Step 1

Step 2 The glucose-6- phosphate is changed into an isomer ,fructose -6- phosphate . This means that the number of atoms is unchanged , but their position have changed(aldose to ketose conversion) this reversible reaction is catalyzed by phosphohexose isomerase (phosphoglucisomerase)enzyme.

Reaction 3 phosphate ester synthesis This reaction is virtually identical to step 1 fructose 6 phosphate is reacted with phosphate from ATP to make fructose 1-6 bisphosphate again this reaction using ATP in the presence of Mg ion. this reaction is irreversible catalyzed by phosphofructokinase enzyme and it is the rate limiting step and has regulating role in glycolysis.

Reaction 4 split molecule in half The six carbon fructose 1-6 bisphosphate is splitted into two triose phosphate carbon compounds which are dihydroxyacetone phosphate and glyceraldehyde 3 phosphate. The split is made between the C3 and C4 of the fructose.this reversible reaction is catalyzed by aldolase .the two cabon atoms compounds are interconverted by phosphotriose isomerase.

The reaction no.5 It is a reversible reaction is first an oxidation involving the coenzyme NAD˖ ,glyceraldehyde 3 phosphate is oxidized to an intermediate acid through the conversion of NAD˖ to NADH + H˖. then an inorganic phosphate (pi)is added in a phosphate ester synthesis to form1,3 bisphosphoglycerate.this and all the remaining reactions occur twice for each glu 6 phosphate (6 carbon),since there are now two molecules of 3 carbon each.

This reaction is catalyzed by glyceraldehyde 3 phosphate dehydrogenase which is atetramer containing SH groups, therefore, may be inhibited by SH poison like iodoacetate which stop glycolysis.so ,iodoacetate is used as prservative of blood sample to prevent in vitro glycolysis.

Reaction 6 hydrolysis of phosphate or synthesis of ATP One of the phosphate groups of 1,3 bisphosphoglycerate undergoes hydrolysis to form 3- phosphoglycerate and phosphate ion is transferred directly to an ADP to make ATP,therefore ,at this stage two molecules of ATP are produced per molecule of glucose ,this is reversible reaction and catalyzed by phosphoglycerate kinase in presence of Mg ion.

Compitition of arsenate with inorganic phosphate in the above reaction to give 1-arseno 3 phosphoglycerate,which is hydrolyzed spontaneously to 3 phosphoglycerate and heat without generating ATP, explain how arsenate toxicity occur for examples in rat.

Reaction 7 isomerization In this reaction the phosphate group moves from position 3 to position 2 of 3-phosphoglycerate producing 2- phosphoglycerate. It is reversible reaction and catalyzed by phosphoglycerate mutase enzyme.

Step 8 Alcohol dehdration (enolation) Dehydration of 2 phosphoglycerate forming phosphoenolpyruvate,this reversible reaction is catalyzed by enolase which is dependent on presence of either Mg ˖² or Mn˖² and inhibited by fluoride which is used as a preservative in vitro in estimation of blood glucose.

Step 9 phosphate ester hydrolysis This is the final reaction in glycolysis,phosphate group of phosphoenolpyruvate is transferred to ADP forming ATP while phosphoenolpyruvate is converted to enolpyruvate which undergoes spontaneous non enzymatic isomerization to pyruvate.at this stage two molecules of ATP are produced per molecule of glucose .this is irreversible reaction catalyzed by pyruvate kinase in the presence of Mg˖²

Fate of pyruvate Depending on availability of aerobic or anaerobic conditions : 1. Under aerobic condition it is transported from the cytoplasm to mitochondria via special pyruvate transporter where it is oxiditively decarboxylated into acetyl Co A (active acetate) by several different enzymes working sequentially in a multienzyme system called collectively as pyruvate dehdrogenase complex system.

The presence of arsenate or defeciency of thiamin inhibit pyruvate dehydrogenase allowing pyruvate to accomulate, also it is inhibited by the product acetyl Co A ,therefore, any source that give rise to acetyl Co A can inhibit pyruvate dehydrogenase system as amino acid and fatty acids. acetyl Co A enter CITRIC ACID cycle for further energy production.

2.Anaerobic condition:pyruvate is reduced to lactate by NADH˖² produced in reaction 5 ,this reversible reaction catalyzed by lactate dehydrogenase with production of NAD˖ allowing glycolysis to proceed in anaerobic condition by regenerating sufficient NAD˖ for reaction 5 to continue.

Therefore,tissues that can function under hypoxic condition can produce lactate as skeletal muscle and RBC

conclusion 1.Glycolysis can be presented simply as: Glucose +2NAD˖ +2ADP +2P→ 2pyruvate+2ATP+2 NADH+2H˖ 2.Glucose with 6 carbon atoms is converted into two pyruvate molecules three carbon each. The ATP produced is as follows: a.Anaerobic condition :2ATP are produced (2ATP produced at reaction 6+2ATP at position 9_2ATP consumed at reaction 1 and 3) i.e 4-2=2

b.Aerobic condition: 2 ATP in anaerobic condition plus 5 ATP are produced at reaction 5 to respiratory Chain (each NADH˖² molecule produce 2.5 ATP) plus 5ATP generated by entrance of two NADH˖²produced from conversion of two molecules of pyruvate into two molecules of acetyl Co A per molecule of glucose to respiratory chain . Plus 20 ATP are produced from the citric acid cycle. So the net ATP produced in aerobic condition are 32 (2+5+5+20=32)

3.Three reactions 1,3 and 9 are irreversible regulating glycolysis while the rest are reversible. 4.Glycolysis reaction can be blocked at position 5 by iodoacetate and at position 8 by fluoride ,so these substances can be used as preservatives of sample for blood glucose estimation.

Clinical aspects 1.Inhibition of pyruvate metabolism leads to lactic acidosis the main cause of this inhibition are: a.Arsenate inhibit pyruvate dehydrogenase complex. b.Thiamin is a coenzyme for pyruvate dehydrogenase therefore its defeciency lead to lactic acidosis.

c. Inherited pyruvate dehydrogenase defeciency presented with lactic acidosis ,particularly after a glucose load. Because of its dependence on glucose as fuel, brain is a prominent tissue where these metabolic defects manifest themselves in neurological disturbances. 2.Inherited aldose deficiency and pyruvate kinase deficiency in erythrocytes cause hemolytic anemia.

3.The exercise capacity of patient with muscle phosphofructokinase defeciency is low ,particularly on high carbohydrate diets. 4.Competition of arsenate with inorganic phosphate to give 1-arseno 3-phosphoglycerate which hydrolyzes spontaneously to give 3-phosphoglycerate+heat,without generating ATP.

Citric acid cycle( krebs cycle) It is a series of reactions discovered by kreps in 1937.occur in the mitochondria that oxidize acetyl Co A to CO2 and H2O in addition to the production of reducing equivalent NADH2 and FADH2 that upon reoxidation in the respiratory chain ATP is formed.. Its dependent on oxygen availability ,therefore, absence i.e anoxia or deficiency i.e hypoxia leads to total or partial inhibition of the cycle respectively.

Importance of kreps cycle 1.It is the final common pathway for the aerobic oxidation of CHO, lipid and protein because glucose ,fatty acids and most aminoacids are metabolized to acetyl Co A or intermediate of the cycle. 2.Its role is central in gluconeogenesis ,lipogenesis and interconverting of aminoacids. 3.Libration of much free energy from oxidation of CHO, lipid and protein. 4.Formation of reducing equivalents which enter the respiratory chain for energy production.

Reactions of kreps cycle 1.Synthesis of citrate 2.Dehydration and rehydration 3.Dehydrogenation and decarboxylation 4.Oxidation and decarboxylation(oxidative decarboxylation) 5.Hydrolysis of succinyl Co A ,synthesis of ATP 6.Dehydrogenation 7.Hydration 8.dehydrogenation

Reaction 1 Condensation of acetyl CoA and oxaloacetate to form citrate and release of CoA –SH this irreversible reaction is catalyzed by citrate synthase .

Reaction 2 Citrate is isomerized to isocitrate by the enzyme aconitase in the presence of iron in ferrous state. this reversible reaction takes place in two steps dehydration of citrate to cis aconitate and rehydration of cis aconitate to isocitrate. The poison fluoroacetate is toxic because fluoroacetyl CoA condenses with oxaloacetate to form fluorocitratae that inhibits aconitase ,causing citrate to accomulate.

Reaction 3 Isocitrate in the presence of NAD˖ is converted to α ketoglutarate ,although this reaction is reversible it is directed more toward the right and is catalyzed by isocitrate dehydrogenase in two steps.dehydrogenation in the presence of NAD˖ to oxalosuccinate (intermediate)+NADH+H Decarboxylation of oxalosuccinate to αketoglutarate and CO2,in the presence of Mn˖² or Mg˖²