards