Carbohydrate Catabolism (glyc to ETC) Flashcards
What cells have an absolute glucose requirement?
Red blood cells, neutrophils, innermost cells of kidney medulla, lens of the eye, cells of brain
uptake depends on glucose concentration
How does the brain cope during periods of starvation?
-it can use ketone bodies for some energy requirements but needs time to adapt.
Stage 1 of carbohydrate catabolism is breakdown to monosaccharides. What enzymes are involved here?
- salivary amylase breaks down starch & glycogen into dextrins.
- pancreatic amylase further breaks them down into monosaccharides
- membrane bound disaccharidases on brush border of SI contain lactase, sucrase & amylase and aid breakdown to monosaccharides.
Why can’t humans hydrolyse cellulose?
-we lack the enzyme to break down the beta 1-4 linkages
There are three types of lactose deficiency. Name them & describe them.
1- primary lactase deficiency= absence of lactase persistent allele, only in adults, most common
2- secondary lactase deficiency= caused by injury to SI, eg coeliac disease, Crohns, usually reversible.
3- congenital lactase deficiency= autosomal recessive gene defect in lactase gene, can’t digest breast milk, v rare
What are symptoms of a lactase deficiency?
-bloating, flatulence, vomiting, diarrhoea.
How are Na+ absorbed into the intestinal epithelial cells ?
- Active transport (low to high conc)- via Na+ dependant glucose transporter SGLT1
- Passive transport (high to low conc)- via GLUT2 into blood
How is glucose taken into blood & then target cells ?
- first into the intestinal cell via active transport ie Na+ Glucose co transporter, then from intestinal cell into blood via passive transport via GLUT2.
- then from blood to target cells via facilitated diffusion using transport proteins (eg GLUT1, GLUT2! GLUT3 etc)
- GLUT4 is insulin regulated
What is stage 2 of carbohydrate catabolism and what is the key thing that happens here? Where does glycolysis happen?
- breakdown to metabolic intermediates
- the release of ‘reducing power’ (NADH) and some ‘energy’
- intracellular, cytosolic (cytoplasm of the cell)
Say the steps of glycolysis.
-6C glucose phosphorylated with ATP to glucose-6-phosphate. This increases reactivity of glucose. Enzyme needed for first phosphorylation= hexokinase
Then from glucose6phosphate to fructose6phosphate
, then fructose6phosphate to fructose 1,6-bisphosphate which is a phosphorylation using ATP. 2nd enzyme=phosphofructokinase-1 ie PFK. THIS IS THE COMMITING STEP because it’s where glucose is first committed to the metabolic pathway.
-then glucose is cleaved into x2 3C molecules
- small amount of NADH captured
-then, substrate level phosphorylation to return the Pi for ATP production.
-again, another molecule of ATP produced (2 for a glucose). Large exothermic reaction so is irreversible
-in the end we have x2 pyruvate, x2 ATP net, x2 NADH. all for x1 glucose molecule
What are the products of glycolysis?
- x2 NADH
- x2 ATP
- x2 pyruvate
Features of glycolysis:
- occurs in all tissues (cytoplasm)
- exergonic, oxidative
- no loss of CO2
- can operate anaerobically with the aid of lactate dehydrogenase
- irreversible pathway
What is the benefit of having many steps in glycolysis?
Hint- metabolic benefit
- chemistry is easier in smaller steps
- efficient energy conservation
- gives versatility (allows interconnections, allows, part can be used in reverse)
- can be controlled
What are the two important intermediates in glycolysis?
1) Glycerol phosphate(made from DHAP, via enzyme glycerol-3-phosphate dehydrogenase): important to triglyceride and phospholipid biosynthesis, produced in adipose and liver.
2) 2,3-bisphosphoglycerate
(2,3-BPG)-made from 1,3 bisphosphoglycerate via enzyme bisphosphoglycerate mutase ): produced in RBC , regulator of Hb O2 affinity i.e promotes release of O2, reduces oxygens affinity for Hb.
What is the enzyme responsible for the production of a) glycerol phosphate
b) 2,3-BPG
a) glycerol-3-phosphate dehydrogenase
b) bisphosphoglycerate mutase
What is glycolysis dependant upon to continue?
- when all NAD is converted to NADH
- this is why when the NAD runs out it must be found another way. (Lactate)
How & why is lactate produced?
NADH+pyruvate-> NAD+Lactate
ENZYME= lactate dehydrogenase
-in skeletal muscle & RBCs, pyruvate -> lactate via LDH to restore NAD+ needed for glycolysis to continue.
-lactate metabolised in liver and heart (in heart, lactate-> pyruvate via LDH for energy as it has lots of O2, in liver lactate-> pyruvate via LDH is NOT used for energy, instead pyruvate is converted back to glucose via GLUCONEOGENESIS) - if enzyme/ vitamins deficient eg thiamine, won’t work.
-main aim to regenerate NAD w/o O2
Define hyperlactaemia & lactic acidosis.
- normal lactate conc in blood is less than 1mM
- hyperlactaemia= 2-5mM, below renal threshold, no change in blood pH
- lactic acidosis=above 5mM, above renal threshold, blood pH lowered
Outline fructose metabolism & location of metabolism. Give a clinical example of this going wrong
- Fructose phosphorylated by fructokinase to fructose-1P.
- Aldolase breaks it down into DHAP & glyceraldehyde. (Metabolised in liver)
- essential fructosuria- fructokinase missing so fructose passes into urine, no other problems
- fructose intolerance= aldolase missing, fructose-1P accumulates in liver, liver damage, must remove fructose from diet.
Outline galactose metabolism & give location of metabolism.
- in liver
- galactose phosphorylated to galactose-1P by galactokinase.
- then, galactose-1P converted to glucose-1P via galactose-1-P uridyl transferase. This requires UDP-glucose conversion to UDP-galactose via UDP-galactose4’-epimerase.
How does galactosaemia develop?
happens bc of deficiency in any of the enzymes.
- galactokinase deficiency=rare, galactose accumulates
- transferase deficiency= common, galactose & galactose-1P accumulate- harms liver, kidney & brain.
- this causes galactose to enter other pathways:galactose covered to galactitol (unwanted) via enzyme aldose reductase in the process converting NADPH -> NADP+
- depletes NADH levels, prevents maintenance of free SH groups on proteins, improper disulphides bond formation, loss of structural & functional integrity, forms cataracts
- remove lactose from diet
Outline the pentose phosphate pathway.
1-starts w glucose-6-P oxidatively decarboxylated to C5 sugar & CO2 via reduction of NADP -> NADPH. ENZYME= glucose-6-P-dehydrogenase
2-then 5C sugar rearranged to Fructose 6 phosphate (the next step in glycolysis) or G-3-P (further down in glycolysis) which then enter glycolysis.
CO2 expulsion means this process is irreversible
*no ATP produced, controlled by NADP/NADPH ratio-if NADPH high, it will stop as the NAD+ will be low.
What are the functions of the pentose phosphate pathway?
1-produces NADPH in cytoplasm which is a reducing power used in lipid & steroid synthesis so lots found in adipose AND maintains free -SH groups, preventing inappropriate bonds.
2-produces C5 sugar for nucleotides so lots found in dividing tissues
How does a glucose-6-phosphate deficiency affect the pathway?
- cannot convert NADP+ to NADPH so there’s a low conc of NADPH in blood.
- this means it cannot maintain a proteins structural integrity & inappropriate S-S bonds formed, cataracts & RBC clump together causing haemolysis.
What is meant by allosteric regulation of enzymes?
- activator/inhibitor binds away from the active site.
- covalently modifies the enzyme(phosp/dephospry) which alters its activity
Metabolic pathways can be regulated. What is the significance of an irreversible step?
- irreversible steps are potential sites of regulation, reduced activity reduces flux of substrates through the pathway directly.
- reversible steps cannot be regulated
Feedback inhibition is a type of product inhibition. Explain its mechanism.
- where the final product of a pathway inhibits the enzyme at the start of the pathway.
- reduces substrate and more intermediates reduce the build up
What is the committing step?
- the step at which the pathway branches
- inhibition of the commuting step allows the substrate to be diverted into other pathways.
How are inhibitory & stimulators regulators activated?
- inhibitory= activated by high energy signals eg ATP, NADH, FAD2H.
- stimulatory= activated by low energy signals eg AMP, ADP NAD+, FAD
Outline hormonal regulation of metabolic pathways. Give 2 examples.
-hormone receptor binding activated signalling pathway.
-Protein kinase phosphorylates or protein phosphotase dephosphorylates.
-alters protein conformation.
eg adrenaline stimulates glycogenesis
eg insulin stimulates glucose utilisation AND inhibits glycogen breakdown
What does the term ‘feed forward’ mean in relation to metabolic pathway regulation?
-substrate at start of pathway provides positive allosteric signal to a later enzyme to activate the pathway
What enzyme is the key regulator of glycolysis? What process does it catalyse? Where is the committing step of glycolysis?
phosphofructokinase-1 (PFK1)
fructose-6P -> Fructose1,6-bisphosphate
-committing step= PFK1
hexokinase is not the committing step as its products are diverted to other pathways eg pentose phosphate pathway
What is the process of phosphoregulation of PFK1 & pyruvate kinase?
Hint; insulin & glucagon.
- insulin (well fed hormone)- activates protein phosphotase1 which activates PFK1
- glucagon (starvation hormone), activates protein kinase A and inhibits PFK1 to save precious glucose from entering glycolysis
Outline metabolic regulation of PFK1.
- high energy signals will inhibit PFK eg ATP,NADH
- low energy signals activate PFK eg AMP,ADP,NAD+
Before pyruvate app an enter the Krebs cycle, it must be converted to Acetyl CoA. How does this occur?
Pyruvate+CoA+NAD+ -> acetyl CoA+CO2+NADH via ENZYME pyruvate dehydrogenase
- happens in mitochondrial matrix, enzymes require vitB cofactors
- irreversible
Outline regulation of the Link reaction (pyruvate dehydrogenase)
- PDH activated by low NAD+, pyruvate, ADP, insulin (dephosphorylation)
- PDH inhibited by Acetyl CoA, ATP, citrate, NADH (phosphorylation)
Outline the Krebs cycle.
allows interconversion to supply other pathways
MITOCHONDRIAL
- oxaloacetate (C4) + Acetyl CoA (C2) -> citrate (C6), co2 removed& NAD-> NADH, co2 removed& NAD-> NADH, GDP to GDT (ATP), FAD -> FADH, NAD-> NADH
- oxidative therefore requires O2 to work
What are the products of the Krebs cycle for 1 glucose molecule?
x2 GTP (ATP) x6 NADH x2 FADH x4 CO2 x1 glucose
How is the Krebs cycle regulated by the different signals?
- activated by low energy signals eg ADP,AMP,NAD+, pyruvate, FAD+, insulin
- inhibited by high energy signals eg ATP, citrate, NADH,FADH
How is reducing power used in ATP synthesis?
1- electron transport
2- oxidative phosphorylation
Outline the electron transport chain.
What’s the difference in energy between NADH &FADH?
- electrons are transferred from NADH & FADH to O2., releasing energy.
- some of this energy is used to drive H+ across the membrane into the inter membrane space. The gradient formed here is called the proton motive force.
- H+ ions must return into the mitochondria down its electrochemical gradient.
- they travel through the ATP synthase using energy from the pmf & this drives ATP synthesis.
- NADH has x3 proton carriers, FADH has x2 therefore 2NADH makes 5ATP compared to 2FADH makes 3ATP
How is oxidative phosphorylation regulated?
-when [ATP] high, [ADP] is low so there’s no substrate for ATP synthase, inward flow of H+ stops & stays in IMS, pumping stops.
Describe inhibition & uncoupling of oxidative phosphorylation.
- inhibitors eg cyanide. Has higher affinity than O2 so acts as a competitive inhibitor. Blocks flow of e into IMS so no pmf created, no ATP made.
- uncouplers increase permeability of inner mitochondrial membrane to H+ so dissipates gradient & reduces pmf, no drive for ATP synthesis
- rest of energy lost as heat eg brown adipose tissue has uncouplers*
Compare & contrast substrate level and oxidative phosphorylation.
Oxidative- needs membrane, indirect coupling (via pmf), needs O2, makes most ATP
Substrate level- requires soluble enzymes (cytosolic), direct (phosphoryl group transfer), can happen for a little w/o O2, only produces a little of ATP
