Here is a term paper on ‘Carbohydrates Metabolism’ especially written for school and college students. This is another test, the stuff is not hanging from here. What’s the issue the?
Whatever a cell does has to be paid for in the currency of energy. If there is no free energy available there is no life. The animal cell does not generate its own energy. It obtains energy ready-made from outside in small parcels which we call foodstuff molecules. The cell knows two methods of getting out the energy of these molecules; it either fragments them or burns them. The first method we refer to as fermentation, the second, oxidation.
NAD+/NADH and ADP/ATP Cycles in Glycolysis:
Glycolysis is the conversion of glucose to lactic acid –
Although molecular O2 is not required for this process, the production of lactic acid from glucose is nonetheless an oxidative process, and the energy released is captured in a useful form (i.e., as ATP). In brief outline, the stoichiometry of glycolysis may be summarized as follows –
Under normal metabolic conditions, the two reactants in this system which usually become limiting are the ADP and NAD+. That is, in contrast to the substrate-level concentrations of other metabolites, they are usually present in only catalytic concentrations. Therefore, it is essential that both ADP and NAD+ be continually generated if glycolysis is to continue uninterrupted and is to accomplish a significant conversion of glucose to lactic acid. The ADP can be constantly replenished since ATP is not stored and is utilized almost concurrently with its formation.
The catalytic quantity of NAD+ required for the glycolytic process can be replenished in two ways:
(1) In the presence of oxygen, re-oxidation via the mitochondrial respiratory chain, or
(2) In the absence of oxygen, re-oxidation by a product of the glucose itself, namely the pyruvate –
By the latter mechanism the net stoichiometry of anaerobic glycolysis is –
Although one usually defines glycolysis as the conversion of glucose to lactic acid, it is important to note that glucose itself is metabolically inert. Glucose is a major organic solute in the extracellular fluids, but intracellular it exists almost entirely as a phosphate ester –
Indeed, this negatively charged derivative may be considered to be locked in the cell. For glucose to be returned to the extracellular compartment, it must lose the phosphate group.
Synthesis of Glucose-6-Phosphate as the Substrate for Glycolysis:
As in glycolysis, many other anabolic and catabolic reactions of carbohydrate metabolism also require glucose-6-phosphate as the primary substrate. This phosphate ester can be formed by two reactions –
Isomerization of Glucose-6-Phosphate to Fructose-6-Phosphate:
In the presence of an isomerase specific for both hexoses, glucose-6-phosphate is converted to fructose-6-phosphate –
Use of ATP to Convert D-Fructose-6-Phosphate to Fructose-1, 6-Diphosphate:
As in the case of the hexokinase catalyzed phosphorylation of glucose, the introduction of a second phosphate into fructose-1-phosphate requires Mg2+ and is practically irreversible.
Splitting of Fructose- 1, 6-Diphosphate into Two Isomeric Phosphotrioses-Dihydroxyacetone Phosphate and Glyceraldehyde-3-Phosphate:
Isomerization of the Triose Phosphates:
Only glyceraldehyde-3-phosphate can serve as a substrate for the subsequent glycolytic reactions. However, the other half of the erstwhile hexose, dihydroxyacetone phosphate, can be converted into its aldehyde isomer –
Because of this reaction, the entire glucose molecule can be degraded by the glycolytic sequence. In summary, the formation of the two trioses from glucose-6-phosphate can be depicted as follows –
Excluding the two phosphorylations, the reactions of glycolysis to this point have involved isomerizations and the reversal of an aldol condensation. No energy has been derived from the process. Overall, the composite oxidation-reduction state of the glyceraldehyde phosphate is the same as for glucose. One may think of glucose having undergone an internal dismutation to produce two aldehydic trioses.
Dehydrogenation of Glyceraldehyde-3-Phosphate (or 3-Phospho-Glyceraldehyde):
The first reaction of the glycolytic sequence in which energy is made available is the oxidation of the 3-phosphoglyceraldehyde to an acid anhydride, 3-phosphoglyceroyl phosphate –
The enzyme catalyzing this reaction, 3-phosphoglyceraldehyde dehydrogenase, has four identical subunits and exhibits negative cooperatively in the binding of the NAD+, which is the coenzymic electron acceptor. Each of the four catalytic sites also contains a cysteinyl side chain whose -SH group binds the 3-phospho-glyceraldehyde covalently.
The sequence of electron-transfer reactions at one of these sites is as follows –
Conservation of the Energy of 3-Phosphoglyceroyl Phosphate as ATP:
As the next step in glycolysis, the phosphate in anhydride linkage with the carboxyl group of 3-phosphoglyceroyl phosphate is transferred enzymatically to ADP –
Isomerization of 3-Phosphoglycerate to 2-Phosphoglycerate:
In a reaction that requires the participation of 2, 3- diphosphoglycerate and Mg2+ as cofactors, 3-phosphoglycerate is converted to 2-phosphoglycerate. The role of the enzyme phosphoglyceromutase can be depicted as follows –
Dehydration of 2-Phosphoglycerate to Phosphoenol Pyruvate:
Catalyzed by enolase, a dehydration is effected between C-2 and C-3 of the 2 phosphoglycerate –
The O—P bond at position 2 has the character of an anhydride bond. That is, the dehydration has created an enolic group whose combination with a phosphate provides the second high-energy phosphate to be made available in glycolysis.
Synthesis of the Second ATP provided by the Glycolytic Sequence:
Catalyzed by pyruvate kinase, the phosphate of phosphoenol pyruvate is transferred to ADP –
Regeneration of NAD+ and the Synthesis of Lactate:
The continuous catabolism of glucose by the glycolytic mechanism depends on the uninterrupted re-oxidation of the NADH resulting from the triosephosphate dehydrogenation. The only avenue for regeneration of NAD+ in an anaerobic system is the transfer of electrons from the NADH to pyruvate –