Vías centrales del metabolismo de los carbohidratos

El estudio de los carbohidratos debe considerar1. Central pathways of carbohydrate metabolism

2. Conversion of compounds to intermediates usable in central pathways

3. Mechanisms of energy (ATP) productiona) Substrate-level phosphorylation

b) Oxidative phosphorylation

c) Other mechanisms of energy transfer

El estudio de los carbohidratos debe considerar4. 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

El estudio de los carbohidratos debe considerar5. Oxygen involvement in energy-generating reactions

a) Aerobic metabolism

b) Anaerobic metabolism

c) Facultative metabolism

6. Metabolic intermediates serving as biosynthetic precursors

El estudio de los carbohidratos debe considerar8. Reactions that replenish biosynthetic intermediates (anapleurotic

reactions)

9. Metabolic and genetic regulatory system

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.

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.

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.

FRUCTOSE BISPHOSPHATE - ALDOLASE OR EMBDEN-MEYERHOF-PARNAS

(EMP)PATHWAY OF GLYCOLYSIS.

Fructose Bisphosphate Aldolase Pathway

Reacciones más importantes de de la ruta de EMP1. 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

Fructose Bisphosphate Aldolase Pathway

Fructose Bisphosphate Aldolase Pathway

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,

Regulación metabólica de las enzimas de la glucólisis y del ciclo de los ácidos tricarboxílicos

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 C 3-C3 cleavage (aldol cleavage) to form three-carbon intermediates.

Inicio de la vía del 6 fosfo gluconato

Phosphoketolase Pathway

Oxidative Pentose Phosphate Cycle

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.

Tarea ¿Cuál es el destino de los carbonos 3 y 4 marcados con

radiactividad de una glucosa que entra a la vía

1. EMB2. Entner-Doudoroff3. Pentosa fosfato4. Fosfogluconato5. Ciclo oxidativo de pentosa fosfato ?

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.

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.

Operation of the TCA cycle under anaerobic conditions.

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).

Pyruvate kinase is not reversible

12. Pyruvate kinase (pykA; pykF). Generates ATP from ADP14. Pyruvate carboxylase. Converts pyruvate to oxaloacetic acid (OAA) via carbon dioxide (CO2) fixation using ATP.15. PEP carboxykinase. Forms phosphoenolpyruvate from OAA using GTP..

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.

The third bypass reaction required for gluconeogenesis involves dephosphorylationof G-6-P

the formation of glucose from pyruvate requires a considerableexpenditure of energy

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.

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.

Gluconeogenesis Regulation The stationary phase induction of PEP carboxykinase requires

Cyclic AMP and

a regulatory signal, (not been fully elucidated)

Glycogen Synthesis