2. El estudio de los carbohidratos debe
considerar
1. Central pathways of carbohydrate metabolism
2. Conversion of compounds to intermediates usable in central pathways
3. Mechanisms of energy (ATP) production
a) Substrate-level phosphorylation
b) Oxidative phosphorylation
c) Other mechanisms of energy transfer
3. El estudio de los carbohidratos debe
considerar
4. Metabolic steps involved in the generation and use of reducing activity
a) Reduction of pyruvate or other substrates to fermentation end products
b) Biosynthetic reactions requiring reducing action
4. El estudio de los carbohidratos debe
considerar
5. Oxygen involvement in energy-generating reactions
a) Aerobic metabolism
b) Anaerobic metabolism
c) Facultative metabolism
6. Metabolic intermediates serving as biosynthetic precursors
5. El estudio de los carbohidratos debe
considerar
8. Reactions that replenish biosynthetic intermediates (anapleurotic
reactions)
9. Metabolic and genetic regulatory system
6. Los carbohidratos no son los únicos compuestos
utilizados como fuente de energía por los
microorganismos
Fatty acids, lipids, amino acids, purines, pyrimidines, and a wide variety of
other compounds can also serve as carbon and energy sources.
7. Los carbohidratos no son los únicos compuestos
utilizados como fuente de energía por los
microorganismos
Generally, utilization of an alternate substrate involves its conversion to an
intermediate intrinsic to one of the central pathways of carbohydrate
metabolism.
8. Vias principales del metabolismo de
carbohidratos
Glucolisis o via de Embden-Meyerhof-Parnas, o via de la Fructosa bisfosfato –
aldolasa
Entner-Doudoroff o Cetogluconato
Ciclo oxidativo de la Pentosa fosfato
Gluconeogénesis y síntesis de glicógeno.
11. Reacciones más importantes de de la ruta
de EMP
1. Phosphorylation of glucose and fructose-6-phosphate by ATP
2. Cleavage of fructose-1,6-bisphosphate to trioses by a specific aldolase
3. Structural rearrangements
4. Oxidation–reduction and Pi (inorganic phosphate) assimilation
14. The enzyme fructose bisphosphate (FPB) aldolase is one of the most critical
steps in the pathway. In the absence of this enzyme, glucose or other hexose
sugars must be metabolized via one of several alternative pathways,
16. Alternate Pathways of Glucose Utilization
Warburg and Christian
• described the oxidation of glucose-6-phosphate to
6phosphogluconate (6-P-G) via G-6-P dehydrogenase).
They also described the decarboxylation of 6-P-G to form
a pentose sugar.
Entner-Doudoroff or Ketogluconate Pathway
• differs from the EMP pathway primarily in the form of
the 6-carbon intermediate that undergoes C3-C3 cleavage
(aldol cleavage) to form three-carbon intermediates.
23. After one turn of the cycle, the net reaction is
Encircled P, phosphate group; G-6-P, glucose-6-phosphate; 6-PG, 6-
phosphogluconate; F-6-P, fructose-6-phosphate; F-1,6-BP, fructose-1,6-
bisphosphate; DOHAP, dihydroxyacetone phosphate; GA-P, glyceraldehyde-3-
phosphate.
24. Tarea
¿Cuál es el destino de los carbonos 3 y 4 marcados con
radiactividad de una glucosa que entra a la vía
1. EMP
2. Entner-Doudoroff
3. Pentosa fosfato
4. Fosfogluconato
5. Ciclo oxidativo de pentosa fosfato ?
25. Diferencias entre la glucólisis y la via de la
pentosa fosfato
Glycolysis generates NADH, which can be reoxidized by linkage to the
electron transport system, or under anaerobic conditions it can be used to
reduce an oxidized substrate, such as pyruvate, to lactate.
26. Diferencias entre la glucólisis y la via de la
pentosa fosfato
The pentose phosphate pathway generates NADPH, which is used primarily
for reducing power in biosynthetic reaction (e.g., the conversion of α-
ketoglutarate to glutamate or the incorporation of acetate into fatty acids)
and is not linked to the terminal respiratory system.
28. GLUCONEOGENESIS
Growth of microorganisms on poor carbon sources,
(L-malate, succinate, acetate, or glycerol),
requires the ability to synthesize hexoses for the production of other
compounds
(cell wall mucopeptides, storage glycogen, and other compounds derived from
hexose, such as pentoses, for nucleic acid biosynthesis).
29. Pyruvate kinase is not reversible
12. Pyruvate kinase (pykA; pykF). Generates ATP from ADP
14. Pyruvate carboxylase. Converts pyruvate to oxaloacetic acid (OAA) via carbon
dioxide (CO2) fixation using ATP.
15. PEP carboxykinase. Forms phosphoenolpyruvate from OAA using GTP..
30. A second irreversible enzyme is
phosphofructokinase
5. Phosphofructokinase (pfkA).
Phosphorylation of F-6-P to FBP using ATP.
16. Fructose-1,6-bisphosphatase. Removes
Pi from F-1,6-bisP to form F-6-P.
31. The third bypass reaction required for gluconeogenesis
involves dephosphorylation
of G-6-P
the formation of glucose from pyruvate
requires a considerable
expenditure of energy
32. Gluconeogenesis Regulation
A major regulatory step is PEP carboxykinase, encoded by pckA in E. coli.
By catabolite repression, a process in which gluconeogenesis is inhibited when
glucose or other carbohydrate carbon sources are available.
33. Gluconeogenesis Regulation
Maximum levels of PEP carboxykinase are induced at
the onset of the stationary phase of growth,
to ensure the synthesis of adequate carbohydrate storage reserves or
to provide metabolites from the upper part of the EMP pathway as the organism
converts proteins to gluconeogenic amino acids.
34. Gluconeogenesis Regulation
The stationary phase induction of PEP carboxykinase requires
Cyclic AMP and
a regulatory signal, (not been fully elucidated)
Divergent pathways from 6-phosphogluconate. The structural genes for the enzymes in E. coli are indicated by their three-letter designations. In the core pathway, glucose-6-phosphate dehydrogenase (zwf, for zwischenferment) oxidizes glucose-6-phosphate to 6-phosphogluconolactone. The lactone is dehydrated to 6-phosphogluconate via lactonase (pgl). In the pentose pathway, 6-phosphogluconate is oxidized to ribulose-5-phosphate and carbon dioxide by 6-phosphogluconate dehydrogenase (gnd). Phosphoribose isomerase maintains ribulose-5-phosphate and ribose-5-phosphate in equilibrium. In the Entner-Doudoroff pathway, 6-phosphogluconate is dehydrated by 6-phosphogluconate dehydratase (edd) to yield 2-keto-3-deoxy-6-hosphogluconate (KDPG). The enzyme KDPG aldolase (eda) cleaves KDPG to form pyruvate and GA-3-P (glyceraldehyde-3-phosphate). Pyruvate decarboxylase action yields ethanol and carbon dioxide. The GA-3-P is metabolized via the triose phosphate portion of the EMP pathway to yield ethanol and carbon dioxide. The net yield in the Entner-Doudoroff pathway is 2 ethanol + 2 CO2.
In this cycle, G-6-P is converted to ribulose-5-phosphate and CO2. Ribulose-5-phosphate is maintained in equilibrium with ribose-5-phosphate (R-5-P) and xylulose-5-phosphate (X-5-P) by the action of ribose phosphate isomerase and ribulose phosphate epimerase. Transketolase converts R-5-P and X-5-P to sedoheptulose-7-phosphate (SH-7-P) and glyceraldehyde- 3-phosphate (GA-3-P). SH-7-P and GA-3-P are converted to F-6-P and erythrose-4- phosphate (E-4-P) via transaldolase. Transketolase also catalyzes the conversion of E-4-P and X-5-P to F-6-P and GA-3-P. By reversal of the FBP aldolase and G-6-P isomerase reactions, GA-3-P and F-6-P may be converted to G-6-P. The G-6-P can then reenter the oxidative pentose cycle.
Glycolytic enzymes
1. Phosphorylase. Degradation of glycogen or starch to G-1-P.
2. Phosphoglucomutase. Isomerization of G-1-P to G-6-P.
3. Hexokinase. Phosphorylation of glucose to G-6-P, using ATP Hexokinase also phosphorylates
fructose to F-6-P using ATP (reaction 3a).
4. Phosphoglucoisomerase (pgi). Isomerization of G-6-P to F-6-P.
5. Phosphofructokinase (pfkA). Phosphorylation of F-6-P to FBP using ATP.
6. Fructose bisphosphate (FBP) aldolase (fbaA). Cleaves FBP to GA-3-P and dihydroxyacetone
phosphate.
7. Triose phosphate isomerase (tpi ). Interconverts GA-3-P and dihydroxyacetone-P.
8. Glyceraldehyde-3-phosphate dehydrogenase (gap). Oxidizes GA-3-P to 1,3-diphosphoglycerate
using nicotinamide adenine dinucleotide (NAD+) and inorganic phosphate (Pi)
to form NADP + H+.
9. Phosphoglycerokinase (pgk). Generates ATP from ADP.
10. Phosphoglyceromutase (pgm). Uses 2,3-diphosphoglycerate to convert 3-phosphoglycerate
to 2-phosphoglycerate.
11. Enolase (eno). Enolization of 2-phosphoglycerate forms high-energy phosphate bond
(∼P; encircled P) in phosphoenolpyruvate (PEP).
12. Pyruvate kinase (pykA; pykF). Generates ATP from ADP.
13. Lactate dehydrogenase. Reduces pyruvate to lactate using NADP + H+.
