BIOC/DENT/PHCY 230 LECTURE 2
description
Transcript of BIOC/DENT/PHCY 230 LECTURE 2
BIOC/DENT/PHCY 230
LECTURE 2
Lactate dehydrogenase
pyruvate + NADH lactate + NAD+
M and H subunits: 5 isozymes
M subunit has a lower affinity for pyruvate
i.e. a higher KM
M-type predominates in anaerobic tissues
M4 predominates in muscle and liver
This isozyme can tolerate high concentrations of lactate
Can either: continue to convert pyruvate to lactate under anaerobic
conditions
OR
recycle lactate to pyruvate
Cori Cycle
Lactate produced in skeletal muscle can be recycled in the liver
H4 predominates in heart (aerobic tissue)
H4 favours lactate to pyruvate
Cardiac cells are permeable to lactate
This isozyme: has a low KM for pyruvate and lactate
AND
is allosterically inhibited by high concentrations of pyruvate
This prevents intracellular pH dropping to levels
which may affect cellular function
Regulation of glycolysis
In general:
Inhibited by: ATP, pyruvate, fatty acids, ketone bodies, citrate.
Co-ordinately regulated with: glycogen metabolism, gluconeogenesis, pentose phosphate pathway, CAC.
Pasteur effect:
no O2
anaerobic yeast
O2
glucose consumption
Increase in G-6-P and F-6-P
Decrease in all intermediates from F-1,6-BP
lactateNAD+
NADH
ATP
ADPAMP
Oscillations in glycolytic intermediates
Start with aerobic yeast culture
NADHNAD+
hexokinase
phosphofructokinase
(PFK)
pyruvate kinase
Regulatory steps of glycolysis:
3 key control points
Hexokinase:
Glucose glucose-6-phosphate
-
increased levels of G-6-P signal high levels of ATP and glycolytic intermediates
Pyruvate kinase:
Inhibited by: ATP, acetyl-CoA, alanine, glucagon
Activated by: fructose-1,6-bisphosphate
phosphoenolpyruvate pyruvate + ATP
Four subunit enzyme
3 isozymes (L, M, A)
pyruvate kinase
+
Phosphofructokinase
Key regulatory enzyme of glycolysis
Homotetramer: Mr = 360,000
reversibly dissociates to dimer (active form)
Inhibited by: ATP, citrate
pH
ATP inhibition:Allosteric inhibition: increases KM
@ low [ATP] PFK in R-state
@ high [ATP] PFK in T-state
R
T
Citrate inhibition:
Citrate enhances allosteric PFK ATP binding
High levels of citrate indicate sufficient CAC intermediates
no need to metabolise (waste) glucose
pH inhibition:
Intracellular pH drops under anaerobic conditions
Caused by a build up of lactate
Low pH decreases PFK activity
lactateNAD+
PFK activation
Activated by: AMP, ADP, F-2,6-BP
[AMP] and [ADP] signal low energy state
Fructose-2,6-bisphosphate is an alternative product of F-6-P metabolism
Low concentrations of F-2,6-BP activate
PFK[F-2,6-BP] increases the affinity of PFK for F-6-P
- acts through decreasing ATP inhibition of PFK
P
PFK-2
PFK-1
[F-2,6-BP] increases as [F-6-P] increases
Why is this important?
Glucose
Glucose-6-P
Fructose-6-P
hexokinase
PFK
-
Fructose-1,6-BPFructose-2,6-BP +
The take home message:
Lactate can be recycled as a fuel
LDH isoenzymes influence how lactate is processed by different tissues
Glycolysis is tightly regulated in accordance with environmental and intracellular conditions
There are three key enzymatic reactions involved in the regulation of glycolysis