Glycolysis
Biochemistry
2017Hayder A Giha
Definition:It is a universal pathway for glucose (glycogen) metabolism, found in the cytosol of all mammalian cells, and give pyruvate and/or lactate as an end product.
Introduction
n Most sugars converted to glucose, so, it is the major carbohydrate utilized by human tissues.
n Glycolysis is the major pathway for glucose metabolism.
n There is a basal requirement for glucose in most of the tissues, e.g. the brain needs is substantial, and erythrocyte needs is total and absolute.
Characteristics of glycolysis
n Site: is the cytosol of all tissuesn Its unique in the sense that, it can use oxygen
(aerobic) when its available, however, it can operate in absence of oxygen (anaerobic).
n Aerobic utilization of glucose demands a set of mitochondrial enzymes (Kerbs cycle & respiratory chain).
The cell
Importance
n As glucose (Glc) enter the cell, it is phosphorylatedat C6, becomes negatively charged, thus Glcremain inside the cell because the cell membrane inside is negatively charged.
n The fate of G6P: a. energy production (glycolysis) b. building of CHO or storage (glycogenesis) c. other pathways ( e.g. Pentose Phosphate Pathway)
n Main source of energy for erythrocytes, and exercising muscles.
Glycolysisn Have 2 phasesI. Energy investment phase (consume energy)
(5 reactions) II. Energy generation phase (energy production)
(5 reactions)
n Three control point (irreversible reactions)
Energy investment phase
n Two phosphorylations; of Glc at C6 & Fructose at C1 I. Hexokinase (all cells) or Glucokinase (liver and B-
cells of pancreas) n Hexokinase: have low Vmax & Km (high affinity),
work at normal or low blood glucose to provide energy for tissues, and inhibited by G6P
n Glucokinase: have high Vmax & Km (low affinity), work after meal (high BG) to store Glu
n Both enzymes catalyze irreversible reactions –suitable for regulation of the pathway
comparisonHexokinase Glucokinase
site All tissues -Liver parenchymal cell-Pancreatic b-cells
Affinity for substrate
High affinity Low Km & V-max
Low affinityHigh Km & V-max
Function To ensure glucose supply for tissues
To remove glucose from blood after meals
Specificity Glucose and other hexoses (low level)
Glucose only
Control Inhibited by glucose 6-phosphate
Affected by nutritional state
Glycolysisn G6P isomerized (phosphoglucose isomerase) to F6Pn F6P phosphorylated by phosphofructokinase-1 (PFK-1)
at C1 to Fructose 1,6 bisphosphate (F1,6P2)n This is the 2nd irreversible reaction, so is a control
(regulation) pointn High energy state (ATP & citrate) inhibit PFK-1, and
the opposite is true (AMP & F2,6P2) activate PFK-1n Small a mount of F6P is converted to Fructose 2,6
bisphosphate (F2,6P2) by phosphofructokinase-2 (PFK-2).
Energy investment phase
n F1,6P2 cleaved by aldolase A into 2 triose-P; glyceraldehyde 3-P (GAP) & dihydroxyacetone-P (DHAP).
n GAP and DHAP are inter-convertible by triose phosphate isomerase
Energy generation phase
n GAP is oxidized and phosphorylated at C1 by GAP dehydrogenase to an energy-rich 1,3 bisphosphoglycerate (1,3-BPG)
Energy generation phase
n 1,3-BPG -------- 3 phosphoglycerate (3-PG) and production of energy at substrate level (ADP ---ATP); by phosphoglycerate kinase
n In RBCs, some 1,3 BPG is converted to 2,3BPG by mutase enzyme (loss of ATP), 2,3 BPG decrease Hb affinity to O2.
n 3-PG (3 phosphoglycerate) --------- 2PG by mutasen 2PG ----------- phosphoenolpyruvate (PEP) by
Enloase (with removal of H2O)This dehydration raise the energy level of P in PEP
n PEP --------- pyruvate by pyruvate kinase (PK), associated with conversion of ADP to ATP (energy at substrate level)(it is irreversible reaction, site for control)
n PK is activated by F1,6P2 and cAMP-dependent protein kinase A
Glycolysis
H - C = O H - C - OH CH2- O - P
Glyceraldehyde 3-phosphate
O II C – O ~ P H - C - OH CH2- O - P
1,3-Biphosphoglycerate
COO- H - C - OH CH2- O - P
3-phosphoglycerate
COO- H - C – O - P CH2OH
2-phosphoglycerate
COO- C – O ~ P CH2
Phosphoenolpyruvate
COO- C - OH II CH2
(Enol) Pyruvate
Glyceraldehyde 3-phosphate Dehydrogenase
P i
NADH NAD ADP ATP
Phosphoglycerate kinase
Mg2
Phosphoglycerate mutase
Enolase
Mg2
H2O
Pyruvate kinase
ADP ATP
Mg2
FL
IA
NAD NADH
n NAD is needed for ATP production at substrate level, but it is limited, so, the produced NADH+H+ need to be oxidized
n Under anaerobic conditions or when no mitochondria, H+ is accepted by pyruvate which is converted to lactate (by lactate dehydrogenase)
n When produced in large quantities, lactate diffuse from cell to blood cause lactic acidosis
Glycolysis
Energy of glycolysis
n 2 ATP are consumed during glycolysis (investment phase) in reactions catalyzed by:
1. Hexokinase (Glucokinase), 1 ATP2. Phosphofructokinase-1, 1 ATP
n Note: if glycogen is the starting point, 1 ATP will be saved, i.e. increased net production of energy at substrate level.
Energy production in glycolysis
n 4 ATP are generated in the energy generation phase;
1. Phosphoglycerate kinase (2 ATP, substrate level)
2. Pyruvate kinase (2 ATP, substrate level).THE NET is 2 ATP
n In addition, two NADH + H+ (Glyceraldehyde 3-phosphate dehydrogenase), (under aerobic conditions, in respiratory chain, each one produce 3 ATP).
Regulation of glycolysis
n Glycolysis is controlled at 3 steps (non-equilibrium reactions) catalyzed by:
1. Hexokinase (glucokinase)
2. Phosphofructokinase-1
3. Pyruvate kinase
Oxidation of pyruvate
n Pyruvate oxidation (under aerobic conditions) occurs within the mitochondria, its transported into the later via pyruvate transporter
n Its oxidatively decarboxylated to Acetyl-CoA, before it enter the citric acid cycle.
n This reaction is catalysed by a complex of enzymes (3 enzymes), designated as, pyruvate dehydrogenase complex (PDH)
n The reaction needs CoA, thiamin diphosphate, lipoic acid, FAD and NAD+
Oxidation of pyruvate
n 2 NADH + H+ are produced for each glucose molecule
n Also a high-energy thio ester group in acetyl-CoA is produced.
The End