Question

1. Write out all steps of the Kreb’s cycle including all metabolites, enzymes, and coenzymes. Identify a point of regulation in this pathway (20 points).

Answer 1

ANS- Kreb cycle- It is the process through which energy is produced for the the cell and also plays an key role in aerobic respiration.It is also called TCA(Tricarboxylic acid) cycle.

• In kreb cycle the acetyl-CoA  molecule is used which was an oxide form of pyruvate that is generated through the glucose metabolism,the process is called glycolysis.
• It takes place in mitochondrial matrix (eukaryotes) and in cytosol of prokaryotes.

STEPS OF KREB CYCLE-

STEP 1-Citrate formation.

Initiation of kreb cycle begins with addition of acety-CoA to the oxaloacetate by the help of an called enzyme citrate synthase, That results into the formation of citrate.

STEP 2-Iso.citrate formation

Now the citrate is isomerized(rearranged) into isomeric form called isocitrate in order to moves its OH group from 3' to 4' position of a molecule , by the help of an enzyme aconitase.

STEP 3-Oxidation of isocitrate to -ketogulatarate.

In this phase isocitrate is converted into -ketogulatarate by the help of enzyme isocitrate dehydrogenase,which catalyzes the oxidation of OH group present at the 4' position of isocitrate in order to form -ketogulaterate and releases the carbon dioxide molecule.

• In this step NADH is roduced from NAD molecule.(NAD------>NADH)

STEP 4--ketogulatrate oxidated into succinyl-CoA

Now the 4 carbon molecule is form called succiny-CoA by the addition of coenzyme A in - ketogulatrate and also CO2 molecule is released during this process.and also At the time of this process ,NAD+ is reduced to NADH + H+ by the help of an enzyme -ketoglutarate dehydrogenase.

STEP 5- Conversion of succinyl- CoA into succinate.

Succinyl-CoA is converted into succinate by the removal of CoA.During this process the released energy is utilized to form guanine triphosphate(GTP) from guanosine diphosphate(GDP) by adding 1 molecule of inorganic phosphate and also GTP molecule used to form ATP molecule, this whole process of conversion and energy production is catalysed by the enzyme called succinyl-CoA synthase.

STEP 6- Fumarate formation from succinate

Now fumarate is generated from the succinate molecule by the removal of 2 hydrogens from it,and also during this process FAD is reduced to FADH2 and the whole reaction is catalyzed by the enzyme called succinate dehydrogenase.

STEP 7-Hydration of fumarate to malate.

fumarate is converted into malate by the addition of hydrogen molecule in fumarate through enzyme fumarse .

STEP 8- Formation of oxaloacetate from malate

Now malate is converted into oxaloacetate by the help of malate dehydrogenase , during this process NAD+ is reduced to NADH + H+.

ENZYMES USED IN KREB CYCLE

• Fumarase
• Malic dehydrogenase
• -ketoglutrate dehydrogenase
• Aconitase
• Citrate synthase

REGULATION OF KREB CYCLE

The regulation of TCA cycle is occur by the influence of these enzymes.

1. Citrate synthase---> This enzyme is allosterially inhibited by the ATP and long- chain of fatty acyl-CoA.
2. Succinate dehydrogenase---> This enzyme inhibition is occur through the oxaloacetae and the availibility of oxaloacetate, as controlled by malate dehydrogenase.
3. Alpha ketoglutrate dehydrogenase--->It is inhibited by succinyl CoA and NADH molecules. and also it is inhibted by the highh energy charge, thats why the rate of cycle is reduced because of the presence of ATP molecules.
4. Isocitrate dehydrogenase---> This stimulated by ATP molecules , which enhances the affinity of enzymes for substrate and also this enzyme is inhibited by iso-citrate dehydrogenase by displacing NAD+.

PRODUCTS OF KREB CYCLE

One Cycle form 7 products-

1. GTP
2. FADH2
3. 3NADH
4. 2CO2 carbon dioxide