Carbohydrate metabolism and TCA cycle -
Bottom Line : In this mitochondrial pathway, the 2-carbon acetyl group derived via glycolysis from glucose or by oxidation of fatty acids, disappears and is replaced by two CO 2 's. The acetyl groups are fully oxidized to generate NADH and FADH 2 and a great deal of energy is generated.
The TCA cycle only occurs under aerobic conditions; it generates energy in the form of GTP equivalent to ATP , NADH, and FADH 2. For a picture of the TCA cycle, see Devlin, Figure Bottom Line: This simple, yet complicated, pathway serves four very different purposes.
It connects with glycolysis in several places. For a picture of the pentose phosphate pathway, see Devlin, Figure We live in an environment that produces a large amount of oxidation in our tissues.
These oxidation reactions can damage our cells. The reducing power of NADPH makes it ideal as a co-factor for many of our anti-oxidant defense systems. This is very important in RBCs which have to maintain the iron in hemoglobin in the reduced state. Note that lesions in the pentose phosphate pathway in RBCs can cause serious reactions to certain medications.
The liver's job is to make sure that levels of blood glucose are sufficiently maintained to support brain and RBC function.
The actual levels of glucose can be regulated by the hormones insulin and glucagon. Gluconeogenesis is almost like glycolysis run in reverse.
A coenzyme is a small molecule that participates in an enzymatic reaction without really getting used up. Initially, the storage fuels or foodstuffs fats, carbohydrates, and proteins are hydrolyzed into smaller components fatty acids and glycerol, glucose and other simple sugars, and amino acids.
The structure of this compound and the manner in which fatty acids are degraded has been considered in Section F , and amino acid metabolism is discussed briefly in Section C. This section is concerned mainly with the pathway by which glucose is metabolized by the process known as glycolysis.
This stage is known variously as the citric acid cycle, the tricarboxylic acid cycle, or the Krebs cycle, in honor of H. Krebs Nobel Prize, , who first recognized its cyclic nature in We can write an equation for the process as if it involved oxygen:. But, as you will see, none of the steps uses molecular oxygen directly.
Hence there must be a stage in metabolism whereby molecular oxygen is linked to production of oxidizing agents that are consumed in glycolysis and in the citric acid cycle.
The coupling of oxygen into the metabolism of carbohydrates is an extremely complex process involving transport of the oxygen to the cells by an oxygen carrier such as hemoglobin, myoglobin, or hemocyanin. These processes are known as oxidative phosphorylation and can be expressed by the equations:.
Oxidative phosphorylation resembles photophosphorylation, discussed in Section , in that electron transport in photosynthesis also is coupled with ATP formation.
Of these, 34 ATPs are produced according to Equation and, as we shall see, two more come from glycolysis. For example, the reactants are either phosphate esters or mixed anhydrides, and the phosphorylating agent is ATP:. First, glucose is phosphorylated to glucose 6-phosphate with ATP.
A second phosphorylation with ATP gives fructose 1,6-diphosphate:. At this stage the enzyme aldolase catalyzes the aldol cleavage of fructose 1,6-diphosphate. One product is glyceraldehyde 3-phosphate and the other is 1,3-dihydroxypropanone phosphate.
The mixed anhydride of phosphoric acid and glyceric acid then is used to convert ADP to ATP and form 3-phosphoglycerate.
Thereafter the sequence differs from that in photosynthesis. Phosphoenolpyruvate is an effective phosphorylating agent that converts ADP to ATP and forms pyruvate:. The net reaction at this point produces more ATP than is consumed in the phosphorylation of glucose and fructose.
What happens thereafter depends on the organism. This reaction is an oxidative decarboxylation reaction. Acetyl CoA enters the Krebs cycle by combining with a four-carbon molecule, oxaloacetate, to form the six-carbon molecule citrate, or citric acid, at the same time releasing the coenzyme A molecule.
The six-carbon citrate molecule is systematically converted to a five-carbon molecule and then a four-carbon molecule, ending with oxaloacetate, the beginning of the cycle. Along the way, each citrate molecule will produce one ATP, one FADH 2 , and three NADH.
The FADH 2 and NADH will enter the oxidative phosphorylation system located in the inner mitochondrial membrane.
In addition, the Krebs cycle supplies the starting materials to process and break down proteins and fats. To start the Krebs cycle, citrate synthase combines acetyl CoA and oxaloacetate to form a six-carbon citrate molecule; CoA is subsequently released and can combine with another pyruvate molecule to begin the cycle again.
The aconitase enzyme converts citrate into isocitrate. In two successive steps of oxidative decarboxylation, two molecules of CO 2 and two NADH molecules are produced when isocitrate dehydrogenase converts isocitrate into the five-carbon α-ketoglutarate, which is then catalyzed and converted into the four-carbon succinyl CoA by α-ketoglutarate dehydrogenase.
The enzyme succinyl CoA dehydrogenase then converts succinyl CoA into succinate and forms the high-energy molecule GTP, which transfers its energy to ADP to produce ATP by substrate-level phosphorylation.
Succinate dehydrogenase then converts succinate into fumarate, forming a molecule of FADH 2. Oxaloacetate is then ready to combine with the next acetyl CoA to start the Krebs cycle again see Figure 4. For each turn of the cycle, three NADH, one ATP through GTP , and one FADH 2 are created.
Each carbon of pyruvate is converted into CO 2 , which is released as a byproduct of oxidative aerobic respiration.
The electron transport chain ETC uses the NADH and FADH 2 produced by the Krebs cycle to generate ATP. Electrons from NADH and FADH 2 are transferred through protein complexes embedded in the inner mitochondrial membrane by a series of enzymatic reactions.
In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP. The role of molecular oxygen, O 2 , is as the terminal electron acceptor for the ETC.
This means that once the electrons have passed through the entire ETC, they must be passed to another, separate molecule. This is the basis for your need to breathe in oxygen. Without oxygen, electron flow through the ETC ceases.
The electrons released from NADH and FADH 2 are passed along the chain by each of the carriers, which are reduced when they receive the electron and oxidized when passing it on to the next carrier. The accumulation of these protons in the space between the membranes creates a proton gradient with respect to the mitochondrial matrix.
Also embedded in the inner mitochondrial membrane is an amazing protein pore complex called ATP synthase. This rotation enables other portions of ATP synthase to encourage ADP and P i to create ATP. In accounting for the total number of ATP produced per glucose molecule through aerobic respiration, it is important to remember the following points:.
Therefore, for every glucose molecule that enters aerobic respiration, a net total of 36 ATPs are produced Figure 6. Gluconeogenesis is the synthesis of new glucose molecules from pyruvate, lactate, glycerol, or the amino acids alanine or glutamine. This process takes place primarily in the liver during periods of low glucose, that is, under conditions of fasting, starvation, and low carbohydrate diets.
So, the question can be raised as to why the body would create something it has just spent a fair amount of effort to break down? Certain key organs, including the brain, can use only glucose as an energy source; therefore, it is essential that the body maintain a minimum blood glucose concentration.
When the blood glucose concentration falls below that certain point, new glucose is synthesized by the liver to raise the blood concentration to normal. Gluconeogenesis is not simply the reverse of glycolysis.
There are some important differences Figure 7. Pyruvate is a common starting material for gluconeogenesis. First, the pyruvate is converted into oxaloacetate. Oxaloacetate then serves as a substrate for the enzyme phosphoenolpyruvate carboxykinase PEPCK , which transforms oxaloacetate into phosphoenolpyruvate PEP.
From this step, gluconeogenesis is nearly the reverse of glycolysis. PEP is converted back into 2-phosphoglycerate, which is converted into 3-phosphoglycerate. Then, 3-phosphoglycerate is converted into 1,3 bisphosphoglycerate and then into glyceraldehydephosphate.
Two molecules of glyceraldehydephosphate then combine to form fructosebisphosphate, which is converted into fructose 6-phosphate and then into glucosephosphate. Finally, a series of reactions generates glucose itself.
In gluconeogenesis as compared to glycolysis , the enzyme hexokinase is replaced by glucosephosphatase, and the enzyme phosphofructokinase-1 is replaced by fructose-1,6-bisphosphatase.
This helps the cell to regulate glycolysis and gluconeogenesis independently of each other. As will be discussed as part of lipolysis, fats can be broken down into glycerol, which can be phosphorylated to form dihydroxyacetone phosphate or DHAP.
Purpose: To review Green tea antioxidants reinforce your knowledge Carbohydeate the Carbohydrae of glycolysis, gluconeogenesis, the pentose phosphate pathway Carbohydrate metabolism and TCA cycle the metbaolism acid Krebs cycle, as well remind you about some metaboliam connections. Meyabolism Line: Glycolysis is an ancient energy-generating pathway used by essentially all cells. Glycolysis generates a small amount of energy, and provides the entry point for the pyruvate dehydrogenase reaction and the TCA cycle. For a good summary of the glycolytic pathway, check Devlin, Figure Please note the structuring into the three stages: a Priming stage; b Splitting stage; c Oxidoreduction—phosphorylation stage. Phosphorylation of glucose and conversion to phosphorylated fructose. New Terms Diabetic foot awareness Triphosphate ATP Acetyl Coenzyme Carbohydrate metabolism and TCA cycle CoA Electron meabolism chain Flavin Adenine Dinucleotide CCarbohydrate Glycolysis Cylce Adenine Dinucleotide NAD Pyruvate Pyruvate dehydrogenase Tricarboxylic Acid TCA cycle. Animals amd energy to carry out all the body processes e. Without energy, an animal is unable to move, to digest its food, to reproduce, to grow, or even to breathe. Energy requirement and balance are more important in food-producing animals with their need to synthesize nutrients e. Carbohydrates are the major energy source in the diet of farm animals.
Mir ist diese Situation bekannt. Ist fertig, zu helfen.