i want 20 multiple choice questions with their model answer below each qusetion that covers different aspects of all of the following information: Objectives

 By the end of this lecture, students are expected to:  Contrast mechanisms of glucose transport into the cells.  Illustrate glycolysis as the major oxidative pathway of glucose.  Summarize the main reactions of glycolytic pathway.  Categorize the rate-limiting enzymes/Regulation of glycolysis.  Conclude the ATP production (aerobic/anaerobic).  Differentiate the unique nature of glycolysis in RBCs.  Demonstrate pyruvate kinase deficiency.  Diagram pyruvate reduction into lactate & Correlate lactic acidosis.Overview of Metabolism

 Metabolism is “the entire set of enzyme- catalyzed transformations of organic molecules in

living cells.

 Two broad classes: Catabolism & Anabolism

 Catabolic (Degradative) Pathways: Transform fuels into cellular energy

 Anabolic (Synthetic) Pathways: Requires inputs of energy to proceed. (Useful energy + small

molecules into complex molecules).

 Pathways that can be either anabolic or catabolic, depending on the energy conditions in the

cell are referred to as amphibolic pathways.

 Metabolic regulation occurs by regulatory signals (hormones, neurotransmitters & availability

of nutrients).Glucose Transport into the cells

 Glucose cannot diffuse directly into cells but enters by one of two transport mechanisms:

1. Na+/Monosaccharide Cotransporter: (Na & ATP dependent)

 Against concentration gradient (from low to high conc.)

 Energy dependent (ATP) & Carrier-mediated

 Coupled to Na+ transport

 Small intestine, renal tubules & choroid plexus

2. Na+ & ATP Independent Facilitated Diffusion:

 Down the concentration gradient (from high to low conc.)

 Energy Independent (Passive transport), Glucose Transporters (GLUT 1-14)Glucose Transport

1. Facilitated Diffusion: 14 Glucose Transporters (Tissue-specific expression pattern)

 GLUT-1 in RBCs and brain

 GLUT-2 in Liver, kidney & pancreas

 GLUT-3 in Neurons

 GLUT-4 Adipose tissue & skeletal muscle

 GLUT-5 Small intestine & testes

 Functions:

 GLUT-1, 3 & 4 Glucose uptake from blood

 GLUT-2 Blood & cells (either directions), GLUT-5 Fructose (not Glucose)Glucose Transport (cont.)

2. SODIUM-DEPENDENT GLUCOSE TRANSPORTERS (SGLT):

 Present in small intestine & kidneys

 Acts via secondary active transport.

 Coupled with the Na2+-K+-ATPase pump.

 Hence, referred “Na2+- Glucose Co-transport” mechanism.

 Against concentration gradient (from low to high conc.)

 Energy dependent (ATP)The Fate of Glucose

Exercising muscle

The fate of glucose varies with physiological conditions, tissues, and organisms.Glycolysis

 Definition: Glycolysis is the sequence of reactions converting glucose to pyruvate or lactate, with the

production of ATP.

 Glycolysis occurs in almost every living cell. This pathway is also called Embden- Meyerhof pathway

(E.M-Pathway).

 It occurs in cytosol.

 Glycolysis occurs in the absence of oxygen (anaerobic) (in RBCs, hypoxia or lack of mitochondria) or in

presence of oxygen (aerobic)in presence of mitochondria.

 Lactate is the end product under anaerobic condition.

 In aerobic condition, pyruvate is formed, which is then oxidized to CO2 & H2O.

 Glucose: the universal fuel for human cells. Sources: Diet (the major sugar in our diet) and internal

glycogen stores.

 Glucose oxidation: after a meal in almost all tissues and during fasting in brain, erythrocytesReactions of Glycolysis

 Divided into two distinct phases:

1. Energy investment phase or priming phase (PREPARATORY PHASE)

❑ This phase consumes 2 ATP.

❑ It consists of the 1st 5 steps of glycolysis

2. Energy generation phase (PAYOFF PHASE)

❑ This phase constitutes the last 5 reactions of Glycolysis.

❑ In aerobic conditions, the two molecules of glyceraldehyde 3-phosphate are converted to 2

pyruvate + 4 ATP + 2 NADH.

❑ In anaerobic conditions, only 4 ATP are generated with conversion of 2 pyruvate to 2 lactate. 2

NADH will not enter ETC.AA. Energy investment phase

1. Glucose phosphorylation

 Phosphorylated sugar molecules do not readily penetrate cell membranes without specific

carriers.

 intracellular Phosphorylation of monosaccharides is very important for trapping

monosaccharides inside the cell keeping the low concentration of un-phosphorylated

monosaccharides inside the cell (phosphorylation keeps the concentration gradient).

 Phosphorylation of monosaccharides is also important for their activation for starting any

pathway.

 Glucose obtained from diet through intestinal hydrolysis of lactose, sucrose, glycogen, or

starch is brought into the hexose phosphate pool through the action of hexokinase.Glucose phosphorylation

 STEP 1:

 In all tissues, the phosphorylation of glucose is catalyzed by hexokinase, one of the three

regulatory enzymes of glycolysis

 Free glucose is phosphorylated to glucose 6 phosphate by hexokinase. It catalyzes a regulatory

step in glycolysis that is irreversible.

 Hexokinase splits the ATP into ADP & Pi, the Pi is added to the glucose.

 There are four isoenzymes of hexokinase (I-IV).NN

N.B. Inactivating Mutations in glucokinase cause rare type of diabetes (MODY type 2) Mature-

Onset- Diabetes of Young 2. It is characterized by impaired insulin secretion & hyperglycemia.Isomerization of Glucose 6-P

 STEP 2:

 Glucose 6 P is a central molecule with a variety of metabolic fates- glycolysis, glycogenesis,

gluconeogenesis and HMP pathway.

 The isomerization of Glucose 6-P (an aldose sugar) to Fructose 6-P (a ketose sugar) is catalyzed

by phosphohexose isomerase. It requires Mg+2 ions.

 The reaction is readily reversible, is NOT a rate limiting or regulated step.Phosphorylation of Fructose 6-P

 STEP 3:

 Fructose 6- phosphate is phosphorylated

to Fructose 1, 6- bisphosphate by

Phosphofructokinase (PFK).

 The PFK reaction is irreversible

rate-limiting step.

 It is controlled by the concentrations

of the substrates ATP & Fructose 6-P.Splitting of F1,6 BP

 Step 4:

 The six carbon Fructose 1, 6- bisphosphate is split to 2 three carbon compounds.

(Glyceraldehyde 3- phosphate & Dihydroxy acetone phosphate) by the enzyme aldolase

(Fructose 1, 6- bisphosphate aldolase).

 The reaction is reversible is not subject to regulation.Isomerization of DHAP

 Step 5:

 Phosphotriose isomerase catalyzes the reversible interconversion of dihydroxyacetone

phosphate (DAHP) & glyceraldehyde 3-phosphate.

 Two molecules of glyceraldehyde 3-phosphate are obtained from one molecule of glucose.B. Energy generation phase

Oxidation of glyceraldehyde 3P

 Step 6:

 Glyceraldehyde 3-phosphate dehydrogenase converts Glyceraldehyde 3-phosphate to 1,3-

bisphosphoglycerate.

 This step is important as it is involved in the formation of NADH +H+ & a high energy

compound 1,3- bisphosphoglycerate, inhibited by iodoacetate & arsenate.

 In aerobic condition only, NADH passes through the ETC and 6 ATP are synthesized by

oxidative phosphorylation.Formation of ATP from 1,3-

bisphosphoglycerate & ADP

 Step 7:

 The enzyme phosphoglycerate kinase acts on 1,3- bisphosphoglycerate resulting in the

synthesis of ATP and formation of 3- phosphoglycerate.

 This step is a substrate-level phosphorylation. (Production of a high-energy P is coupled

to the conversion of substrate to product, instead of resulting from oxidative

phosphorylation). Unlike most other kinases, this reaction is reversible.

 This is the 1st ATP generation step in glycolysis.Substrate-level phosphorylation vs.

Oxidative phosphorylation

 Phosphorylation is the metabolic reaction of introducing a phosphate group into an organic

molecule.

 Oxidative phosphorylation:

 The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP coupled to

the transfer of electrons from reduced coenzymes to molecular oxygen via the electron

transport chain (ETC); it occurs in the mitochondria.

 Substrate-level phosphorylation:

 The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP (or GDP to

GTP) coupled to cleavage of a high-energy metabolic intermediate (substrate). It may occur in

cytosol or mitochondria.Isomerization of 3- Phosphoglycerate

 Step 8:

 3- Phosphoglycerate is converted to 2- Phosphoglycerate by phosphoglycerate mutase.Dehydration of 2- Phosphoglycerate

 Step 9:

 The high energy compound phosphoenol pyruvate (PEP) is generated from 2-

Phosphoglycerate by the enzyme enolase.

 This enzyme requires Mg+2 or Mn+2 and is inhibited by fluoride.2nd substrate level phosphorylation

 Step 10:  The enzyme pyruvate kinase catalyses the transfer of high energy phosphate from PEP to ADP, leading to the formation of ATP.

 This step is also a substrate level phosphorylation. It is irreversible rate limiting reaction.  This is non-oxidative phosphorylation.