Glycolysis Flashcards
Where is glucose a primary fuel?
brain, red blood cells and the renal medulla
Is glycolysis aerobic or anaerobic?
anaerobic
Glycolysis definition
Metabolic pathway that converts glucose into pyruvate and a hydrogen ion. Occurs in a hypoxic environment
Where does it take place?
cytoplasm
brief 3 stages
energy investment, 6C splitting and energy harvest
What mediates the control?
supply and demand, supply in terms of selecting the best fuel, demand in terms of the energy needs of the cell
What does glycolysis produce as a waste product?
lactate
How is glucose taken up into the cell?
GLUT1-4 mediated facilitated diffusion and SGLT1-2 mediated secondary active transport
Differences in tissue uptake + main transporter
Liver, endocrine pancreas, dependent on plasma glucose concentration- GLUT2- insulin independent
peripheral tissues- depends on energy needs of tissues and is regulated in tissues that can use non-carbohydrate substrates- GLUT 4- insulin dependent
When is uptake independent of substrate concentration?
Km is less than physiological
When is uptake highly dependent on substrate concentration?
Km is more than or equal to physiological
The three stages of glycolysis?
glucose priming, splitting of P’ated i/mediate, oxidoreduction phosphorylation
First reaction in glycolysis + enzyme
glucose phosphorylation
ATP–> ADP , glucose –> glucose 6-phosphate
hexokinase/glucokinase
Differences between hexokinase and glucokinase
hexokinase - most tissues regulatory step broad substrate specificity low kM (high affinity) low vMax
glucokinase
- liver, pancreatic islets
- similar specificity
- different regulation
- high km
- high vmax
second reaction in glycolysis + enzyme
reversible isomerisation reaction
glucose 6-phosphate into fructose 6-phosphate
enzyme= phosphoglucose isomerase
Third reaction in glycolysis + enzyme
fructose 6-phosphate into fructose 1,6- bisphosphate
ATP—> ADP
phosphofructokinase-1
fourth reaction + enzyme
fructose-1,6-bisphosphate –> glyceraldehyde 3-phosphate + dihydroxyacetone phosphate
aldolase
Why does the proportion of glyceraldhyde 3-phosphate and dihydroxyacetone phosphate vary?
They can be changed from one form to the other using triose phosphate isomerase
at equilibrium, 96% of triose phosphate is DHAP
What if formed in the fifth reaction?
1,3-biphosphoglycerate
How does the oxidation of glyceraldehyde-3-phosphate take place?
reduction of cofactor NAD+ and Pi to form NADH and an H+
same time OAA becomes malate, thanks to NADH
Where is the original energy of glucose?
1,3-bisPGA used directly for the synthesis of ATP
NADH used indirectly for the synthesis of ATP
6th step + enzyme
1,3- bisphosphoglycerate to 3-phosphoglycerate
ADP to ATP
phosphoglycerate kinase
What type of reaction is the 6th step?
substrate level phosphorylation
7th step + enzyme
3-phosphoglycerate to 2-phosphoglycerate
phosphoglycerate mutase
8th step + enzyme
phosphoglycerate to phosphoenolpyruvate with release of water
enolase
9th step + enzyme
phosphoenolpyruvate to pyruvate
ADP to ATP
Pyruvate kinase
How many products for one molecule of glucose?
2x pyruvate
2 xATP
2x NADH
What happens to pyruvate in the aerobic pathway?
TCA cycle, oxidative phosphorylation
What happens to pyruvate in the anaerobic pathway?
pyruvate forms lactate, using NADH
Which muscle fibres require glycolysis the most?
skeletal muscle type IIb (fast twitch fibres)
Why is the phosphorylation of glucose in the first step so important?
Forms G6P which then maintains low glucose concentration in the cell, promoting continuous transport of glucose into the cell through plasma membrane transporters
also prevents glucose from leaking out, as the cell lacks transporters for G6P
hexokinase is inhibited by its product
Difference in Km between hexokinase and glucokinase
glucokinase has a lower affinity for glucose, with the Km in the vicinity of normal glycemia
Importance of second stage
GGP converted into fructose-6-phosphate
allows the reaction to drive forward because of low conc of F6P, which is constantly consumed in the next stage of glycolysis
isomerisation to a keto sugar is necessary for the carbanion stabilisation
What happens if F6P concentration is high? + why
reaction readily runs in reverse, explained by Le Chatelier’s Principle
Importance of third step
the glycolytic process becomes irreversible, as phosphofructokinase 1 is coupled to the hydrolysis of ATP (an energetically favourable step)
forms the rate limiting step
second phosphorylation event allows the formation of two charged groups, ensuring the prevention of free diffusion from the cell
destabilising the molecule, to allow next step to happen with ease
What inhibits PFK?
A high ratio of ATP to ADP
H+ allosterically
what activates PFK?
Fructose 2,6-bisphosphate, which is also produced from fructose 6-phosphate by PFK1
more F6P leads to more F-2,6-BP, whose binding increases the affinity of PFK1 for F6P and minimises inhibitory effect of ATP
Pyruvate kinase function
catalyses last step in glycolysis
transfer of phosphate group from phosphoenolpyruvate to ADP, to produce one pyruvate and one ATP
Two conformations of pyruvate kinase + what binds
R state- high substrate affinity, stabilised by PEP and fructose 1,6-bisphosphate
T state- inactivated form, low substrate affinity, bound and stabilised by ATP and alanine- causing the phosphorylation of pyruvate kinase and inhibition of glycolysis
What is the cofactor? + experiment that confirmed this
magnesium 2+
in yeast cells, interaction of yeast pyruvate kinase with PEP and allosteric effector fructose 1,6-bisphosphate was shown to be enhanced by presence of MG2+
How does Fructose -1,6-bisphosphate allow a feed forward mechanism?
FBP is a glycolytic intermediate produced early on in the reaction
binds to allosteric binding site at domain C of pyruvate kinase and changes the conformation of the enzyme
the higher the conc of FBP, the greater the allosteric activation and the magnitude of pyruvate kinase activity
What process is able to utilise pyruvate to form new proteins?
Transamination
Explain transamination
chemical reaction that transfers an amino acid group to a ketoacid to form new amino acids.
alanine transaminase catalyses the addition of a nitrogen group (from alpha amino acid) to pyruvate to form a ketoacid and then form alanine
Glycerol-3-phosphate shuttle definition
mechanism that regenerates NAD+ from NADH
Stages of using galactose as a fuel
- galactose to galactose 1-phosphate (ATP to ADP) catalysed by galactokinase
- galactose 1-phosphate uridyltransferase then catalyses the conversion into glucose 1-phosphate
- glucose-1-phosphate isomerises and forms glucose-6-phosphate, which is then able to enter the glycolytic pathway
stages of using fructose as a fuel
- fructose becomes fructose 6-phosphate, catalysed by hexokinase
able to enter the pathway
galactosaemia definition
accumulation of galactose in blood, cannot be metabolised properly
