Carb metab Flashcards
What are aldoses
Monosaccharide with aldehyde group at C-1
What are ketoses
Monosaccharide with ketone group at C-2
What is a monosaccharide
Basic sugar unit, e.g. glucose, galactose, fructose
What is a disaccharide
2 mono units linked via glycosidic bond (lactose and sucrose)
What is an oligosaccharide?
3-10 mono units
What is a polysaccharide?
Multiple mono units (starch)
What is alpha-amylase and where is it found
Random hydrolyses alpha-1,4 glycosidic bonds, except those that act as branch points (near alpha-1,6 linkage)
Found in saliva and pancreatic juice
What is sucrase-isomaltase complex?
- Sucrase hydrolyses sucrose to form glucose and fructose
- Maltase hydrolyses maltotriose and maltose
- Isomaltase hydrolyses alpha-1,6 linkages
What is glucoamylase complex
Hydrolyses alpha-1,4 glycosyl units, starting from the non-reducing end of chains
How is sucrose digested
Hydrolysis of sucrose into glucose and fructose by sucrase
How is lactose digested
Hydrolysis by lactase.
What are the effects of lactase deficiency
No lactose absorption in lower intestine, bacterial fermentation of lactose producing lactic acid and gases, lactose and lactic acid is osmotically active, draws water into the lumen resulting in diarrhoea
Describe the absorption of glucose and fructose in intestines.
SGLT1 (active transport):
- Na+ dependent cotransporter, works with Na+/K+ ATPase pump
- energy from Na+ ionic conc gradient, secondary active transport
- actively transports glucose and galactose into epithelial cells
GLUT 5:
Tansports fructose via facilitated diffusion- passive transport
Monosaccharides exit epithelial cells into bloodstream via GLUT2 (facilitated diffusion, passive)
Describe the transport of glucose in the liver and pancreatic beta cells
GLUT 2, low affinity transporter
in pancreas: regulate the secretion of insulin
in liver, remove excess glucose in the blood
Describe the transport of glucose in the brain and nerve tissues
GLUT3: high affinity transporter
Allows for basal glucose uptake
Describe the transport of glucose in the heart and skeletal muscles
GLUT 4, regulated by insulin, high affinity transporter-> in the presence of insulin, increase in translocation of GLUT4 to the cell surface
What is the glycaemic index
Describes blood glucose response after consumption of a carbs containing test food relative to a carbs containing reference food
High GI food-> blood glucose level increases rapidly after consumption
How does oral rehydration therapy function
ORS solution contains glucose, sodium chloride, potassium chloride and trisodium citrate
Solution retains fluid and salts within body via osmotic pressure, prevent excretion, replace lost fluid and salts
SGLT 1 cotransports glucose and sodium into epithelial cells, so when glucose is absorbed in the intestines, water and sodium is absorbed as well
What is the purpose of glycolysis in catabolism
Occurs when ATP is required, even in the absence of O2
Only source of ATP for cells without mitochondria (e.g. RBC)
Describe the action and regulation of hexokinase
Forms G6P from glucose using 1 ATP and giving 1 ADP
Inhibited by product, G6P
What is the difference between hexokinase and glucokinase
Glucokinase is a type of hexokinase only found in liver cells and beta pancreatic cells, has a higher Km value and thus lower affinity to glucose.
Glucokinase only functional at high glucose levels, to slow down glycolysis in the liver, as glucose is used for glycogenesis to generate glycogen
Describe the action and regulation of phosphofructokinase-1
Reacts with F-6-P to form F-1,6-biphosphate
Uses 1 ATP and gives 1 ADP
Positive regulation
- AMP
- F26BP
Negative regulation
- ATP
- Citrate
Describe the action and regulation of glyceraldehyde 3-P dehydrogenase
Glyceraldehyde 3-phosphate—-> 1,3-Bisphosphoglycerate
Using 1 NAD-> NADH+H+ and 1 Pi
Describe the action and regulation of phosphoglycerate kinase
SUBSTRATE LEVEL PHOSPHORYLATION
Using 1 ADP to give 1 ATP
1,3-bisphosphglycerate-> 3-phosphoglycerate
Describe the action and regulation of pyruvate kinase
SUBSTRATE LEVEL PHOSPHORYLATION
Using 1 ADP to give 1 ATP
Phosphoenolpyruvate (PEP)-> Pyruvate
Negative regulators
- alanine
- ATP
- Acetyl CoA
- Phosphorylation of pyruvate kinase by protein kinase A (deactivates pyruvate kinase) in fasting state: glucagon
Positive regulators
- F16BP
- Dephosphorylation of pyruvate kinase by phosphatase in fed state: insulin
What is the net output of glycolysis
2 pyruvate
2 ATP
2 NADH
Describe the fate of pyruvate under aerobic conditions
Pyruvate produced in cytoplasm
Passes through impermeable mitochondria membrane via transporter
Converted to Acetyl CoA in mitochondria by Pyruvate dehydrogenase complex (PDC) and enters TCA cycle
Describe the reaction that pyruvate undergoes with PDC
Pyruvate+CoA+NAD+—-PDC—> CO2+ acetyl-CoA+ NADH
How many enzyme components are present in the PDC
3:
E1- Pyruvate dehydrogenase
E2- dihydrolipoyl transacetylase
E3- dihydrolipoyl dehydrogenase
How is PDC regulated?
Product inhibition by:
- NADH
- Acetyl CoA
Covalent modification:
Positive regulation:
- Phosphatase dephosphorylates E1 to activate PDC, activated by Ca2+ and Mg2+
Negative regulation:
- Kinase phosphorylates E1 to deactivate PDC
Inhibited by pyruvate, ADP and Ca2+
Activated by Acetyl-CoA and NADH
What are the effects of PDC deficiency
High pyruvate and lactate in blood, slightly low glucose
What are the possible treatments of PDC deficiency
Thiamine supplement: Part of PDC
High fat, low carb diet: FA metabolised into Acetyl-CoA without forming pyruvate
Oral sodium bicarbonate: to neutralise acidic environment caused by lactate
Describe the fate of pyruvate under anaerobic conditions
Pyruvate Lactate
NADH+H+—->NAD+
NAD returned to glycolysis
State the 2 paths for the fate of NADH under aerobic conditions
In the heart and liver: malate-aspartate shuttle
In the skeletal muscles and brain: Glycerophosphate shuttle
Describe the Malate-aspartate shuttle
NADH transported into mitochondria via malate-aspartate shuttle
- NADH becomes NAD+ in cytosol when oxaloacetate becomes malate
- Malate transported into mitochondria, forms NAD+ from NADH and becomes oxaloacetate
-e- transferred from malate to NAD+ in mitochondria to form NADH, NADH used in ETC
Oxaloacetate becomes aspartate, glutamate becomes alpha-kg
Describe the glycerphosphate shuttle
NADH transported to mitochondria via shuttle
electron transferred to dihydroxyacetone-P to form Glycerol 3-P, diffuses into the mitochondria
G3P donates electron to FAD to give FADH2.
FADH2 enters electron transport chain for more ATP generation
What is the function of the 2,3 bisphosphoglycerate (2,3BPG) shunt
RBC has no mitochondria, so anaerobic glycolysis is the only ATP source.
2,3 BPG is an allosteric regulator of Hb affinity for O2.
When conc. of 2,3BPG is high, oxygen is more readily released.
When low oxygen pressure at high altitudes, increase synthesis of 2,3BPG in RBCs, increased unloading of oxygen to tissues.
High 2,3BPG in chronic obstructive airway disease
Describe mechanism of 2,3BPG shunt
1,3BPG — BPG mutase —> 2,3 BPG — 2,3 BPG phosphatase —> 3PG
Describe fructose metabolism in liver, kidney and small intestine
Mainly in liver, kidney and small intestine:
fructose—fructokinase —> fructose-1-P— Fructose-1-phosphate aldolase (aldolase B) —> glyceraldehyde—triose kinase—>glyceraldehyde-3-P
ATP consumed in first and third step
Second step produces dihydroxyacetone-P which can change into glyceraldehyde-3-P
Describe fructose metabolism in muscles
Fructose—hexokinase—> Fructose-6-P
What is essential fructosuria?
Fructokinase deficiency
Fructose excreted in urine, no accumulation of toxic metabolites, benign
What is hereditary fructose deficiency
F1P aldolase (aldolase B) deficiency
cellular accumulation of F1P, inhibits glycogenolysis and gluconeogenesis!!
Avoid fructose (treatment)
Describe metabolism of galactose
Galactose—galactokinase—> galactose 1P—galactose 1-phosphate uridylyltransferase—> glucose-1-phosphate—>glucose 6-phosphate
How does galactose 1-phosphate uridylyltransferase work?
Substitutes galactose in galactose-1-phosphate with glucose from UDP-glucose to give UDP-galactose.
UDP-glucose is regenerated by epimerase to form UDP-galactose
What causes type 1 (classical) galactosemia?
No galactose-1P uridylyltransferase activity
- galactosemia
- galactouria
- galactose-1P accumulation
mental retardation, cataract formation, liver damage, kidney failure
What is type II (nonclassical) galactosemia?
Mutation in galactokinase
- galactosemia
- galactouria
cataract formation
What is type III galactosemia
Epimerase mutation
- benign form no symptoms
- severe form, UDP glucose not regenerated, type I symptoms
Where does TCA take place
in the mitochondria matrix
What enzyme catalyses the reaction of oxaloacetate to form citrate?
citrate synthase
Acetyl CoA—> Coenzyme A
Acetyl group donated to form citrate
What regulates citrate synthase
Citrate inhibits citrate synthase (product inhibition)
What enzyme catalyses the reaction of isocitrate to form alpha-kg
Isocitrate dehydrogenase
(oxidative decarboxylation)
NAD+—> CO2+NADH+H+
What regulates isocitrate dehydrogenase
Positive regulators:
- ADP
- Ca2+
Negative regulators:
-NADH
What enzyme catalyses the reaction of alpha-kg to form succinyl-CoA
Alpha-kg dehydrogenase
(oxidative decarboxylation)
CoA-SH+NAD+—>CO2+NADH+H+
What regulates alhpa-kg dehydrogenase
Positive regulator:
-Ca2+
Negative regulator:
-NADH
Which enzyme catalyses the reaction of succinate to form fumarate?
succinate dehydrogenase
FAD—>FADH2
Which enzyme catalyses the reaction of L-Malate to form oxaloacetate?
malate dehydrogenase
NAD+—>NADH+H+
What regulates malate dehydrogenase?
NADH is an inhibitor
Which step of TCA involves substrate level phosphorylation?
Succinyl-CoA—succinate thiokinase—>succinate
GDP+Pi—> GTP+CoA
Name the enzyme complex that allows for the link between glycolysis and TCA cycle via acetyl CoA
PDC
Pyruvate—PDC—> acetyl coA
Why is TCA cycle amphibolic
Catabolic: oxidation of fuel molecules converge at TCA cycle, contributes e carriers for ATP production
Anabolic: intermediates used for synthesis of other molecules
What are the anabolic substrates in the TCA
Citrate: FA synthesis
alphakg and OAA: amino acid
Succinyl-CoA: heme
Malate: gluconeogenesis
What are anaplerotic reactions
Reactions to replenish intermediates removed for biosynthesis
Name one example of an anaplerotic reaction
Conversion of pyruvate to OAA by pyruvate carboxylase
Activated by acetyl-CoA and requires biotin
Biotin is a vitamin synthesised by intestinal bacteria, required in small amounts in humans
Where does ETC take place
On inner mitochondria membrane
What is Complex I
NADH-CoQ reductase
What is complex II
Succinate-Co Q reductase
What are the two mobile electron carriers in the ETC
Coenzyme Q (CoQ) ubiquinone
Cytochrome C (complex III to IV)
What is Complex III
Co Q-cytochrome c reductase
What is complex IV
Cytochrome c oxidase
Complex V
ATP synthase
How are electrons transported
Through prosthetic groups
- FMN Flavin mononucleotide (Complex I)
- Iron-sulfur clusters (FeS) (Complex I and III)
- Heme (containing Fe) within the cytochromes (Complex III and IV)
- Cu (complex IV)
How does Fe in heme and FeS accept and release e-
Charge varies between +2 and +3
What is the net output of the TCA
6 NADH, 2 FADH2, 2GTP for each glucose molecule–> 2 acetyl CoA molecules
Where does NADH and FADH2 enter the ETC
NADH enters at complex I (NADH-CoQ reductase)
FADH2 enters at complex II (succinate CoQ dehydrogenase)
Where is H+ pumped out of matrix across inner mitochondria membrane?
Complex I (4H+) Complex III (4H+) Complex IV (2H+)
How many ATPs are produced by NADH and FADH2
NADH: 2.5 ATPs
FADH2: 1.5 ATPs
What 2 transport systems are used to transport ADP and Pi into mitochondria matrix
1) Pi-H+symport, both ions transported in together, driven by proton gradient.
2) ADP-ATP translocase- export ATP and import ADP
How does complex V (ATP synthase) work
Rotation of c-ring due to passage of protons from inter-membrane space through F0 pore to the matrix mechanically induces conformational changes in the F1 headpiece that synthesizes ATP from ADP and Pi
What are uncoupling proteins
They form channels in the inner mitochondria membrane to conduct protons back into the mitochondrial matrix, disrupts proton gradient while releasing heat without ATP production.
Where is thermogenin found
In brown adipose tissue, allows for non-shivering thermogenesis in babies especially
What is dinitrophenol?
Carries H+ across inner mitochondria membrane to dissipate H+ gradient-> no energy to drive ATP synthesis, electron transport continues, but energy from H+ gradient lost as heat
What is Leigh’s syndrome?
Nerodegenerative disorder
- Complex mutation
- Pyruvate carboxylase mutation-> pyruvate not converted to OAA (anaplerotic reaction) TCA breakdown
- PDC mutation, pyruvate not converted to Acetyl CoA-> accumulation of pyruvate, converted to lactate
LEADS to
High pyruvate and lactate in blood
Where does pentose phosphate pathway (HMP shunt) take place
In the cytoplasm
What is the function of the HMP shunt
Provides Ribose-5-phosphate for nucleotide and nucleic acid synthesis
Provides NADPH for:
- biosynthesis
- counter oxidative stress
- detox reactions by CYP450
- generation of superoxide/nitric oxide to counter bacteria infection in cells
What are the products of the oxidative phase of PPP/HMP shunt
NADPH and 5C sugars
When R5P and NADPH is balanced, no non-oxidative phase takes place.
G6P—G6PD—>6phosphogluconolactone
NADP+—> NADPH+H+
6-phosphogluconate—6-phosphogluconate dehydrogenase—>R5P
NADP+—>NADPH+H+ +CO2
What is the purpose of non-oxidative reaction in HMP
When NADPH is needed, less/no need for R5P
R5P converted to glycolytic intermediates
What happens if R5P is needed and NADPH not needed
Oxidative phase inhibited and glycolytic intermediates converted back to R5P via reversible non-oxidative reactions
What is glutathione? oxidising or reducing agent
Reducing agent, and antioxidant
How does glutathione eliminate oxidants like H2O2
reduced glutathione (GSH) is oxidised by H2O2 to form oxidised glutathione (GSSG) under action of glutathione peroxidase
How is PPP related to GSH
GSSG is converted back to GSH by NADPH
GSSG+NADPH+H+—> 2GSH+NADP+
Effects of G6PD deficiency?
G6PD deficiency, no NADPH produced, less GSSG converted back to GSH, less GSH to remove harmful peroxides, proteins and lipids in RBC oxidised, impairs cell membrane RBC lysis
antimalaria drug (primaquine), fava beans stimulate peroxide formation
What is the difference between the function of hepatic and muscle glycogen
Hepatic glycogen:
- store of glucose for fasting situations to maintain blood glucose levels
Muscle glycogen:
-store of glucose which can be rapidly mobilised and metabolised via glycolysis under anaerobic conditions
What is the difference between the fate of glucose-6P from hepatic and muscle glycogen
Muscle: Glucose-6P enters glycolysis, no glucose-6P phosphatase present in muscles
Hepatic: glucose-6P—Glucose-6P phosphatase—> glucose enters bloodstream
What is the key enzyme for glycogenolysis
Glycogen phosphorylase
Glycogen(n) +Pi—-> glycogen (n-1)+G-1P
What enzyme produces G6P from G1P and vice versa
Phosphoglucomutase
What is a limit branch?
Branch of about 4 glucose, where glycogen phosphorylase can no longer act
What are the 2 functions of debranching enzyme
- Transglycosylase
Transfers alpha1->4 linked tri/quartrosaccharid unit from limit branch to nonreducing end of another branch - Hydrolyse last remaining alpha 1->6 linked glycosyl residue in the branch to yield GLUCOSE
What is the key enzyme for glycogenesis
Glycogen synthase
UDP-glucose+PPi+glycogen (n)—Glycogen synthase—> glycogen (n+1) + UDP
glycosyl residues added from UDP-glucose to non-reducing ends of glycogen chain via alpha,1->4 linkages
What does UDP-glucose pyrophosphorylase do?
Glucose-1P+UTP—-UDP-glucose pyrophosphorylase—> UDP-glucose+PPi
What happens when chains reach 11 residues
Branching enzyme cleaves 6-8 residue piece and reattached to another glycosyl unit by alpha-1,6 glycosidic bond
Glycogen is highly-branched molecule
How is muscle glycogen synthase regulated
+ Glucose-6-phosphate
How is muscle glycogen phosphorylase regulated
+ AMP, Ca2+
- ATP, Glucose-6-phosphate
How is liver glycogen phosphorylase regulated?
-ve glucose
What is the effect of glucagon on glycogen
Activate glycogen breakdown in LIVER to maintain blood glucose levels during fasting
Activates phosphorylase kinase to phosphorylate (activate) glycogen phosphorylase and various kinases to phsophorylate (deactivate) glycogen synthase
What is the effect of insulin on glycogen
Activate glycogen storage in LIVER AND MUSCLE during fed state
Activates phosphoprotein phosphotase I (PPI) which removes phosphate group from both glycogen synthase (activates) and glycogen kinase (deactivates)
What is the effect of epinephrine on glycogen
Activate glycogen breakdown in liver (to maintain blood glucose levels) and muscle (to provide energy for muscle movement)
Activates phosphorylase kinase to phosphorylate (activate) glycogen phosphorylase and various kinases to phsophorylate (deactivate) glycogen synthase
What is an allosteric activator of hepatic PP1?
Glucose
Why is phosphofructokinase deficiency considered a glycogen storage disease?
PFK deficiency results in F6P not being converted to F16BP, so glycolysis cannot take place. Thus there is no need for Glucose-6P and glycogenolysis will not take place.
What is gluconeogenesis?
Synthesis of glucose from non-carb sources (even chain fatty acids {metabolised to acetyl CoA} and acetyl CoA {cannot be converted back to pyruvate} not precursors)
What is the purpose of gluconeogenesis?
To maintain blood glucose especially in fasting state/ especially for tissues with little/ no mitochondria (brain, lens of eye, rbc)
Where does gluconeogenesis take place?
90% in liver, 10% in kidney, where there is glucose-6-phosphatase (glucose-6P—> glucose)
Takes place mainly in cytosol, minority in mitochondria
Is gluconeogenesis a reversal of glycolysis?
No. Different key enzymes.
Name the key enzyme of gluconeogenesis that converts Glucose 6-phosphate to glucose
Glucose-6-phosphatase
Name the key enzyme of gluconeogenesis that converts fructose 1,6 bisphosphatase to fructose 6-phosphatase
Fructose-1,6-bisphosphatase
Name the key enzyme of gluconeogenesis that converts OAA to Phosphoenolpyruvate (PEP)
Phosphoenolpyruvate carboxykinase
Describe the mechanism by which pyruvate enters gluconeogenesis pathway
Pyruvate formed in cytosol, transported to mitochondria
Pyruvate—pyruvate carboxylase—>OAA
OAA transported to cytosol through malate-aspartate shuttle
OAA—> PEP—> Glucose
Name the precursors for gluconeogenesis
Glycerol
Lactate
Glucogenic amino acids
Where is glycerol from, and how does it enter gluconeogenesis pathway
TG in adipocytes broken down during lipolysis-> release FA and glycerol-> transported to liver
glycerol—glycerol kinase—> Glycerol 3-P—>dihydroxy acetone phosphate
phosphate provided by ATP
Glycerol 3-P converted to DHAP by NAD
Where is lactate from, and how does it enter gluconeogenesis pathway
Lactate is produced by anaerobic glycolysis via lactate dehydrogenase from pyruvate.
Lactate—lactate dehydrogenase—> pyruvate,
producing NADH
pyruvate—pyruvate carboxylase—> OAA
When does the cori cycle take place?
When ATP demand at tissues is greater than capacity of mitochondria’s oxidative phosphorylation.
Anaerobic glycolysis takes place in skeletal muscle/RBC, lactate transported to liver for gluconeogenesis, glucose produced transported back to muscles
Where are glucogenic amino acids from, and how does it enter gluconeogenesis pathway
During prolonged fasting, proteolysis occurs.
Glucogenic amino acids enter TCA cycle at different points, converted to OAA
How is alanine converted to pyruvate?
By alanine aminotransferase (ALT) via transamination.
alpha kg converted to glutamate via transamination catalysed by ALT at the same time
Name one key allosteric regulator of both glycolysis and gluconeogenesis in the liver
Fructose-2,6-bisphosphate
What is the effect of insulin on fructose-2,6-bisphosphate levels?
When insulin is produced in wellfed state, activates phosphatase, dephosphorylates and activates PFK2, F6P—> F26BP
What is the effect of glucagon on F26BP levels
When glucagon is produced in starving state, activates kinase, phosphorylates and activates fructobisphosphatase 2, converts F2,6BP to F6P, lower F26BP
How does F26BP regulate FBPase and phosphofructokinase (PFK)
It is an inhibitor of FBPase and an activator for PFK
What are the inhibitors for FBPase
- AMP
- F26BP
What are the activators for phosphofructokinase?
- AMP
- F26BP
What are the inhibitors for phosphofructokinase?
- ATP
- Citrate
What is type 1 glycogen storage disease?
Von Gierke’s disease, Glucose-6 phosphatase deficiency.
G6P cannot be converted to Glucose
Gluconeogenesis impaired, low glucose levels, hypoglycemia
Glycogenolysis impaired, liver glycogen phosphorylase activated by glucagon, but Pi trapped within G6P
What does fructose bisphosphatase deficiency cause?
F16BP cannot be converted to Fructose 6 Phosphate.
Gluconeogenesis cannot take place.
Hypoglycemia, metabolic acidosis upon fasting. Lactate and pyruvate accumulation
What does pyruvate carboxylase deficiency cause?
Pyruvate cannot be converted into OAA for gluconeogenesis, OAA obtained only from amino acids
Hypoglycemia.
Anaplerotic reaction breakdown, TCA disrupted
Pyruvate undergo anaerobic glycolysis, lactate, lactic acidosis
