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Bez nadpisu

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KAPITOLA 8 Energetick metabolismus I glykol za a kva en pent zov cyklus glukoneogeneze lokalizace reakc v bu ce fyziologick aspekty bioenergetiky – PowerPoint PPT presentation

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Title: Bez nadpisu


1
KAPITOLA 8
Energetický metabolismus I
  • glykolýza a kvašení
  • pentózový cyklus
  • glukoneogeneze
  • lokalizace reakcí v bunce
  • fyziologické aspekty bioenergetiky

2
Glycogen, starch, sucrose
Major pathways of glucose utilization in cells of
higher plants and animals. Although not the only
possible fates for glucose, these three pathways
are the most significant in terms of the amount
of glucose that flows through them in most cells.
Storage
Glucose
oxidation via pentose phosphate pathway
oxiadation via glycolysis
Ribose-5-phosphate
Pyruvate
3
Three possible catbolic fates of the pyruvate
formed in the payoff phase of glycolysis.
Pyruvate also serves as a precursor in many
anabolic reactions, not shown here.
4
The two phases of glycolysis. For each molecule
of glucose that passes through the preparatory
phase (a), two molecules of glyceraldehyde-3-phosp
hate are formed both pass through the payoff
phase (b). Pyruvate is the end product of the
second phase under aerobic conditions, but under
anaerobic conditions pyruvate is reduced to
lactate to regenerate NAD. For each glucose
molecule, two ATP are consumed in the
prepa-ratory phase and four ATP are produced in
the payoff phase, giving a net yield of two
molecules of ATP per one of glucose converted to
pyru-vate. Keep in mind that each phosphate
group, represented here as P, has two negative
charges (- PO32-).
5
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6
Entry of glycogen, starch, disaccharides, and
hexoses into the preparatory stage of glycolysis.
7
Covalent regulation
Covalent and allosteric regulation of glycogen
phosphorylase in muscle. (a) The enzyme has two
identical subunits, each of which can be
phosphorylated by phosphorylase b kinase at
Ser14 to give phosphorylase a, a reaction
promoted by Ca2. Phosphorylase a phosphatase,
also called phosphoprotein phosphatase-1,
remo- ves these phosphate groups, inactivating
the enzyme. Phosphorylase b can also be
activated by noncovalent binding of AMP at its
alosteric sites. Conformational changes in the
enzyme are indicated schematically. Liver
glycogen phosphoryla- se undergoes similar a and
b interconver- sions, but has different
regulatory mecha- nisms.
Allosteric regulation
8
(b) The three-dimensional structure of the enzyme
from muscle. The two subunits (gray and blue)
of the glycogen phosphorylase a dimer, showing
the location of the phosphates (orange) attached
to the Ser14 residues (red) in each. In
phosphorylase b, the amino-terminal peptide
containing Ser14 is disordered. However, with
the attachment of negatively charged phosphate
group at Ser14 this peptide folds toward several
nearby (positively charged) Arg residues (pink),
forcing compensatory changes in regions distant
from Ser14 and activating the enzyme. AMP, the
allosteric activator of phosphorylase b, binds
very near Ser14. On the back side of the enzyme
is a deep channel that admits the substrate
glycogen to the active site, which is 3,3 nm
away from the allosteric site. (c) A close-up
view of the region around the phospho-Ser
residue note its proximity to the interface
between dimers.
9
Hormonal regulation of glycogen phosphorylase in
muscle and liver. A cascade of enzymatic
activations leads to activation of glycogen
phosphorylase by epinephrine in muscle and by
glucagon in liver. When catalysts activate
catalysts large amplifications of the initial
signal results.
10
The oxidative reactions of the pentose phosphate
pathway, leading to D-ribose-5-phosphate and
producing NADPH.
11
Secondary pathways for glucose metabolism through
UDP-glucuronate.
12
The nonoxidative reactions of the pentose
phosphate pathway convert pentose phosphates back
into hexose phosphates, allowing the oxidative
reactions to continue. The enzymes transaldolase
and transketolase are specific to this pathway
the oher enzymes also serve in the glycolytic or
gluconeogenetic pathways.
13
A simplified schematic diagram showing the
pathway leading from six pentoses (5C) to five
hexoses (6C).
14
The pathway from phosphoenol-pyruvate to
glucose-6-phosphate is common to the biosynthetic
conversion of many different precursors into
carbohydrates in animals and plants.
15
Alternative paths from pyruvate to
phosphoenolpyruvate. The paths differ depending
upon the gluco-neogenetic precursor (lactate or
pyruvate) and are determined by cytosolic
requirements for NADH in gluconeogenesis.
16
The opposing pathways of glycolysis and
gluconeogenesis in rat liver. The three bypass
reactions of gluconeogenesis are shown in orange.
Two major sites of regulation of gluconeogenesis
are also shown.
17
Oxaloacetate enters the cytosol and serves as the
starting material for gluconeogenesis and the
synthesis of sucrose, the transported sugar in
plants.
The conversion of stored fatty acid to sucrose in
germinating seeds begins in glyoxysomes, which
produce succinate and export it to mitochondria.
There it is converted to oxaloacetate by enzy-mes
of the citric acid cycle.
18
Triacylglycerols stored in seeds are oxidized to
acetyl-CoA and dihydroxyacetone phosphate during
germination both are substrates for
gluconeogenesis in plants. Recall that acetyl-CoA
is not a substrate for gluconeogenesis in animals.
19
The glycogen-branching enzyme glycosyl-(4?6)-trans
ferase (or amylo (1?4) to (1 ? 6)
transglycosylase) forms a new branch point during
glycogen synthesis.
20
Initiating the synthesis of a glycogen particle
with a protein primer, glycogenin. Steps 1
through 5 are described in the text. Glycogenin
is found within glycogen particles, still
covalently attached to the reducing end of the
molecule.
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