Week 3 Carbohydrate Metabolism 1 Part B GLYCOLYSIS Flashcards
GLYCOLYSIS
2 stages
•Investment stage
—Uses ATP
•Pay off stage
—Creates ATP
Overall:
1 x 6 carbon glucose is split into 2 x 3 carbon pyruvate
INVESTMENT STAGE (+ SPLITTING)
Glucose
V
Glucose 6- phospahte (6C)
V ^
Fructose 6-phosphate (6C)
V
FRUCTOSE 1,6 Bisphosphate (6C)
/. \
Dihydroxyacetone Glyceraldehyde
Phosphate (3C). 3 phosphate (3C)
PAY OFF STAGE
Dihydroxyacetone. —> Glyceraldehyde
Phosphate <— 3 - phosphate
V
1,3 bisphosphoglycerate
V
3 phosphoglycerate
X2. V
Phosphoenolpyruvate
V
Pyruvate
STEP 1: PHOSPHORYLATION OF GLUCOSE (1ST PRIMING)
Irreversibly “traps” glucose in the cell
•Lowers [glucose] in the cell
•Highly thermodynamically favourable
•Uses ATP
•Under substrate level control
HEXOKINASE
HEXOKINASE VS GLUCOKINASE
•Hexokinase has allosteric enzyme kinetics.
•Low activity at low [glucose]
•Only high activity at high [glucose]
•Ensures glucose is only irreversibly entered into glycolysis at high [glucose]
•Ensures CHO utilization in muscle and adipose tissue influenced by glucose availability
•In liver – glucokinase ensures utilization of glucose even at low [glucose].
•Key for glucose uptake from portal system
•Key to regulating glucose metabolism and supply (see later lectures
STEP 2: ISOMERISATION OF G-6-P
Changing to a pentagonal ketose allows easier further phosphorylation (see step 3).
•Allows for creation of potentially symmetrical molecule.
•Reversible
•Direction and rate = under substrate level control;
STEP 3: PHOSPHORYLATION OF F-6-P (2ND PRIMING)
Irreversible
•The committed step of glycolysis
•Uses ATP
•requires Mg2+ as cofactor
•PFK-1 = KEY enzyme of control – SEE LATER
•ATP = powerful inhibitor
•Produces a “symmetrical” molecule for splitting
STEP 4: SPLITTING – ALDOL CLEAVAGE
Reversible
•Two halves are not quite the same
= DHAP + GAP
Need to have both halves in the form of GAP
STEP 5: TRIOSE PHOSPHATE INTERCONVERSION
Reversible
•Allows for continual breakdown using one pathway
= end of the investment stage
FROM HERE ON – EVERYTHING IS DUPLICATED
STEP 6: OXIDATION OF GAP
Reversible
•First energy yielding step of glycolysis
•NAD+ reduced to NADH
•Generates a high energy phosphate compound (1,3 BisPG)
= allows for ATP production in subsequent steps
STEP 7: FIRST ATP PRODUCTION
Reversible
•Substrate level phosphorylation of ADP
•1,3 BisPG (high energy compound) donates Phosphate to form ATP
STEP 8: REARRANGEMENT
STEP 9: DEHYDRATION
STEP 10: 2ND ATP PRODUCTION
Irreversible
•Substrate level phosphorylation of ADP
•Loss of phosphate from PEP leads to tautomerization which helps drive the reaction
•Pyruvate kinase (PK) requires Mg2+ as cofactor
•PK = highly regulated enzyme
SUMMARY OF GLYCOLYSIS
Glucose + 2NAD+ + 2ADP + 2Pi
2 Pyruvate + 2 NADH + 2 ATP
Glycolysis occurs in the cytosol of the cell and breaks down glucose (6C) into 2 x pyruvate molecules (3C)
•Glycolysis consists of an investment stage and a pay off stage. I.e. you have to speculate to accumulate
•2 ATP are invested to gain 4 ATP = Net gain of 2 ATP
•There are 3 irreversible steps in glycolysis
•Step 1 – Hexokinase
•Step 3 – PFK1
•Step 10 (last step) – Pyruvate Kinase
•These are the control steps of the pathway and we will learn more on these in Part D
2 ATP can be generated (net) per glucose, without oxygen
•BUT, need to recycle NAD+ used in the conversion of glyeraldehyde 3-phosphate to 1,3 bisphosphoglycerate
•Achieved by dehydration reaction forming lactate
STEP 11” - FORMATION OF LACTATE
(ANAEROBIC CATABOLISM)
Glucose + 2 Pi + 2 ADP
2 Lactate + 2 ATP + 2H2O
Takeaway
In the liver Galactose and Fructose can enter glycolysis by alternative routes.
•Entry of Fructose into glycolysis, bypasses the key control steps of the pathway and can lead to an “overload” of carbohydrate
•Excess glycogen storage in liver
•Creation of fat by de novo lipogenesis
•Excess export of glucose out into the system (high blood glucose)
•The anaerobic breakdown of glucose (e.g. in muscle) requires the additional step converting pyruvate to lactate to recycle NADH back to NAD+ and keep glycolysis going.
•This lactate can be recycled back to glucose in the liver by gluconeogenesis (see later lecture), using the Cori cycle.
CONTROL OF GLYCOLYSIS
•Irreversible reactions with 3 key enzymes:
Hexokinase
Phosphofructokinase
(PFK-1)
Pyruvate kinase
•Activity of enzymes controlled by hormones and other metabolites
HEXOKINASE VS GLUCOKINASE
Hexokinase has allosteric enzyme kinetics.
•Low activity at low [glucose]
•Only high activity at high [glucose]
•Ensures glucose is only irreversibly entered into glycolysis at high [glucose]
•Ensures CHO utilization in muscle and adipose tissue influenced by glucose availability
•In liver – glucokinase ensures utilization of glucose even at low [glucose].
•Key for glucose uptake from portal system
•Key to regulating glucose metabolism and supply (see later lectures
The Key enzyme in control of glycolysis
•ATP is powerful inhibitor
47
Inhibition enhanced by:
–Glucagon
–Citrate
–Catecholamines (in liver)
Inhibition reversed by:
–Insulin
–AMP
–Fructose 6-phosphate
–Catecholamines (in muscle)
Catecholamines = epinephrine (adrenalin) and norepinephrine (noradrenalin) – e.g. released in fight or flight response
PYRUVATE KINASE
Activated by:
•Fructose 1,6 Bisphosphate
•Insulin (via counteracting glucagon)
Inhibited by:
•ATP
•Glucagon
•Alanine (signalling an abundance of building blocks)
Anaerobic catabolism
—Converted to lactate
•Aerobic catabolism
—Converted to Acetyl CoA (into mitochondria)
—Via TCA cycle and oxidative phosphorylation
•Synthetic
—Converted to alanine (amino acid)
•Gluconeogenic
•Converted back to form glucose
Takeaway
The 3 irreversible steps of glycolysis are the control points of the pathway
•PFK1 = the key regulatory enzyme in glycolysis.
•Switches glycolysis on or off
•PFK1 inhibited by ATP (energy availability)
•PFK1 also under lots of other allosteric control
•Insulin, catecholamines,
