Metabolism 3 (Regulating glycolysis) Flashcards
What are the 2 types of response to changing environmental conditions?
Rapid response
= regulation of enzyme activity
Long-term response
= changes in gene expression & protein synthesis
What are the 4 factors responsible for controlling fluxes through metabolic pathways?
> substrate availability
conc of enzymes
allosteric regulation
covalent modification
How is glucose taken up by cells?
Uptake of glucose into cells is regulated by GLUT transporter proteins
- work via conformational changes
How does the 1st step of glycolysis affect glucose?
How does it do this?
Why is this useful?
Traps glucose inside the cell
Glucose moves into cell via GLUT
- > Hexokinase adds phosphate = G6P
- > can no longer pass back out of cell
Glucose kept inside cell where it can be metabolised by glycolysis
Which reactions in glycolysis are the rate limiting steps?
What are the enzymes involved in these steps?
The irreversible reactions:
- Hexokinase
- Phosphofructokinase
- Pyruvate kinase
What are the mechanisms are used to control the rate of limiting steps?
Concs of key rate-limiting enzymes in glycolysis are regulated by hormones
- insulin + glucagon
Which enzymes does insulin affect + what affect does it have?
Upregulates expression of :
>hexokinase
>phosphofructokinase
>pyruvate kinase
Which enzymes does glucagon affect and what affect does it have?
Inhibits:
> hexokinase
> phosphofructokinase
> pyruvate kinase
Describe how insulin up regulates the expression of hexokinase
- insulin enters cell via insulin receptor
- activates transcriptional activator
- TF binds to promoter of genes
- increases expression of Hexokinase gene
- –> increases glycolysis
What is allosteric control?
Inhibition/activation of enzyme by a small regulatory molecule that interacts at an allosteric site other than active site
= ability of enzyme to catalyse a reaction is modified
Describe the process of product inhibition of hexokinase
Glucose converted to G6P via hexokinase
-> hexokinase is inhibited by G6P
(allosterically)
What is glucokinase?
What does it facilitate?
How does it differ from hexokinase?
(AKA Hexokinase IV)
Enzyme sequestered in the nucleus of liver cells by a regulatory protein
Phosphorylation of glucose to G6P
NOT inhibited by G6P
In terms of glucokinase, what happens when blood glucose is >5mM?
Glucokinase = active
localised in cytosol
In terms of glucokinase, what happens when blood glucose is <5mM?
Why is this important?
Glucokinase = inactive
(localised in nucleus)
Prevents liver from taking glucose that might be needed by other cells when glucose is scarce
- liver can carry out gluconeogenesis
What is the most important control site in the mammalian glycolytic pathway?
Why is this?
Phosphofructokinase (PFK)
It catalyses a unique step
What is PFK controlled by in muscle?
The energy state of the cell
ATP:AMP ratio
How is glycolysis in resting muscles controlled through allosteric regulation?
Not as much glycolysis:
Hexokinase inhibited by
-ve feedback from G6P accumulation
PFK inhibited by high energy charge
= high ATP:low AMP
How is glycolysis in contracting muscles controlled through allosteric regulation?
More glycolysis:
ATP used up
PFK stimulated by low energy charge
= low ATP:high AMP
How is PFK controlled?
By citrate, fructose 2,6-bisphosphate + ATP
As citrate accumulates –> inhibits PFK
ATP inhibits PFK
2,6-bisphosphate comes from fructose 1,6 bisphosphate via alternative pathway
How is pyruvate kinase regulated via allosteric control?
Upregulated: via feed forward mechanism mediated by fructose 1,6 bisphosphate
Downregulated: ATP + Acetyl CoA inhibits
How is glycolysis controlled through covalent modification of pyruvate kinase?
Phosphorylated state of pyruvate kinase regulates catalytic activity
Phosphorylated = less active
De-phosphorylated = more active
What are the 2 catalytic subunits of the bifunctional enzyme that synthesises fructose 2,6 bisphosphate?
Phosphofructokinase 1 (PFK1)
+ fructose 2,6 bisphosphate
How is pyruvate kinase phosphorylated?
Via kinases + ATP
How is pyruvate kinase dephosphorylated?
Via phosphotases
Why are metabolic pathways formed from step-wise processes?
> cell gets greater control of reaction
gets rid of products that could lead to a reversible reaction
metabolic processes produce heat -> small amounts of heat protect cell
How do GLUT transport glucose into cells?
T1 configuration = open to outside of cell
-> glucose binds
-> conformational change
T2 configuration = open to inside of cell
-> glucose release
-> conformational change
T1 configuration
What are the 3 GLUTs?
GLUT1
GLUT2
GLUT4
Where is GLUT1 found and what is its function?
Red blood cells
Controls basal glucose uptake
Where is GLUT2 found and what is its function?
Liver cells
Pancreatic beta cells
Uptake glucose at rate proportional to amount of glucose
Remove excess glucose from blood
Where is GLUT4 found and what is its function?
What is it regulated by?
Muscle cells
Adipocytes
Removes excess glucose from blood
Insulin
Describe how insulin is stimulated to be released from pancreatic beta cells
- Blood glucose >5mM
- Glucose enters cell via GLUT2
- Glycolysis occurs to produce ATP + pyruvate
- Pyruvate undergoes oxidative phosphorylation in mitochondria -> produces lots of ATP
- ATP inhibits K+ exiting cell = cell is depolarised
- Ca2+ enters cell
- Insulin-containing vesicles fuse with plasma membrane + release insulin into blood
Describe glucose uptake in muscle cells when blood glucose is >5mM
- High insulin in blood from beta cells
- Insulin binds to receptor on cell
- Cascade
- GLUT4 fuse with plasma membrane
- Glucose enters cell via GLUT4
Describe how PFK is activated by fructose 2,6 bisphosphate
F6P is converted into F 2,6 BP by PFKII
F 2,6 BP activates PFKI
What happens to fructose 2,6 bisphosphate when glucose levels drop?
F 2,6 BP is converted into F6P by F 2,6 BPase
F6P can then continue along the glycolytic pathway
Why does citrate inhibit PFK?
As there’s lots of metabolites downstream
-> NO need to continue breaking down glucose
What are the 3 things G6P can be used in?
Glycolysis
Forming glycogen
Pentose phosphate pathway
