1 Central metabolic pathways: Pathways that provide precursor metabolites to all other pathways...
-
Upload
dulcie-arnold -
Category
Documents
-
view
227 -
download
5
Transcript of 1 Central metabolic pathways: Pathways that provide precursor metabolites to all other pathways...
1
Central metabolic pathways:Pathways that provide precursor
metabolites to all other pathways
Carbohydrates metabolic pathways
and carboxylic acids
☺Embden – Meyerhof –
Parnas Pathway= EMP = glycolysis☺Pentose phosphate pathway(PPP)☺Entner Doudoroff pathway(ED)
Similarities of the three pathways:
• Convert glucose to phosphoglycer-
aldehyde
• Phosphoglyceraldehyde is
converted to pyruvate through the
same reactions
Carbohydrates pathways and
Krebs Cycle
Central metabolic pathways:Three substrate level
phosphorylations
Six oxidation reactions → NADH
and FADH2
Pyruvate → depends on cell
Respiration → oxidized to acetyl-
CoA → CO2
Fermentation → alcohol, organicacids, solvents.
The oxidation of NADH and FADH2
Respiration → oxidized viaelectron transport → formation of∆P
Fermentation → oxidized incytosol by an organic acceptor →no production of ATP
Glucose 6-P-GluconateATP
G6P
NADPHATP
PPP ED
ATP
CO2 NADPH
Pentoses
Pi
H2O
F6PATP
FBP
Gluconate
Fumarate
Succinate
ATP, NADH
ATP
CO2 NADH
citrate
Cis- aconitate
Isocitrate
FADH2
ATP
succinyl CoA
CO2 NADH
2
α-ketoglutarate
NADPH
oxalosuccinate
CO2
PGLAD
PEP
Pyruvate
Acetyl-CoA
Oxaloacetate
Malate
NADH
Citric Acid Cycle
3
Embden – Meyerhof – ParnasPathway
Catalyzes the splitting of the
glucose molecule (C6) into two
phosphoglyceraldehyde (C3)molecules
Catalyzes the oxidation of
phosphoglyceraldehyde (C3) topyruvate
Figure 1. Glycolysis
Glycolysis as an anabolic pathway
The glycolytic pathway serves not onlyto oxidize carbohydrate to pyruvate and tophosphorylate ADP, but also providesprecursor metabolites for many otherpathways.
G6P → polysaccharides, aromatic aminoacids, pentosa phosphates
F6P → amino sugars (muramic acid andglucosamine)
DHAP → phospholipids
PGA → serine, glycine, cystein
PEP → aromatic amino acids, muramicacid
4
Glycolytic pathway can be reversed
from PEP only to F 1,6 bP (not at all to
pyruvate).
This is due to the pyruvate kinase and
phosphofructokinase reactions are
physiologically irreversible → high free
energy in the phosphoryl donors
Regulation of glycolysis pathway:
Two key enzymes :
1. phosphofructokinase
2. fructose 1,6 bisphosphate
phosphatase
♣phosphofructokinase activity will
be stimulated when the ADP levels
are high (ATP levels are low) →
glycolysis is stimulated
♣phosphofructokinase activity is
inhibited by PEP (end - product
inhibition).
When glycolysis is stimulated by ADP,
the reversal of glycolysis is slowed by
high levels of AMP → AMP inhibits the
fructose 1,6 bisphosphate reaction
Fructose 1,6 bisphosphatase regulates
pyruvate kinase
5
Pentose Phosphate Pathway
In this pathway:
Producing pentose phosphate
→ precursors to the ribose and
deoxyribose in the nucleic acid
Providing erythrose phosphate
→ precursors to the aromatic
amino acids
Reactions of Pentose phosphatepathway:
�Oxidation-decarboxylation reactions
products : CO2, 2 NADPH, 5 carbon
sugar (ribulose –5– phosphate).
�Isomerization reactions → precursors
to the stage 3
• some of the ribulose-5-phosphate
is isomerized to ribose-5-phosphate
and to xylulose-5-phosphate
Two type of reactions:
Transfer of two-carbon fragment
from ketose to an aldose →
transketolase
Transfer of three-carbon fragment
from ketose to an aldose →
transaldolase
The rule : the donor is always a ketoseand the acceptor is always analdose
�Sugar rearrangement reactions →
glyceraldehyde phosphate.
6
Thiobacillus novellus and Brucella
abortus use only an oxidative
pentose phosphate pathway to
grow on glucose
Stage 2 and 3 of the pentose phosphate
pathway are reversible synthesize
pentose phosphates from
phosphoglyceraldehyde
The pentose phosphate pathway and
glycolysis interconnect at
phosphoglyceraldehyde and F6P
organisms growing on pentoses can
make hexose phosphates
Figure 3. Pentose phosphate pathway
7
Entner--Doudoroff Pathway
♣Only found in prokaryotes.widespread, particularly amonggram-negative bacteria, aerobic
♣Yield of the pathway: 1 molglucose 2 pyruvate, 1 ATP, 1NADH and 1 NADPH.
♣Difference to pentose phosphate= some of 6P–gluconates aredehydrated to 2 keto-3-deoxy-6-P-gluconate (KDPG) instead of toribulose-5-phosphate.
Figure 4. Entner-Doudoroff pathway
8
The Citric acid cycle:
This cycle is present in most
heterotrophic bacteria growing
aerobically, but organisms that
grow on C1 compounds (methan,
methanol and so on) carry out a
reductive pathway
They are 4 oxidations per acetyl-
CoA producing 2 NADH, 1
CITRIC ACID CYCLE
NADPH and 1 FADH2, and 1
substrate-level phosphorylation
producing ATP.
CoA
9
The Citric acid cycle is feedback
inhibited by several end product:
♠In Gram – bacteria: citrate
synthase is allosterically
inhibited by NADH.
♠In facultativ anaerob by α-
ketoglutarat.
♠In Gram + the citrate synthase is
inhibited by ATP
The Citric acid cycle provide
precursors to 10 of the 20 amino
acid found in protein:
Succinyl-CoA: L- lysine and L-
methionin, tetrapyroles
(prosthetic group in several
protein, Cyt & Chlorophylls)
10
Oxaloacetate : aspartate, which
itself is the precursor to five
other amino acid.
Fumarate: aspartate
α-ketoglutarat :glutamate, which
itself is the precursor to three
other amino acid.
Modification of the citric acid cycleinto a reductive cycle duringfermentation Growth.
The solution is to convert the citricacid cycle from an oxidative into areductive pathway.
11
Pyruvate
Acetyl-CoA
Oxaloacetate Citrate
malate
Fumarate
succinate
Succinyl~CoA
[cis-akonitate]
isocitrate
oxalosuccinate
α-ketoglutarat
Glucose-6-P
The Glyoxylate Cycle
Phosphoenolpyruvate
The Glyoxylate cycle is required by
aerobic bacteria to grow on fatty
acids and acetate (Plants and
protozoa also have this cycle)
What regulates the fate of the
isocitrate?
In E. coli: the isocitrate hydrogenase
activity is partially inactivated by
phosphorylation when cell are grown
on acetate.
Acetate also induces the enzymes ofGlyoxylate cycle.
Isocitrate lyase requires a highintracellular concentration of isocitrate
Gluconeogenesis
Growing microorganisms on poor
carbon source, such as L-malate,
succinate, acetate or glycerol
requires the ability to synthesize
hexoses needed for the production of
cell wall mucopeptides, storage
glycogen and other compounds
derived from hexose, such as
pentoses, for nucleic acid
biosynthesis
12
13
Hexose synthesis involves a reversal
of carbon flow from pyruvat
(gluconeogenesis).
It is not allow a carbon flow from
pyruvat to hexoses directly because:
1. Pyruvat kinase is not reversible
because the free-energy
requirement is to great. PEP is
formed by PEP-carboxykinase (Mg2+
dependent, with GTP as phosphate
donor)
2. Reaction with phosphofructokinase
is irreversible. Fructose-1,6-
bisphophatase dephosphorylates
FBP to yield F-6-P and Pi.
3. Glucose – 6- phosphatase removes
Pi from G-6-P to yield glucose.
2 pyruvate + 4ATP + 2GTP + 2NADH
+ 2H+ + 4H2O
→Glucose + 2NAD+ + 4ADP + 2GDP
+ 6Pi
14
Gluconeogenesis enzymes
1. Pyruvate carboxylase: pyruvat →
OAA
2. PEP carboxykinase: OAA + GTP →
pyruvat
3. Fructose-1,6-bisphosphatase
4. Glucose-6-phosphatase
5. ATP-glucose pyrophosphorylase: G-
1-P +ATP →ADP-glucose
6. Glycogen Synthase: α-1,4-glycan +
ADP-glucose →glycogen / starch
15
Regulation:
carboxykinase, which is regulated
by catabolite repression (inhibited
when glucose or other carbohydrate
carbon source are available).
PEP carboxykinase is induced at the
stationary phase of growth and
requires cAMP and regulatory
signal.
Regulation:
1. Allosteric Control of enzymes activity
in Catabolic PathwaysThe major regulatory step is PEP
Alosteric regulation in Emden-Meyerhof-
Parnas, Gluconeogenesis and TCA is
predominantly effected by
intermediates.
This is due, in part,to the interrelation of
the Emden-Meyerhof-Parnas, pentose
phosphate and TCA pathways at the
level of intermediates of Emden-
Meyerhof-Parnas
16
♣The effector intermediates may
either be inhibitors, activators, or
both (affecting different enzymes)
♣accumulation of Fru 1,6 biP
serves to activate both ADP-
glucose pyrophosphorylase and
pyruvate kinase
♣accumulation of PEP serves to
inhibit phosphofructokinase but
activates pyruvate dehydrogenase
17
2. Posttranslational Covalent
Modification of proteins
♠Covalent modification of prokaryotic
proteins involves phosphorylation at
various residues, uridylylation,
adenylylation, methylation, fatty
acylation and proteolysis
♠Example: Isositrat Dehydrogenase
For growth on acetate or fatty
acids, E.coli require the functioning
of the glyoxylate bypass.
During growing on acetate, isocitrate
dehidrogenase is inhibited by
approximately 75% compared to during
growth on most other carbon.
18
This inhibition is caused by the
phosphorylation of a single serine
residue of isocitrate dehydrogenase,
which completely inactivates the
enzyme.
Phosphorylation of isocitrate
dehydrogenase is carried out by an
ATP-dependent IDH kinase/
phosphatase, which also catalyzes
dephosphorylation of phospho-
isocitrate dehydrogenase.