Question

hi, trying to understand the proccess of crebs cycle..

a. If Isocitrate dehydrogenase is inhibited by axcess of citrate, and alpha ketogluterate by axcess of Suc-CoA, what exactly happeneds with the already made citrate and Suc-CoA? do they go back to being Acetyl coA and Alphaketogluterate respectively (and also become into fatty acids, amino acids and purins)?

b. I also don't get how calcium signals for more production of Acethyl coA? who gets this signal? is that the Icam somthing...?

c. in HIF1 degradation in the proteosome, what exactly pVHL and PHD2 do? like what is done individually and what tougether?

d. does HIF1 stop crebs cycle? I only know it encourages anaerobic respiration.. so what proccess does it stop if so? is directly or indirectly?

e. is there any other important substances in the crebs cycle? i remember something said about Fumarate and Succinate is there something worth remembering?

thanks in advance, I am very very lost and my summery is just confusing me.

Answer 1

Defination of Kreb's Cycle:-

Kreb's Cycle, also known as the TCA cycle.

It is an aerobic process consisting of eight definite steps. In order to enter the Kreb's Cycle pyruvate must first be converted into Acetyl-CoA by pyruvate dehydrogenase complex found in the mitochondria.

In the presence of oxygen organisms are capable of using the Kreb's Cycle. The reason oxygen is required is because the NADH and [FADH2] produced in the Kreb's Cycle are able to be oxydized in the electron transport chain (ETC) thus replenishing the supply of NAD+ and [FAD].

Steps:-In order for pyruvate from glycolysis to enter the Kreb's Cycle it must first be converted into acetyl-CoA by the pyruvate dehydrogenase complex which is an oxidative process wherein NADH and CO2 are formed. Another source of acetyl-CoA is beta oxidation of fatty acids.

1. Acetyl-CoA enters tthe Kreb Cycle when it is joined to oxaloacetate by citrate synthase to produce citrate. This process requires the input of water. Oxaloacetate is the final metabolite of the Kreb Cycle and it joins again to start the cycle over again, hence the name Kreb's Cycle. This is known as the committed step
2. Citrate is then converted into isocitrate by the enzyme aconitase. This is accomplished by the removal and addition of water to yield an isomer.
3. Isocitrate is converted into alpha-ketogluterate by isocitrate dehydrogenase. The byproducts of which are NADH and CO2.
4. Apha-ketogluterate is then converted into succynl-CoA by alpha-ketogluterate dehydrogenase. NADH and CO2 are once again produced.
5. Succynl-CoA is then converted into succinate by succynl-CoA synthetase which yields one ATP per succynl-CoA.
6. Succinate coverts into fumerate by way of the enzyme succinate dehydrogenase and [FAD] is reduced to [FADH2] which is a prosthetic group of succinate dehydrogenase. Succinate dehydrogenase is a direct part of the ETC. It is also known as electron carrier II.
7. Fumerate is then converted to malate by hydration with the use of fumerase.
8. Malate is converted into oxaloacetate by malate dehydrogenase the byproducts of which are NADH.

Diagram:-

ANSWER (C):-  Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor that activates the cellular response to hypoxia. The HIF1 subunit is constantly synthesized and degraded under normoxia, but degradation is rapidly inhibited when oxygen levels drop.

The hypoxia-inducible transcription factor (HIF) is expressed at very low levels in cells under normal oxygen tension, but is rapidly induced upon exposure to hypoxia , triggering the activation of a genetic program that enables the metabolic adaptation of cells . HIF is a heterodimeric factor composed of a hypoxia-regulated subunit (HIF1 or HIF2) and constitutively expressed HIF1(also known as aryl hydrocarbon receptor nuclear translocator, ARNT) . Although the subunit is constantly transcribed and translated, it is also degraded in an oxygen-dependent mechanism. It is only with dwindling oxygen levels that HIF1 or HF2 expression is readily detected . In the presence of oxygen, HIF prolyl-hydroxylases (PHD) hydroxylate two proline residues, in a reaction that requires molecular oxygen and -ketoglutarate as co-substrates. These hydroxyproline residues are recognized by the Von Hippel-Lindau tumor suppressor protein (pVHL), one of the components of a E3 ubiquitin-ligase complex that also contains elongins B and C, cullin2, and Rbx, which conjugates ubiquitin to HIF. This results in the oxygen-dependent targeting of HIF to the proteasome. Decreased oxygen concentration results in impaired prolyl-hydroxylation, reduced targeting of HIF to the proteasome and the accumulation of HIF in the nucleus, where it activates a plethora of genes devoted to improving the delivery of oxygen and enhancing the production of ATP by glycolysis, among other actions.

Answer (d):- There are metabolites of the Krebs cycle. The oxidation of succinate by succinate dehydrogenase (SDH) has been shown to be of importance in the classical activation of macrophages. This leads to reverse electron transport (RET) in complex I of the electron transport chain (ETC) driving the production of ROS, which in turn leads to activation of HIF1. Increased levels of cytosolic succinate can inhibit the prolyl hydroxylase domain enzymes via product inhibition, also potentiating HIF1 stabilization. This prevents the hydroxylation of proline residues on HIF1, and so it is not ubquitinated and targeted for proteasomal degradation . Instead, it can heterodimerize with its binding partner the aryl hydrocarbon nuclear translocator (ARNT/HIF-1). The HIF-1 complex can translocate to the nucleus and bind hypoxia response elements in the promoters of HIF target genes. HIF also represses mitochondrial function through upregulation of pyruvate dehydrogenase kinase 1 (PDK1) . PDK1 phosphorylates and inhibits pyruvate dehydrogenase (PDH) and so pyruvate cannot be converted into acetyl-CoA in order to enter the mitochondria and feed the Krebs cycle .

Answer (e):- Fumarate:- Oxidation of Succinate to Fumarate ,The succinate formed from succinyl-CoA is oxidized to fumarate by the flavoprotein succinate dehydrogenase . In eukaryotes, succinate dehydrogenase is tightly bound to the inner mitochondrial membrane (in prokaryotes, to the plasma membrane); it is the only enzyme of the citric acid cycle that is membrane-bound.

The reversible hydration of fumarate to L-malate is catalyzed by fumarase .

Succinate:-  

Succinyl-CoA, like acetyl-CoA, has a strongly negative free energy of hydrolysis of its thioester bond . In the next step of the citric acid cycle, energy released in the breakage of this bond is used to drive the synthesis of a phosphoanhydride bond in either GTP or ATP, and succinate is also formed in the process