KAPITOLA 10
Energetický metabolismus III
• katabolismus a anabolismus
mastných kyselin
• glyoxalátový cyklus
• močovinový cyklus
• lokalizace a fyziologické aspekty
KAPITOLA 10
Energetický metabolismus III
• Fatty acids catabolism and anabolism
• Glyoxylate cycle
• Urea cycle
• Localization and physiological aspects
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The glyoxylate cycle and its
relationship to the citric acid
cycle. The orange reaction
arrows represent the glyoxylate
cycle, and the blue arrows, the
citric acid cycle. Notice that
the glyoxylate cycle bypasses
the two decarboxylation steps
of the citric acid cycle, and that
two molecules of acetyl-CoA
enter the glyoxylate cycle
during each turn, but only one
enters the citric acid cycle.
The glyoxylate cycle was
elucidated by Hans Kornberg
and Neil Madsen in the
laboratory of Hans Krebs.
The reactions of the glyoxylate cycle (in glyoxysomes) proceed simultaneously with, and mesh with,
those of the citric acid cycle (in mitochondria), as intermediates pass through the cytosol between these
compartments.
Regulation of isocitrate dehydrogenase
activity determines the partitioning of
isocitrate between the glyoxylate cycle and
the citric acid cycle. When isocitrate
dehydrogenase is inactivated by phosphorylation (by a specific protein kinase),
isocitrate is directed into biosynthetic
reactions via the glyoxylate cycle; when the
enzyme is activated by dephosphorylation
(by a specific phosphatase), isocitrate enters
the citric acid cycle, and ATP production
results.
Fatty acid entry into mitochondria via the acyl-carnitine/carnitine transporter.
After its formation at the outer surface of the inner mitochondrial membrane, fatty acylcarnitine moves into the matrix by facilitated diffusion through the transporter. In
the matrix, the acyl group is transferred back to CoA, freeing carnitine to return to the
intermembrane space via the same transporter. The acyltransferase I and II enzymes
are bound to the outer and inner surfaces, respectively, of the mitochondrial inner
membrane. This entry process is the rate-limiting step for oxidation of fatty acids in
mitochondria.
Stages of fatty acid oxidation.
Stage 1: A long-chain fatty acid is oxidized
to yield acetyl residues in the form
of acetyl-CoA.
Stage 2: The acetyl residues are oxidized to
CO2 via the citric acid cycle.
Stage 3: Electrons derived from the oxidations of stages 1 and 2 are
passed to O2 via the mitochondrial
respiratory chain, providing the
energy for ATP synthesis by
oxidative phosphorylation.
Regulation of metabolite flow from
pyruvate through the citric acid
cycle. The pyruvate dehydrogenase
complex is allosterically inhibited at
high [ATP]/[ADP], [NADH]/[NAD+],
and [acetyl-CoA]/[CoA] rations, all of
which indicate the energy-sufficent
metabolic state. When these rations
decrease, allosteric activation of
pyruvate oxidation results. The rate
of flow through the citric acid cycle
can be limited by the availability of
the substrates oxaloacetate and
acetyl-CoA or by the depletion of
NAD+ by its conversion to NADH,
which slows the three oxidation steps
for which NAD+ is the cofactor.
Feedback inhibition by succinyl-CoA,
citrate, and ATP also slows the cycle
by inhibiting early steps. In muscle
tissue, Ca2+ signals contraction and
stimulates energy-yielding metabolism
to replace the ATP consumed by
contraction.
The fatty acid oxidation (-oxidation)
pathway.
(a) In each pass through this sequence, one acetyl
residue (shaded red) is removed in the form of
acetyl-CoA from the carboxyl end of palmitate
(C16), which enters as palmitoyl-CoA.
(b) Six more passes through the pathway yield
seven more molecules of acetyl-CoA, the
seventh arising from the last two carbon atoms
of the 16-carbon chain. Eight molecules of
acetyl-CoA are formed in all.
Oxidation of
unsaturated FA
Comparison of  oxidation of
fatty acids as it occurs in
animal mitochondria and in
animal and plant peroxisomes.
The peroxisomal system differs in two respects:
1) in the first oxidative step
electrons pass directly to O2,
generating H2O2, and
2) the NADH formed in  oxidation cannot be reoxidized, and
the peroxisome must export
reducing equivalents to the
cytosol. (These eventually are
passed on to mitochondria.)
Fatty acid oxidation in glyoxysomes occurs by the peroxisomal pathway. In mitochondria, acetyl-CoA is further oxidized via the citric acid cycle.
Acetyl-CoA produced by peroxisomes and glyoxysomes is exported; the acetate from glyoxysomes serves as a biosynthetic
precursor.
Electrons removed from fatty acids during  oxidation pass into the mitochondrial respiratory chain
and eventually to O2. The structures I through IV are enzyme complexes that catalyze portions of the
electron transfer to oxygen. Fatty acyl-CoA dehydrogenase feeds electrons into an electrontransferring flavoprotein (ETFP) containing an iron-sulfur center, which in turn reduces a lipidsoluble electron carrier, ubiquinone (UQ, or coenzyme Q). -Hydroxyacyl-CoA dehydrogenase
transfers electrons to NAD+, and the resulting NADH is reoxidized by NADH dehydrogenase
(Complex I of the respiratory chain). Propionate produced from odd-chain fatty acids is
converted to succinate. Succinate dehydrogenase, which acts in the citric acid cycle, feeds electrons
into the respiratory chain at Complex II. Cytochrome c (cyt c) is a soluble electron carrier that
transfers electrons between Complexes III and IV.
Formation of ketone bodies from acetyl-CoA.
Under circumstances that causes acetyl-CoA accumulation
(starvation or untreated diabetes, for example), thiolase
catalyzes the condensation of two acetyl-CoA molecules to
acetoacetyl-CoA, the parent of the three ketone bodies. These
reactions all occur within the mitochondrial matrix. The six-carbon
compound -hydroxy- -methylglutaryl-CoA (HMG-CoA) is
also an intermediate of sterol biosynthesis, but the enzyme that
forms HMG-CoA in that pathway is cytosolic. HMG-CoA lyase is
present in the mitochondrial matrix but not in the cytosol.
The role of  oxidation in the
conversion of seed triacylglycerols
into glucose in germinating seeds.
Ketone body formation
and export from the liver.
Conditions that increase
gluconeogenesis (diabetes,
fasting) slow the citric
acid cycle (by drawing off
oxaloacetate) and enhance
the conversion of acetylCoA to oxaloacetate. The
released
coenzyme
A
continued  oxidation of
fatty acids.
The acetyl-CoA carboxylase reaction.
Acetyl-CoA carboxylase has three functional
regions; biotin carrier protein; biotin carboxylase,
which activates CO2 by attaching it to a nitrogen in
the biotin ring in an ATP-dependent reaction; and
transcarboxylase, which transfers activated CO2
from biotin to acetyl-CoA, producing malonyl-CoA.
The long, flexible biotin arm carriers the activated
CO2 from the biotin carboxylase region to the
transcarboxylase active site, as shown in the
diagrams below the reaction arrows.
The four-step sequence used
to lengthen a growing fatty
acyl chain by two carbons.
Each malonyl group and
acetyl (or longer acyl) group is
activated by a thioester that
links it to the fatty acid
synthase,
a
multienzyme
complex.
1) The first step is the condensation of an activated acyl group (an acetyl group is
the first acyl group) and two carbons derived from malony-CoA, with the
elimination of CO2 from the malonyl group; the net effect is extension of the acyl
chain by two carbons. The b-keto product of this condensation is then reduced in
three more steps nearly identical to the reactions of  oxidation, but in the
reverse sequence:
2) The -keto group is reduced to an alcohol
3) The elimination of H2O creates a double bond, and
4) The double bond is reduced to form the corresponding saturated fatty acyl
group.
Subcelllar localization of lipid metabolism in yeast and in vertebrate animal cells differs from that
in higher plants. Fatty acid synthesis takes place in the compartment in which NADPH is available
for reductive synthesis (i.e., where the [NADPH]/[NADP+] ratio is high).
The acetyl group shuttle
for transfer of acetyl
groups from mitochondria
to the cytosol for fatty
acid synthesis. (The outer
mito-chondrial membrane
is freely permeable to all
of these compounds.)
Acetyl groups pass out of the mitochondrion as
citrate; in the cytosol they are delivered as acetylCoA for fatty acid synthesis. Malate returns to the
mitochondrial matrix, where it is converted to
oxaloacetate. An alternative fate for cytosolic
malate is oxidation by malic enzyme to
generate cytosolic NADPH; the pyruvate
produced returns to the mitochondrial matrix.
Overwiew of the catabolism of
aminoacids. The separate path
taken by the carbon skeleton and
the amino groups are emphasized
by the bold divergent arrow.
The glucose-alanine cycle.
Alanine serves as a carrier of
ammonia equivalents and of the
carbon skeleton of pyruvate from
muscle to liver. The ammonia is
excreted, and the pyruvate is used to
produce glucose, which is returned to
the muscle.
The "Krebs bicycle", composed of the urea cycle on the right, which meshes with the asparateargininosuccinate shunt of the citric acid cycle on the left. Fumarate produced in the cytosol by
argininosuccinate lyase of the urea cycle enters the citric acid cycle in the mitochondrion and is
converted in several steps to oxaloacetate. Oxaloacetate accepts an amino group from glutamate
by transamination, and the aspartate thus formed leaves the mitochondrion and donates its
amino group to the urea cycle in the argininosuccinate synthetase reaction. Intermediates in the
citric acid cycle are boxed here.
The urea cycle and the reactions that feed amino group into it.
Note that the enzymes catalyzing these reactions are distributed between the mitochondrial matrix and the cytosol. One amino group enters
the urea cycle from carbamoyl phosphate (step 1), formed in the matrix; the other (entering at step 2) is derived from aspartate, also formed
in the matrix via transamination of oxaloacetate and glutamate in a reaction catalyzed by aspartate aminotransferase. The urea cycle itself
consists of four steps:
1) Formation of citrulline from ornithine and carbamoyl phosphate. Citrulline passes into the cytosol
2) Formation of argininosuccinate through a citrullyl-AMP intermediate
3) Formation of arginine from argininosuccinate. This reaction releases fumarate, which enters the citric acid cycle
4) Formation of urea. The arginase reaction also regenerates the starting compound in the cycle, ornithine.
The nitrogen cycle.
The total amount of nitrogen
fixed annually in the biosphere
exceeds 1011kg.
Overview of amino acid biosynthesis.
Precursors from glycolysis (red), the
citric acid cycle (blue),and the pentose
phosphate pathway (purple), and the
amino acids derived from them are
boxed in the corresponding colors..
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