Biochemistry sheet (11)

Transcription

1 Biochemistry sheet (11) Made by: wael abu-anzeh corrected by: abd. salman DATE: 3/11/2016

2 Some explanations about the previous lectures: 1- Glucose is the only form of sugar that circlet in the blood, so the fructose and galactose should be converted to the glucose form. 2- Glucose and galactose transport via passive absorption "without direct expenditure of metabolic energy" and compete on the same carrier the sodium carrier, while the fructose transports via facilitated diffusion "it has to be in high concentration in order to be absorbed" and this criteria is beneficial to the Diabetes's patients which enables them to eat a little amount of fructose containing food such as vegetables and fruit. 3- Cellulose is not considered as a source of glucose for the human being because we don t have the cellulase enzyme "which can degrade the (β 1-4 glycosidic linkage)" 4- Cellulose is considered as a source of glucose for animals (e.g. cows and cattle) because they have some types of bacterial inside their stomachs "which can degrade the (β 1-4 glycosidic linkage)" Glycolysis Glycolysis: Derived from the Greek stem glyk="sweet," and the word lysis="dissolution" Learning bjectives: there are five learning objectives for this chapter 1- What is the overall pathway in glycolysis? 2-ow Is the 6-Carbon Glucose Converted to the 3-Carbon Glyceraldehyde-3 Phosphate? 3-ow Is Glyceraldehyde-3-Phosphate Converted to Pyruvate? 4-ow Is Pyruvate Metabolized Anaerobically? 5-ow Much Energy Can Be Produced by Glycolysis? Glycolysis in short: is the process by which the glucose "the 6 carbon atom molecule" is converted to two pyruvate "the 3 carbon atom molecule" and yield or produce 4 ATP molecules but because it use or consume 2 ATP molecules during the process, the net product is 2ATPs only. The pyruvate molecules then will rather undergoes one of three enzymecatalyzed reactions, depending of the type of cell and its state of oxygenation.

3 Questions: Site of glycolysis? It takes place in the cytosol not in the mitochondria Why it is important in spite that it gives two ATPs only? Because not all the body cells have mitochondria (e.g. red blood cells, cornea of the eye, skin, the adrenal cortex of the adrenal gland) but they have cytosol, so the main source of energy for these cells is the glycolysis process. Which organs rely on glycolysis?

4 Cells that have a little or no mitochondria like the RBCs which depend completely on the glycolysis and the end product of the glycolysis will be lactate because the pyruvate will not enter the citric acid cycle. The glycolysis process is made up of 3 stages: -the first stage is called the investment stage االسحثوار" "هزحلة because we consume ATPs. -the second stage is called the splitting stage where the fructose-1, 6- bisphosphate to two triose phosphates. -the third stage is called ATP production stage where it produces 4 ATP molecules but hence we consumed 2 ATPs in the investment stage the net product is 2 ATPs. Glycolysis: a series of 10 enzyme-catalyzed reactions by which glucose is oxidized to two molecules of pyruvate gl ycolysis C C 3 CC Gluco se Pyruvate There is net conversion of 2ADP to 2ATP C ADP + 2 P i 2 C 3 CC ATP Glucose Pyruvate Fates of Pyruvate: Pyruvate is most commonly metabolized in one of three ways, depending on the type of organism and the presence or absence of 2

5 10 Reactions of glycolysis: 1- Phosphorylation of glucose to give glucose-6-phosphate: by addition of ATP and hexokinase enzyme which use Magnesium 2+ or MN+2 as a cofactor. 2- Isomerization of glucose-6-phosphate to give fructose-6-phosphate: they are isomers for each other. 3- Phosphorylation of fructose-6-phosphate to yield fructose-1,6- bisphosphate. 4- Cleavage of fructose-1,6,-bisphosphate to give glyceraldehyde-3- phosphate and dihyroxyacetone phosphate(dap) 5- Isomerization of dihyroxyacetone phosphate "ketone" to give glyceraldehyde-3-phosphate"aldhyde": the body prefers to deal with aldoses more than ketoses. 6- xidation of glyceraldehyde-3-phosphate to give 1,3- bisphosphoglycerate"which bind with hemoglobin ". 7- Transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP to give 3-phosphoglycerate 8- Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate: by the presence of mutase enzyme which transfer the phosphate group from one carbon atom to another carbon atom from the same compound. 9- Dehydration "removal of water" of 2-phosphoglycerate to give phosphoenolpyruvate 10- Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate Any enzyme's name that ends with Kinase = transfer the phosphate group from the ATP to sugar or from the sugar to the ADP or AMP. Any enzyme's name that ends with Phosphatase = removal of phosphate group Glycolysis - Reaction1: phosphorylation of alpha-d-glucose by hexokinase.

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7 The main purpose of Phosphorylation process is to make the glucose negatively charged so the enzymes can attach to it because the neutral glucose can move freely inside the cell,"صا ع" so phosphorylation process traps the glucose inside the cells. exokinase enzyme has high affinity for glucose and present in every cell body except the liver which contains glucokinase instead. Phosphorylation of glucose is endergonic and driven by the free energy of hydrolysis of ATP "coupled reactions " The net change of the reaction is exergonic. Glucose + P Glucose-6-phosphate + 2 G o' = kj mol -1 i ATP + 2 ADP + Pi G o' = kj mol -1 Glucose + ATP Glucose-6-phosphate + ADP G o' = kj mol -1 catalyzed by hexokinase (in non-hepatic cells) A large conformational change takes place when substrate is bound to the enzyme exokinase and hexokinase-glucose complex "induced fit theory " The ATP-dependent phosphorylation of glucose to form glucose 6- phosphate is catalyzed by hexokinase. The phosphorylation has two goals: First, the hexokinase reaction converts nonionic glucose into an anion that is trapped in the cell, since cells lack transport systems for phosphorylated sugars.

8 Second, the biologically inert glucose becomes activated into a form capable of being further metabolized. Four mammalian isozymes of hexokinase are known (Types I - IV), with the Type IV isozyme often referred to as glucokinase. Glucokinase is found in liver cells. The high Km of glucokinase for glucose "low affinity" means that this enzyme is saturated only at very high concentrations of substrate (glucokinase has lower affinity to glucose than hexokinase and works as gulcostate since all the absorbed sugar goes to the liver which will later on distribute sugar to other organs). Reaction 1 is considered a irreversible reaction. When we eat excess amount of glucose, the liver will transform the glucose in to glycogen and store it in the liver and muscles. If we eat feather glucose then the glucose concentration will still be high, and the body will transform the glucose in to fat by irreversible process. exokinase is inhibited by accumulation of glucose6-phosphate or when there is high ATPs concentration inside the cell "end product inhibition". Reaction 2: isomerization of glucose-6-phosphate to fructose-6-phosphate (reversible) It occurs by two steps, this isomerization is most easily seen by considering the open-chain forms of each monosaccharide; it is one keto-enol tautomerism followed by another.

9 1 1 2 C C Glucose-6-phosphate (An enediol ) Fructose-6-phosphate Reaction 3: phosphorylation of fructose-6-phosphate (irrevsible) C C 2 P 3 C 2 P 3 C 2 P 3 2 C 2 C 6 C 2 P 3 1 C 2 phosphofructokinase + ATP Mg D-Fructose-6-phosphate C 2 P 3 1 C 2 P 3 + ADP -D-Fructose-1,6-bisphosphate Reaction 3 " most important control reaction": is called the rate limiting step or the committed step because the product of this reaction (alpha-d-fructose-1, 6- bisphoshate) is only produced by the glycolysis process. What makes reaction 3 the most important control step, and why reaction 1 is not the most important one since both of these reaction are irrersible? Because the end product of reaction 1 is involved in other pathways (e.g. glycogen synthesis pathway and pentose phosphate pathway) so we can't control or inhibit this reaction because it will affect other process apart from glycolysis. The enzymes of this reaction are allosteric enzymes "they can be activated or inhibited" Phosphofructokinase: a key regulatory enzyme in the metabolism of glucose in reaction 3 and it is : a tetramer and subject to allosteric feedback the tetramer is composed of L and M subunits M4, M3L, M2L2, ML3, and L4 are exist muscles are rich in M4; the liver is rich in L4

10 ATP is an allosteric effector; high levels inhibit the enzyme, low levels activate it fructose-2,6-bisphosphate is also an allosteric effector Phosphofructokinase is an allosteric enzyme Reaction 4: cleavage of fructose-1,6-bisphosphate to two triose phosphates: C 2 P 3 C= C 2 P 3 Fructose-1,6-bisphosphate aldolase C 2 P 3 C= C 2 C C C 2 P 3 Dihydroxyacetone phosphate D-Glyceraldehyde 3-phosphate The body deals better with aldoses better than ketoses, because the ketoses will become acids ( keto acids ) which are toxic and if they reach the brain it will cause coma and other problems. Reaction 5: isomerization of triose phosphates catalyzed by triosephosphate isomerase reaction involves two successive keto-enol tautomerizations only the D enantiomer of glyceraldehyde 3-phosphate is formed C 2 C= C 2 P 3 Dihydro xyaceto ne phosphate C C- C 2 P 3 An enedi ol intermediate C C C 2 P 3 D-Glyceraldehyde 3- phosphate

11 Reaction 6: oxidation of the D-glyceraldehyde-3-phosphate to 1,3 bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase : the -C group is oxidized to a carboxyl group the oxidizing agent, NAD+, is reduced to NAD the overall reaction involves an exergonic oxidation and an endergonic phosphorylation the overall reaction is slightly endergonic oxidation: C- to C- - G o' = kj mol -1 phosphorylati on: C- - C- to C-- P- - - to C-- P- - - G o' = kj mol -1 G o' = +6.2 kj mol -1 Reaction 7: transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP" begins of ATP production " this reaction is called substrate-level phosphorylation C-P 3 C + C 2 P 3 1,3-Bisphosphogl ycerate - - P-- AMP ĀDP phosphogl ycerate kinase C - C Mg 2+ C 2 P 3 3- Phosphoglycerate P-- P-- AMP - - ATP This reaction is the sum of the endergonic phosphorylation of ADP and the exergonic hydrolysis of the mixed phosphate anhydride

12 The net product of ATP at this level is zero, because it produces 2ATP molecules which equal the same number of ATPs consumed at investment stage. Reaction 8: isomerization of 3-phosphoglycerate to 2-phosphoglycerate : Mutase enzyme transfers the phosphate group from one carbon atom to another in the same compound. Reaction 9: dehydration of 2-phosphoglycerate: C C - P 3 C 2 2-Phosphoglycerate C - enolase C P Mg 2+ C 2 Phosphoenolpyruvate Dehydration=removal of water, hydration= addition of water Reaction 10: phosphate transfer to ADP "irreversible" Stage 1: transfer of the phosphate group

13 C - C P 3 C 2 Phosphoenolpyruvate P-- AMP - ADP C - C- + C 2 Enol of pyruvate pyruvate ki nase Mg P-- P-- AMP - - ATP stage 2: enolization to pyruvate C - C- C 2 Enol of pyruvate C - C= C 3 Pyruvate reaction 10 is the sum of an exergonic hydrolysis and an endergonic phosphorylation of ATP " net product" hydrolysis PEP + 2 Pyruvate + P i G o' = kj mol -1 phosphorylati on ADP + P i ATP + 2 G o' = kj mol -1 PEP + AD P Pyruvate + ATP G o' = kj mol -1 phosphenol-pyruvate is important because It is one of the high energy phosphates because it has higher energy than ATP.(the table of the previous lecture) Any kinase enzyme requires magnesium as a cofactor. Summing these 10 reactions gives the net equation for glycolysis : C NAD P ADP Glucose 2 C 3 CC - Pyruvate gl ycolysis + 2 NAD + 2 ATP

14 Energetics of Glycolysis: negative and positive but, taken together, occur with a large decrease in free energy Three reactions exhibit particularly large decreases in free energy; the enzymes that catalyze these reactions are sites of allosteric control: hexokinase phosphofructokinase "most important one " pyruvate kinase Control Points in Glycolysis irrevsible reactions Step 1: exokinase is inhibited by glucose-6-p Step 3: phosphofructokinase is inhibited by ATP Step 10: pyruvate kinase is inhibited by ATP

15 Summary: In the final stages of glycolysis, two molecules of pyruvate are produced for each molecule of glucose that entered the pathway These reactions involve electron transfer, and the net production of two ATP for each glucose There are three control points in the glycolytic pathway Anaerobic Metabolism of Pyruvate Pyruvate does not accumulate in cells, but rather undergoes one of three enzymecatalyzed reactions, depending of the type of cell and its state of oxygenation: reduction to lactate (lactate fermentation) reduction to ethanol (ethanol fermentation) oxidative decarboxylation to acetyl-coa A key to understanding the biochemical logic behind two of these fates is to recognize that glycolysis needs a continuing supply of NAD "oxidized form of NAD" if no oxygen is present to reoxidize NAD to NAD+, then another way must be found to reoxidize it in other words the main target of transforming of pyruvate to lactate is to recycle NAD T NAD to continue the glycolysis process Lactate Fermentation In vertebrate muscle under anaerobic conditions, the most important pathway for the regeneration of NAD+ is reduction of pyruvate to lactate C 3 CC - + NAD + + Pyruvate lactate dehydrogenase C 3 CC - + NAD + Lactate

16 lactate dehydrogenase (LD) is a tetrameric isoenzyme consisting of and M subunits; 4 predominates in heart muscle, and M4 in skeletal muscle Accumulation of lactate will accumulate + and forms acid at the muscles and cause painful feeling during sever exercise "stiffness or cramps". Pyruvate to Lactate While lactate fermentation allows glycolysis to continue, it increases the concentration of lactate and also of + in muscle tissue C Glucose lactate fermentati on 2 C 3 CC Lactate When blood lactate reaches about 0.4 mg/100 ml, muscle tissue becomes almost completely exhausted. Glucose to Lactate Lactate fermentation occurs with a significant decrease in free energy: Glucose + 2ADP + 2P i + 2NAD + 2Pyruvate + 2ATP + 2NAD G ' (kj mol- 1 ) Pyruvate + 2NAD Lactate + 2NAD Glucose + 2ADP + 2P i 2Lactate + 2ATP

17 Pyruvate to Ethanol Yeasts and several other organisms "bacterial" regenerate NAD+ by this two-step pathway: decarboxylation of pyruvate to acetaldehyde: reduction of acetaldehyde to ethanol: C 3 C + NAD + + Acet al dehyde alcohol dehydro genase C 3 C 2 + NAD + Ethanol ولهذا السبب حن اضافو السكز على الوخلالت Structures of Thiamine and TPP "vitamin B1"

18 Pyruvate to Ethanol An ethanol fuel plant in West Burlington, Iowa "good for the environment " FETAL ALCL SYNDRME can be detected by measuring the level of acetaldehyde in the blood stream of a يولد الطفل قبل موعده woman: pregnant

19 lactic acid الذي DENTAL CARIES نحج بسب جحل ل البكحز ا الوحىاجده جحث االسناى و اللثو للسكز ات و جحىل ها الى قىم بححل ل طبقو االنول الح جحو االسناى. Summary: Pyruvate is converted to lactate in anaerobic tissues, such as actively metabolizing muscle. NAD+ is recycled in the process In some organisms, pyruvate is converted to ethanol in a process requiring thiamine pyrophosphate as a coenzyme.