CARBOHYDRATE METABOLISM

UNIT-2

SYLLABUS :

Glycolysis - Pathway, energetics and significance, Citric acid Cycle - Pathway, energetics & significance, HMP Shunt and significance Glucose-6-phosphate dehydrogenase (G6PD), Glycogen Metabolism pathway and Glycogen storage disease (GSD), Gluconeogenesis pathway and its significance, Hormonal regulation of blood glucose level and Diabetes mellitus.

INTRODUCTION

• Carbohydrates are the major source of energy for all the living cells.
• Carbohydrate metabolism are the combined process of anabolism and catabolism of carbohydrates, in which synthesis, breakdown and inter-conversion of carbohydrates take place and release or generate energy (ATP).
• It is simpler than that of fat and amino acids therefore it is used as immediate source of energy.

GLYCOLYSIS

• It is defined as, it is the sequence of the reaction converting glucose into pyruvate and lactate with the production of ATP.
• Glyco $\rightarrow$ Glucose/sugar, Lysis $\rightarrow$ Breakdown
• It was discovered by two Bio-chemist Embeden and Mayerhof. So, it is also called E.M. Pathway.
• It takes place in the cytosol of cells.

This pathway is divided into three phases:-

1. Energy Investment Phase
2. Splitting Phase
3. Energy generation phase

I. Energy investment phase :- It requires 2 ATP molecules for phosphorylation.

• In this step, Glucose is phosphorylated to Glucose-6-phosphate with the help of enzyme hexokinase, and utilization of one ATP.
• Glucose-6-phosphate isomerised into fructose-6-phosphate in the presence of phosphohexo isomerase.
• Fructose-6-phosphate is phosphorylated to fructose-1,6-biphosphate by phosphofructokinase enzyme.

II. Splitting Phase :-

• Now, fructose-1,6-biphosphate splits into two equal 3 carbon containing compounds i.e. Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (DHAP) by Aldolase enzyme.
• DHAP undergoes isomerisation to Glyceraldehyde-3-phosphate under phosphotriose isomerase.

III. Energy generation phase :-

• Glyceraldehyde-3-Phosphate is converted into 1,3-Biphosphoglycerate with the help of enzyme dehydrogenase.
• 1,3-Biphosphoglycerate is convert into 3-phosphoglycerate with the help of enzyme phosphoglycerate kinase.
• These two step generate 2 NADH and ATP.
• 3-phosphoglycerate is convert into 2-phosphoglycerate with the help of enzyme phosphoglycerate mutase.
• 2-phosphoglycerate converts into phosphoenol pyruvate (PEP) by enzyme enolase.
• Now, PEP convert into pyruvate with the help of enzyme pyruvate kinase and generate ATP.

SIGNIFICANCE (Glycolysis)

• Occured in all the cells of the body.
• Only source of energy in erythrocytes (RBCs).
• Anaerobic glycolysis.
• Reversible, also used for gluconeogenesis.
• Pyruvate used for TCA cycle etc..

CITRIC ACID CYCLE

• It is also known as Krebs cycle or TCA (Tricarboxylic Acid) cycle.
• Albert Szent-Gyorgyi and Hans Krebs established the components and reactions of TCA cycle.
• It is aerobic process and occurs in mitochondrial matrix.
• It is the most important metabolic cycle which generate high amount of energy which fulfill about 65-70% of ATP requirements.

STEPS

• It starts when pyruvate from aerobic glycolysis convert into acetyl co-A which further enters into mitochondria and starts the citric acid cycle.

• It involves the formation of citrate, in which acetyl co-A reacts with oxaloacetate in the presence of citrate synthase and form citrate.
• Now, citrate isomerised and form isocitrate in the presence of enzyme aconitase.
• This isocitrate converts into \alpha$-ketoglutrate by releasing $CO_2 in the presence of isocitrate dehydrogenase and generate one $NADH_2$ energy.
• Now, This \alpha$-ketoglutrate converts into succinyl-CoA in the presence of enzyme dehydrogenase and generate one $NADH_2 (3 ATP) energy.
• This succinyl-CoA converts into succinate in the presence of enzyme Succinyl-CoA synthetase and generate one GTP (ATP).
• Now, Succinate converts into fumerate in the presence of enzyme dehydrogenase and generate $FADH_2$ (2 ATP).
• This fumerate further converts into Malate in the presence of enzyme fumarase.
• Now, in last step, malate again form oxaloacetate in the presence of malate dehydrogenase and generate one $NADH_2$. Now, this oxaloacetate continue the cycle.

Energetics

• In step no. III, IV and VIII = $3 NADH_2 \times 3 ATP = 09 ATP$
• In step V = 1 ATP
• In step VI, $1 FADH_2 = 02 ATP$
• Total = $09 + 1 + 2 = 12 ATP$
• There are 2 pyruvate molecules, so $12 \times 2 = 24 ATP$
• Pyruvate $\rightarrow$ Acetyl Co-A = total 2 NADH = 6 ATP.
• Total with pyruvate = 30 ATP gained.

Significance

• Most important metabolic pathway for energy supply to body, about 65-70% of ATP is synthesized.
• Also helps in the synthesis of Non-essential A.A.
• Common Oxidative pathway for carbohydrates & fats.

HMP SHUNT

• Hexose Mono Phosphate shunt is also known as Pentose phosphate pathway.
• It is an alternative pathway for glucose oxidation (i.e. alternate to glycolysis and TCA).
• This pathway occurs in cytosol of cells.
• This pathway is divided into two phases:-
  1. Oxidative phase
  2. Non-oxidative phase

STEPS

I. Oxidative phase :-

• It involves the synthesis of NADPH.
• It starts when Glucose 6-Phosphate converts into 6-Phosphoglucolactone in the presence of G-6-P dehydrogenase and form NADPH.
• This 6-phosphoglucolactone converts into 6-phosphogluconate through Glucolactone hydrolase.
• Now, This 6-phosphogluconate converts into Ribulose-5-Phosphate, in the presence of Phosphate dehydrogenase and synthesized NADPH.

II. Non-oxidative phase :-

• It mainly involves the synthesis of pentose sugars.
• It starts when Ribulose-5-Phosphate converts into Xylulose-5-Phosphate in the presence of enzyme ribulose 5-phosphate-3-epimerase.
• On other hand, Ribulose-5-Phosphate converts into Ribose-5-phosphate in the presence of enzyme ribose-5-phosphate ketoisomerase.
• Now, sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate formed by the transfer of carbon from xylulose-5-phosphate to ribose-5-phosphate, in the presence of enzyme transketolase.
• Now, Sedoheptulose-7-phosphate and Glyceraldehyde-3-P convert into fructose-6-phosphate and erythrose-4-phosphate in the presence of transaldolase enzyme by transferring of carbon.
• Now, again erythrose-4-phosphate converts into Glyceraldehyde-3-P, which further form/convert fructose-6-phosphate (if needed).

SIGNIFICANCE

• It is unique and generate two important products:
  1. Pentose sugars $\rightarrow$ It is useful for the synthesis of Nucleic acids (RNA, DNA) and many nucleotides such as ATP, $NAD^+$, FAD and Co-A etc.
  2. NADPH $\rightarrow$ It is useful for the biosynthesis of fatty acids, steroids, amino acids etc..
• It helps in the engulfment of foreign particles (phagocytosis).
• Also helps to preserve the transparency of lens.

GLYCOGEN METABOLISM PATHWAY

• Glycogen is a one form of glucose i.e. stored form of glucose in liver & muscles, which is also a main source of energy.
• Glycogen is mainly stored in liver (6-8%) and muscles (1-2%) but due to more muscle mass, the quantity of glycogen in muscles (250 gm) is about three times more than that in liver (75 gm).
• Glycogen metabolism is a process of synthesis and breakdown of glycogen & takes place in cytosol of cells.
• The main function of glycogen:-
  • to maintain blood-glucose level
  • supply energy during starvation
  • muscle glycogen is utilised to supply ATP during muscle contraction.

GLYCOGENESIS

• It is the synthesis of glycogen from glucose.
• Glycogenesis :- Glyco $\rightarrow$ Glycogen, Genesis $\rightarrow$ formation/synthesis

STEPS

• It mainly involve four steps:-

I. Synthesis of UDP-Glucose

• Glucose is converted into Glucose-6-P in the presence of enzymes hexokinase (in muscles) and glucokinase (in liver).
• Glucose-6-P converts into Glucose-1-P in the presence of enzyme phosphoglucomutase which further form UDP-Glucose in the presence of UDP-glucose pyrophosphorylase.

II. To initiate glycogenesis

• Glycogen synthesis is initiated by small fragment of pre-existing glycogen (act as primer) or glycogenin (a protein) accepts glucose from UDP-glucose.
• First glucose molecule is transfer with the help of enzyme glycogen initiator synthase to glycogenin, then glycogenin itself capture glucose.

III. Glycogen synthesis by Glycogen synthase

• $1,4-\alpha$-glycosidic linkages are formed by the glycogen synthase enzyme.

IV. Formation of branches in Glycogen

• Now, glycogen are formed by adding branches with the help of enzyme glycogen synthase and glucosyl transferase.
• The overall reaction of the glycogen synthesis for the addition of each glucose residue can be written as:
  $[Glucose]_n + Glucose + 2ATP \rightarrow [Glucose]_{n+1} + 2ADP + Pi$
• In this, two ATP molecules consumed.

GLYCOGENOLYSIS

• It is the conversion of stored glycogen in liver and muscle into glucose, in the presence of glucagon and epinephrine.

GLUCOSE-6-PHOSPHATE DEHYDROGENASE (G6PD) DEFICIENCY

• It is an X-linked genetic disorder mostly occuring in males.
• In this disease, deficiency of G6PD occurs, which causes breakdown of premature RBCs in body (also known as haemolysis).

CAUSES

• It may be caused due to any type of bacterial or viral infection.
• It is a heredity condition which is passed from one or both parents to their child.
• It may also cause mild to severe jaundice in newborns.

Symptoms: Haemolytic anaemia, paleness, jaundice, dark urine, fatigue, breathlessness, tachycardia--

• It can be treated by removing the factors responsible for haemolytic conditions.

GLYCOGEN STORAGE DISEASES (GSD)

• These are genetic disorder of glycogen metabolism occuring due to accumulation of large amount of glycogen and its metabolites.
• Von Gierke's disease: It is the deficiency or complete absence of enzyme glucose-6-phosphatase in liver, kidney & intestine. It may caused enlarged liver.
• Pompe's disease: It is a fatal disease arising due to decrease in lysosomal $\alpha$-glucosidase. Cardiomegaly occurs if glycogen accumulates in heart and death is evident due to cardio respiratory failure.
• Cori's disease: It is the stored of metabolites of glycogenolysis in liver due to deficiency or absence of de-branching enzymes called limit dextrinosis.
• Anderson's disease: In this, an intermediate product like amylopectin deposits in liver, spleen and heart. Also occurs due to absence of branching enzymes.

GLUCONEOGENESIS

BIOLOGICAL OXIDATION

CHAPTER-2 UNIT-2

SYLLABUS :
Electron transport chain (ETC), and its mechanism, Oxidative phosphorylation & its mechanism, and substrate level phosphorylation. Inhibitors ETC and Oxidative phosphorylation/uncouplers.

Biological Oxidation

• It is the transfer of electrons which produce energy (in the form of ATP) from NADH and $FADH_2$ (co-enzymes).
• It is the combination of oxidation-reduction transformations of substances in living organism.

ELECTRON TRANSPORT CHAIN (ETC)

• It is also known as 'Electron transport system'.
• It is a series of protein complexes, that involve the transfer of electrons through redox reactions, which generate proton gradients that leads to the formation of ATP.
• It is the most essential part of oxidative phosphorylation.
• It occurs in the inner site (membrane) of the mitochondria.

Components:

• It consist of four protein complexes for ETC and one for oxidative phosphorylation.
• It also consist of some enzymes.
• Mostly constituents embedded in cristae (infoldings).

Complexes:

1. Complex I $\rightarrow$

• It is NADH Co-Q oxidoreductase / NADH dehydrogenase.
• It is flavoprotein, contains FMN and also linked with FeS proteins.

• It transport 2e- from NADH to Co-Q and transport 4H+ proton into intermembrane space.

• Complex I $\rightarrow$ It transport $2e^-$ from NADH to Co-Q and transport $4H^+$ proton into intermembrane space.

• Complex II $\rightarrow$ It transport $2e^-$ from $FADH_2$ to Co-Q.

• Complex III $\rightarrow$ It is Co-Q $H_2$ cytochrome c oxidoreductase. It take electron from Co-Q and also transport $4H^+$.

• Complex IV $\rightarrow$ It is Cytochrome C oxidase. It take electron from Cyt-c to mitochondrial matrix. It pumped $2H^+$ proton.

• Complex V $\rightarrow$ It is ATP synthase. It transport $H^+$ protons from intermembrane space to mitochondrial matrix and generate ATP from ADP.

MECHANISM

• Firstly NADH and $FADH_2$ are obtained from glycolysis and citric acid cycle.
• Now, NADH release $2e^-$ to complex I which further transported to Co-Q and also transport $4H^+$ protons.
• Then, these electron ($e^-$) released from complex IV and formed water (H_2O$). $[2H^+ + 1/2 O_2 + 2e^- \rightarrow H_2O]
• These electrons generate proton gradient.
• Approx. $10H^+$ protons transported to intermembrane space, then these protons transported to matrix through complex V.
• During this transport, ATP synthase convert ADP to ATP (energy) through oxidative phosphorylation.
• $NADH \rightarrow 3 ATP / 2.5$
• $FADH_2 \rightarrow 2 ATP / 1.5$

SIGNIFICANCE

• Regenerating Electron Carriers:- NADH and $FADH_2$ Oxidise into NAD and FAD which again utilised in glycolysis & TCA cycle.
• Generating Proton Gradient:- Generate proton gradient which used to make ATP.

OXIDATIVE PHOSPHORYLATION

• It is the final step in the formation of ATP.
• It is defined as it is the process in which ADP is phosphorylated to ATP by using ATP synthase and Electron transport chain (ETC).
• Oxidative phosphorylation involves ETC and chemiosmotic hypothesis.
• mechanism same as ETC (mainly complex V).
• This hypothesis was proposed by Peter Mitchell. $ADP + Pi \rightarrow ATP$ (Pi = inorganic phosphate)

Inhibitors

• These are those substances which obstruct or inhibits the electron transport chain by binding to any ETC components.
• eg. Cyanide poisoning - Cyanide is most potent ETC inhibitors which binds to $Fe^{3+}$ ions of cytochrome oxidase.

Most probable sites of ETC inhibitions:

• NADH $\rightarrow$ Co-Q (fish poison rotenone, antibiotic piericidin A, barbiturates etc).
• B/W Cyt. b $\rightarrow$ Cyt. c
• Cyt. oxidase

UNCOUPLERS

• Also known as uncouplers of oxidative phosphorylation.
• These are those compounds which have ability to uncouple or delink the process of ETC (electron transport chain) and Oxidative phosphorylation.