Energy Metabolism Continued Flashcards
Sources of pyruvate
Lactate- mostly from muscles, produced in great quantities during exertion. Lactate is released from the muscles to the blood and travels to the liver for conversion to pyruvate and, ultimately to glucose
Why is regulation required?
Ensure products are available when needed
To maintain the steady stet levels of all intermediates
To ensure synthesis and breakdown pathways are active at different times
Necessary because..
Intermediates are used for other pathway
If the pathway is not in fine balance it may not be capable of responding to small changes in signalling
Regulation of glycolysis
The regulatory enzymes of the glycolytic pathway are hexokinase, phosphofructokinase, pyruvate kinase. Each catalyses irreversible reactions and so can turn pathway on or off.
Glycolysis regulation strategies
Allosteric effects:
- rapidly reversible binding of molecules at sites which are distant from the active site.
- Changes the shape of the enzyme
- Operates on the millisecond timescale
- can increase of decrease enzyme activity
Covalent modification:
- eg reversible attachment of phosphorylation groups, usually to serine, threonine or tyrosine by specific protein kinases.
- removed by specific phosphates
- eg proteolytic cleavage of precursor protein active enzyme
- effects in seconds
Transcriptional control:
- increase amount of enzyme
- operates over mints/ hours
Feed back control:
- product of pathway feeds back to inhibit enzyme that makes
- often the first enzyme in pathway is inhibited by the ultimate product
- operates on millisecond timescale
Feed forward control:
- product of one reaction feeds forward to activate an enzyme further along the pathway
Hexokinase
Liver enzyme (hexokinase) doesn’t bind glucose well when levels are low
Brain can use glucose even when levels are extremely low
GLUT2 transporter brings equilibrium
Inhibition can be overcome by increased glucose levels
Inhibition by high F-6-P levels- promotes binding and sequestration by GK - RP
Liver acts to monitor blood glucose levels due to equilibrium to prevent high blood glucose (hyperglycaemia)
Phosphofructokinase
Regulated by energy levels - uses ATP, produced ADP
Regulatory site which is distant from active site binds ATP and other allosteric regulatory molecules
ATP is only a a substrate for PFK but also an end product of glycolysis
When ATP levels are high ATP binds to allosteric site on PFK and induces a conformational change
Low levels of ATP- active PFK
High levels of ATP- inactive PFK
Inhibition by citrate - PFK
Inhibition of PFK-1 (glycolysis) by high levels of citrate
Oxidation of fatty acids and ketone bodies results in high levels of citrate which feed into TCA cycle to produce NADH and ATP
Preserves glucose to be used for ATP synthesis in brain
Regulation of PFK-1 by pH
Tissues can produce lactate from glycolysis but generally don’t due to the presence of oxygen and mitochondria.
Lactic acidosis occurs due to extreme muscle use and lack of oxygen caused either by overproduction or under use of lactate.
Due to lack of oxygen, ATP production drops, dependence on glycolysis for ATP production
Increase in glycolysis to produce ATP
Increase in lactate production
Lactic acidosis
As lactate is taken into the blood from muscles, it is coupled with the transport of H+ ions into the blood stream causing a drop in blood pH
Low pH inhibits PFK-1 activity to shut down glycolysis to stop further lactate production and stop blood pH from dropping further
Fructose 2,6-Biphosphate
The most potent allosteric activator for PFK -1 - it increases the affinity if PFK-1 for F-6-P and blocks ATP dependent inhibition
A powerful inhibitor of FBPase-1
Therefore promotes glycolysis and inhibits gluconeogenesis
PFK-1 Is virtually inactive in the absence of F26BP
PFK-1 Activity is 20x higher in presence of F26BP
In the absence of F26BONthenFBPase-1 enzyme is active
FBPase-1 activity is inhibited by F26BP
tightly controlled
regulatory control of PFK-2/FBPase-2
Under hormonal control leading to reversible phosphorylation
This is one way hormone regulation controls flux through glycolysis
Pyruvate kinase
Several isoenzyme of pyruvate kinase
All subject to inhibition by high levels of ATP, Acetyl coA l, and fatty acids
Liver isoform also subject to regulatory phosphorylation
Liver isoform subject to hormonal regulation
Accumulation F-1, 6-BP activates PK activity (feed fwd)
Negatively regulated by high alanine levels (feed back)
Glycogen synthesis
Not linear molecule of glucose- highly branches polymer of glucose molecules.
Branches structure containing 1-4 or 1-6 glucose- glucose linkages.
The straight chains are comprised of 1-4 linkages with branch points resulting from 1-6 linkages every 8-12 residues.
Glycogen synthase can only add new glucose residues on the non- reducing end of the chain using a 1-4 linkage
Glycogen exists in cells as large granules. Within these granules are all the enzymes required for the synthesis and breakdown of glycogen in response to blood glucose levels.
- glucose-6-P is starting material for glycogen- phosphoglucomutase converts to glucose -1-P
- converts to UDP-glucose - glucose donor for glycogen synthesis
Glycogen breakdown- glycogenolysis
3 enzymes required to breakdown glycogen
- glycogen phosphorylase
- glycogen debranching enzyme
- phosphoglucomutase
Glycogen has many non-reducing ends that can be broken down simultaneously- large amounts of glucose can be released very quickly. Each chain has 12-14 glucose residues
Low blood glucose results in inhibition of phosphatase which results in inactive synthase enzyme
Therefore less glycogen
Blood glucose level
High blood glucose activates phosphatase (inhibits kinase), then active synthase, more glucose stored as glycogen and blood glucose falls
Products from glycogen breakdown
Glycogen phosphorylase cannot act on residues close to branch point
Glucose -1-P and glucose
G-1-P needs to be converted to G-6-P using phosphoglucomutase
G-6-P in liver can be converted to glucose in ER lumen
G-6-P in muscle can enter glycolysis to provide energy for muscle contraction
Acetyl co-A production
Acetyl Co-A goes through TCA cycle to produce more energy.
Pyruvate dehydrogenase is the link between glycolysis and the TCA cycle
Oxidative decarboxylation (release of CO2)
2 X 3C pyruvate produced 2 X 2C Acetyl CoA
