Carbohydrate metabolism Flashcards
what are concepts of metabolism
- Metabolism is the sum of chemical transformations
- Metabolic pathways involve enzymes
- Catabolism – degradation of biomolecules
- Anabolism – synthesis of biomolecules (aa, sugars, FA -> prot, lipids, Nucleic acids)
- Metabolism is the sum of reactions
- Catabolism is the breakdown of molecules
- Anabolism is the synthesis (biosynthesis) of molecules
- Rate-limiting steps determine the overall speed of a pathway
- Rate-limiting steps represents points of regulation, and are exergonic
- Being (strongly) exergonic makes reactions essentially irreversible
what are the types of pathways
- most catacolic pathways end up at the final common product, in this example acetate
- anabolic start from common product
what are the rate limiting steps
- Rates of biochemical pathways depend on the activities of enzymes that catalyze each step
- In any pathway, the rate of most steps is limited by substrate availability (enzyme is in excess)
*so much enzyme present that the pathway will never be saturated
- However, reactions catalysed by one or more enzymes in any pathway will be limiting
*enzyme esists at a concentration that can be saturated, can only function at certain rate
- These rate limiting steps will set the overall speed of the pathway
how are pathways regualted
- regulated at rate limiting steps
- Rate-limiting steps are often exergonic and irreversible under cellular conditions
- This allows the cell to regulate the overall rate of metabolic pathway without regulating every single enzyme involved
*regulate a few key points, too energetically demanding to reg all
ways to regulate:
- regualte role of translation of mRNA, pause translation or degrade
- ubiquitination (attach many ubiquintin mol onto enzyme you want degraded
- combine enzyme with regulatory protein (prot attaches to shut off enzyme)
- allosterif effectors ot inc or dec rate
- enzyme binding to substrate, stops ability to continue rxn
- put it into nucleus so cant acess substrate
**important
glycolysis
- production of ATP from glucose: preparatory phase
Boxed reactions are highly exergonic: hexokinase, phosphofructokinase-1, pyruvate kinase
These are the steps in glycolysis that are regulated
- both first ans last reaction are regualted, hexokinase and pyruvate kinase
how is hexokinase inhibited
- allostericaly inhibited by its product
Hexokinase catalyzes the reaction that allows entry of glucose into glycolysis
*hexokinase 2 and 3 are the same as hexokinase 1
In muscle, hexokinase I is expressed
- normally has maximal activity
- if [Glucose 6-P] increases, enzyme is inhibited (-ve feedback)
In liver, hexokinase IV (glucokinase) is expressed
- an isozyme (different gene), therefor can be independently regulated
- lower affinity for glucose: we dont want liver competing for glucose that needs to be in muscle
- inhibited by fructose 6-P not glucose 6-P
- inhibition by fructose 6-P effected through glucokinase regulatory protein
comment on the K0.5 value of hexokinase IV
- relatively poor
- The muscle enzyme (I) does not increase its rate when blood [glucose] is higher than optimal (~5 mM), not opperative at full capacity
- Hexokinase IV has a much higher Km (~10 mM),
- With hexokinase IV, the liver responds directly to increasing blood [glucose] with increased turnover
- when dec blood glucose, hexokinase 1 less impacted then hexokinase IV
How can enzymes be regulated
- alter K0.5 change enzyme response to conc of substrate
- in high activity r state, half V max is reach with very low substrate concentration
- need way more substract in lo activity strate to rach Vmax, and only a bit of substrate in high activity in high activity
Alter Vmax
- enzyme interacts the same
- inc rate at which enzyme works, K0.5 is still the same but Vmax is higher
- when we inhibiit, k0.5 same (same amount substrate needed) but 1/2 Vmax i much lower
*
how is hexokinase IV regulated
- When [fructose-6-P] is high, glucokinase regulatory protein sequesters hexokinase IV in the nucleus
High [glucose] weakens the enzyme/regulator interaction, encouraging cytosolic localization
Glucose promotes cytoplasmic localization and fructose6phos does nuclear localization
Glucose comes thru blood tream, need transporter to bind into cytoplasm, no glucose in nucleaus
Fructose6phos interacts with regualtory protein in cytoplasm, this causes conformational change of regualtory exposing binding site for hexokinase 4, binds it in cytoplasm causing another conformational chnge exposing nucelar localization signal then transports into nucleus
explain phosphofructokinase 1 allostery
Glucose-6-P has several possible fates in the cell
Phosphorylation by PFK-1 commits fructose 6-phosphate (in equilibrium with G6P) to glycolysis
- ATP and citrate inhibts
- AMP, ADP a and fructose 2.6 bisphospahte activates
Allosteric regulation of PFK-1 is complex:
- ATP binds to an allosteric site on PFK-1 and lowers affinity for fructose 6-P
- ADP and AMP relieve inhibition by ATP
- Citrate increases the inhibition by ATP
- fructose 2,6-bisphosphate is a strong activator
how is PFK-1 regualted by ATP
- High [ATP] greatly reduces the affinity of PFK-1 for fructose 6-phosphate
- When [ATP] is low, higher F6P affinity allows PFK-1 to be more active
how is pyruvate kinase inhibited by ATP
- Pyruvate kinase (PK) catalyzes the last step in glycolysis
- PK transfers Pi from phosphoenolpyruvate to ADP
- This yields pyruvate and a molecule ofATP
- High [ATP] allosterically inhibits PK, decreasing its affinity for PEP
what inhibits pyruvate kinase besides ATP
Acetyl-CoA and long-chain fatty acids also inhibit PK
- important fuels for citric acid cycle
- when plentiful, so is ATP
• Other allosteric modulators of PK:
- alanine (-)
- F 1,6-BP accumulation (+)
summarize the pyruvate kinase regulation
(liver and normal tissues)
in liver:
- extra form of regulation
- reg by PKA, if glucagon is too high it activtes PKApulling phos off atp to render pyruvate kinase inactive
- pyruvate phosphatase (PP) does off using water to pull off phosphat gorup
in all glycolytical tissues inc liver:
- is F16BP pos inc pyruvate kinase
- acetyl coA, long chain fatty acids and ATP allosterically inhibit
- alanine is neg inhibitor bc of transamination reaction
why si pyruvate kianse unqiue
- kinase typically grab phos from ATP and sticks onto target
- in this case pyruvate kinase takes phosphate off phosphoenol pyruvate and phosphorylates the ADP
what are the precursors to gluconeogensis
The main precursors for gluconeogenesis are
– lactic acid (via pyruvate)
– glycerol (from lipids)
– some amino acids (termed glucogenic a.a.)
how do steps in gluconeogenesis relate to steps in glycolysis
In gluconeogenesis, 7 of 10 steps are the glycolytic reactions but run in reverse
- the 3 irreversible steps of glycolysis are bypassed
- Here, different enzymes catalyze one or more different steps to enable the reverse reaction
*the 7 steps are just run in reverse, still not regulated
summarize step 1 gluconeogenesis
- 2 reactions generating phosphoeol pyruvate
- Conversion of pyruvate to phosphoenol- pyruvate (PEP) (bypass for step 10 of glycolysis)
- why two pathways? depends on source of pyruvate, if not underoing anerobic exercise already have pyruvate to directly import into mitochondira
- if excersiing and most pyruvate is consumed, have lots of lactate and use that as source for pyruvate, use lactate dehydrogenase
explain step 8 of gluconeogenesis
Fructose 1,6 bisphosphatase (FBPase-1) converts F 1,6-BisP to F 6-P (bypass for step 3 of glycolysis)
• This reaction is a phosphatase reaction (exergonic)
explain step 10 of gluconeogenesis
Glucose-6-phosphatase catalyzes the dephosphorylation of Glucose 6-phosphate (bypass for step 1 of glycolysis)
• This enzyme is expressed in few tissues (liver, kidney, small intestine) = gluconeogenic tissues
*we are dephosphorylating w/ phosphatase
how are glycolysis and gluconeogenesis regulation regulated
its coordinated
-Glycolysis and gluconeogenesis are opposing cellular processes
Running both in parallel would simply waste energy
Regulation of these two processes is therefore coordinated
how does Fructose 2,6-BP allosterically regulates PFK-1 and FBPase-1
- in a reciprocal manner
- pos enhances PFK-1 and neg enhances phosphatase
- if we ahve this molecule, the accumulation of fructose 2,6 bisphose you inc glycolysis and dec gluconeogenesis
- if in gluconeogenic tissue, a decrease of frutose 2,6 bis phos you relase inhibition on phosphatase *take away activation provided in its presence
graph representation of how Fructose 2,6-bisphosphate greatly increases phosphofructokinase-1 activity
Phosphofructokinase-1 has low affinity for fructose-6-P in the absence of F2,6BP
* in absense of F26BP, wihtout this mol need sig conc of substrate before you see any activity
In the presence of F2,6- BP, affinity increases more than 100x (note log scale)
graphical representation of how Fructose 2,6-bisphosphate drastically reduces fructose 1,6-bisphosphatase activity
Fructose bisphosphatase-1 has high affinity for fructose- 1,6-BP in the absence of F2,6BP
In the presence of Fructose 2,6-BP, affinity for Fructose 1,6-BP decreases more than 10x
explain Fructose 2,6-bisphosphate control
Fructose 2,6-BP concentration is controlled by two opposing enzyme activities:
– phosphofructokinase-2 (PFK-2)
– fructose 2,6-bisphosphatase (FBPase-2)
*these two enzymes are actualy the same protien
explain PFK-2/ FBPase-2 regulation
PFK-2/FBPase-2 is a bifunctional enzyme where the two activities are reciprocally regulated:
– Phosphorylation by protein kinase A (in response to glucagon) activates FBPase-2 and inactivates PFK-2
– Dephosphorylation by phosphoprotein phosphatase (in response to insulin) activates PFK-2, and inactivates FBPase-2
– Xylulose 5-phosphate (pentose pathway) also allosterically upregulates phosphoprotein phosphatase
*only 1 of 2 domains can be active at once, either kinase or phosphatase never both active at once
*presence of glugacon activates
THIS FIGURE ARROW GOING THE WRONG WAY FROM PYRUVATE TO LACTATE
- SHOULD BE NAD+ TO NADH
what does acetyl CoA dof or step 10 glycolysis/ step 1 glucoenogensis
- reciprocally regulates
- indicates energy levels are being met can can slow down glycolysis
- Pyruvate kinase is allosterically activated by F1,6-BP – the first molecule committed to glycolysis
- Pyruvate kinase is allosterically inhibited by ATP, acetyl-CoA, long chain fatty acids and alanine (1 step from pyruvate)
- These all signal abundant energy
- The liver has a different pyruvate kinase isoform
*comes from same gene but will be process differently (alternavtive splicing maybe)
- This isoform is phosphorylated by PKA in response to the hormone glucagon (which signals low blood sugar)
- This slows liver PK, reserving scarce sugar for organs that need it
- liver is the last organ that needs glucose,
** Acetyl CoA activates pyruvate carboxylase, ADP
