Reciprocal Regulation of Glycolysis and Gluconeogenesis PDF

Preview

Summary

How to turn glycolysis and gluconeogenesis on and off and what stimulates/ inhibits the stages 1, 3 and 10 of these processes....

Description

RECIPROCAL REGULATION OF GLYCOLYSIS AND GLUCONEOGENESIS These processes can occur at the same time as each process releases a set amount of energy in the cell = SPONTANEOUS EXERGONIC REACTIONS if sum of these set energy releases. Allosteric enzymes, however, prevent these reactions from occurring at the same time. GLUCONEOGENESIS:   

X4 ATP molecules used X2 GTP used Exergonic (release energy)

GLYCOLYSIS: X2 ATP molecules produced.

OVERALL – X2 ATP, X2 GTP so if at same time, use up ATP and none, new formed.

IF CELL HAS HIGH ENERGY CHARGE (HIGH ATP: AMP) – FAVOURS GLUCONEOGENEIS = stops producing ATP molecules = halts glycolysis. But it begins to make new glucose molecules via gluconeogenesis as this USES UP THE EXCESS ATP FROM GLYCOLYSIS (X4 ATP +X2 GTP used in gluconeogenesis). Use this to form glycogen. GLUCONEOGENESIS ON: STEP 10 - PYRUVATE CARBOXYLASE:  ACETYL COA ACTIVATES– increases efficiency of this enzyme in converting pyruvate to oxaloacetate.  This is also needed for TCA cycle so if we have excess acetyl CoA, there’s plenty to go around for the TCA cycle and therefore we can use the excess for gluconeogenesis.

STEP 3 – FRUCTOSE-1,6 -BPASE: 

CITRATE ACTIVATES – intermediate of TCA cycle, again with this molecule in excess it tells the cell that there are plenty of molecules to go round for the TCA cycle so don’t need to make any more glucose and can switch to gluconeogenesis.

GLYCOLYSIS OFF: STEP 3 – PFK-1: inhibit PFK1, inhibit HEXOKINASE TOO.   

ATP INHIBITS – inhibited by high ATP: AMP, ATP bids to allosteric site on PFK1 which inhibits its activity. ATP builds up. G-6-P INHIBITS = F-6-P + G-6-P are in equilibrium, an increase in one increases the other = both now at high concentration = inactivates HEXOKINASE (STEP 1).

STEP 3 – CITRATE AND H+ IONS



H+ IONS INHIBITS – in skeletal muscles cells when the rate of glycolysis exceeds the rate of oxidative phosphorylation = build-up of lactic acid (fermentation) which results in an increase in H+ ions. Also inhibits PFK1 activity. NOT TRUE FOR LIVER CELLS = H+ doesn’t affect PFK1 activity as liver cells can transform lactate into pyruvate



CITRATE INHIBITS IN LIVER CELLS – binds to PFK1 = tells cell to stop glycolysis. Plenty of ATP = plenty of pyruvate which under aerobic conditions forms citrate in CAC cycle. Lots of citrate = no more pyruvate needed.

STEP 10 – PYRUVATE KINASE:  



ATP INHIBITS – inhibits like for PFK1. ALANINE INHIBITS – lots of ATP = plenty of pyruvate = plenty of alanine. So, there is a negative feedback loop in which a high alanine conc. = decrease in pyruvate kinase activity due to inhibition. PHOSPHORYLATION INHIBITS IN LIVER CELLS – L isoenzyme form can be phosphorylated. This signals low glucose levels so switches over to gluconeogenesis (via the glucagon cyclic AMP cascade).

So, lots of ATP = gluconeogenesis in order to use glycerol, lactate and amino acids to form glucose.

IF CELLS HAVE A LOW ENERGY CHARGE (LOW ATP: AMP) – GLYCOLYSIS FAVOURED Under these conditions, we want to be able to make a net amount of ATP to increase its level inside our cells. This means glycolysis will be activated but gluconeogenesis will be turned off.

GLUCONEOGENESIS OFF: STEP 10 – PYRUVATE CARBOXYLASE 

ADP INHIBITS – inhibits PEP carboxylase too.

STEP 3 – F-1,6-BPASE:  

AMP INHIBITS – now have lots of AMP in cell instead of ATP. AMP binds to F-1,6BPase and inhibits it. Local signal. F-2,6-BPase INHIBITS – inhibits the phosphatase (which takes away the Pi) and activates the kinases (PFK1, adds a Pi which is needed to make pyruvate). Global signal.

ALL ABOUT F-2,6-BPase : Side reaction of F-6-P is the production of F-2,6-BP via the PFK2 enzyme. The opposite reaction is catalysed by F-2,6-BPase. When blood sugar is high, we want to have lots of F-2,6-BP (as inhibits gluconeogenesis, stimulates glycolysis to reduce amount of glucose by storing as fat). Overrides effects of ATP on gluconeogenesis.

PFK2 and F-2,6-BPase are part of the SAME PROTEIN. The complete protein is a bifunctional/tandem enzyme.   

REGULATORY DOMAIN – 1-32 A.A that we can phosphorylate to regulate which enzyme we are using. KINASE REGION (35 – 250 A.A) – is the PFK2 enzyme which adds a Pi to F-6-P to become F2,6-BP. PHOSPHATASE DOMAIN (250 – 450 A.A) – is the F-2,6BPase enzyme which takes away the Pi degrading the F-2,6-BP.

DOMAIN KINASE PHOSPHATASE

ENZYME PFK2 F-2,6-BPase

GLYCOLYSIS ON OFF

GLUCONEOGENESIS OFF ON

When phosphorylated – kinase domain off, phosphatase domain on (as has a Pi) = phosphorylated tandem enzyme degrades F-2,6-BP back to F-6-P = GLUCONEOGENESIS. When unphosphorylated – kinase domain on, phosphatase off (as no Pi) = production of F-2,6-BP = GLYCOLYSIS. SERINE RESIDUE is what is phosphorylated. WHAT CONTROLS PHOSPHORYLATION OF THE TANDEM ENZYME? Prescence of GLUCAGON will TURN ON GLUCONEOGENESIS and the presence of F-6-P (and the absence of glucagon) will TURN ON GLYCOLYSIS. DURING NIGHT/FASTING:    

Rise in glucagon Triggers cyclic AMP cascade  phosphorylation of tandem enzyme by protein kinase A (PKA) PKA adds a Pi group to REGULATORY DOMAIN = turns off kinase, turn on phosphatase = low levels of F-2,6-BP = GLUCONEOGENESIS ON  In addition glucagon stimulates PEP carboxykinase and F-1,6-BPase.

RISE IN INSULIN SECRETION/GLUCOSE PRESENT (after a meal?)    

Triggers a different signal pathway Activates the protein phosphatase (PP1) = removes Pi group from REGULATORY DOMAIN Turn on kinase, turn off phosphatase = high levels of F-2,6-BP = GLYCOLYSIS ON, glucose stored as fat.  In addition insulin activates PFK1 + pyruvate kinase

GLYCOLYSIS ON: when ATP levels are low, glycolysis is favoured. STEP 3 – PFK1  

AMP ACTIVATES – as lots of AMP and low ATP, F-1,6-BPase is inactivated in gluconeogenesis and PFK1 is activated in glycolysis. F-2,6-BP ACTIVATES – overrides ATP signal and turns on PFK1 in HIGH glucose concentrations.

STEP 10 – PYRUVATE KINASE 

F-1,6-BP ACTIVATES – PFK1 has been activated so there is a build-up of F-1,6-BP, positive feedback loop so an increase in F-1,6-BP = increase in pyruvate kinase activity.

NOTES: HIGH BLOOD GLUCOSE/ENERGY NEEDED = glycolysis in liver + muscle = energy but also produces acetyl CoA so the glucose is converted to fatty acids and glycerol. LOW BLOOD GLUCOSE/TOO MUCH ENERGY = gluconeogenesis in liver + kidneys = more glucose for muscle and brain.

G-6-PASE ON – when high G-6-P as more of this turns off hexokinase.

GLUCOGENESIS ACTIATED – build-up of acetyl CoA = low blood sugar

LIVER PYRUVATE KINASE – increase in F-1,6-BP activates, if glucagon secreted F-1,6-BP won’t be present as dephosphorylated to F-6-P. (PKA adds a Pi to this enzyme and turns it off in the presence of glucagon).

Insulin and glucagon effects is just in the liver == global signals ATP + AMP occur in both muscle and liver == local signals....