Topic 6: Energy Production - Carbohydrates

Question 1

What is the general structure of carbohydrates?

Answer 1

• General formula: (CH2O)n
• Contain aldehyde or ketone group
• Exist as mono, di and polysaccharides

Question 2

What is the function of carbohydrates?

Answer 2

• Most are used as a fuel by tissues

- Small amounts are stored as glycogen

Question 3

How are carbohydrates digested?

Answer 3

• Salivary amylase & pancreatic amylase: hydrolyse polysaccharides (starch & glycogen) to release glucose, maltose and dextrins
• Lactase, glycoamylase and sucrase: digest maltose, dextrins, lactose, sucrose to release monosaccharides (glucose, fructose and galactose)

Question 4

How are carbohydrates absorbed?

Answer 4

Glucose, galactose and fructose are actively transported into absorptive cells lining the gut, then diffuses into blood and then diffuses into tissues using glucose transport proteins

Question 5

Why is cellulose not digested in the human GI tract?

Answer 5

Human GI tract doesn’t produce enzymes that can hydrolyse B-1,4 linkages so cellulose cannot be digested

Question 6

What is the biochemical basis of the clinical condition of lactose intolerance?

Answer 6

• Low active of lactase reduces ability to digest the lactose present in milk products
• Lactose persists into the colon where bacteria can break it down
• Presence of lactose in the lumen of the colon increases osmotic pressure of contents
• Water is drawn into lumen
• Causing diarrhea

Question 7

What is the glucose dependency of different tissues?

Answer 7

• Tissues that can only use glucose (RBC, neutrophils, kidney medulla, lens of the eye): 40g
• Brain & central nervous system (prefers glucose): 140g

Question 8

What are the ways of regulating metabolic pathways?

Answer 8

• Product inhibition
• Committing step
• Allosteric regulation

Question 9

What are the principles of regulation of metabolic pathways through product inhibition?

Answer 9

• Increasing product displaces equilibrium towards reactants

- Pathway intermediates build up so flux through the pathway slows down

Question 10

What are the principles of the regulation of metabolic pathways through the committing step?

Answer 10

Inhibition of committing step allows substrate to be diverted into other pathways, preventing build up

Question 11

What are the principles of the regulation of metabolic pathways through allosteric regulation?

Answer 11

• Activator or inhibitor bins at regulatory site, affecting catalytic activity
• Covalent modification like phosphorylation introduces bulky negatively charged group, altering structure of protein, altering its activity

Question 13

How is glycerol phosphate derived from glycolysis?

Answer 13

• Important for triglyceride and phospholipid biosynthesis
• Produced from dihydroxyacetone phosphate in adipose tissue and liver using glycerol 3-phosphate dehydrogenase
• Lipid synthesis in liver requires glycolysis

Question 14

How is 2,3-bisphosphoglycerate derived from glycolysis?

Answer 14

• Produced from 1,3-Bisphosphoglycerate in RBC
• Using bisphosphoglycerate mutase
• Important regulator of O2 affinity of haemoglobin

Question 18

What are the key features of glycolysis?

Answer 18

• Starting material, end-products and intermediates are C6 or C3
• No loss of CO2
• Some C3 intermediates are used by the cell for specific functions
• Glucose oxidized to pyruvate and NAD+ is reduced to NADH
• Exergonic process with a negative G value
• All intermediates are phosphorylated and some is able to undergo substrate level phosphorylation
• 2 moles of ATP are required to activate the process and 4 moles of ATP are produced by the process, net yield is 2 moles of ATP

Question 21

Why is lactic acid production important in anaerobic glycolysis?

Answer 21

When supply of oxygen is inadequate and in cells without mitochondria, pyruvate is reduced to lactate.

Question 22

How is the blood concentration of lactate controlled?

Answer 22

Under normal physiological conditions, rate of lactate production = rate of utilization so plasma concentration remains relatively constant.

In excess, lactate can be converted back to pyruvate using lactate dehydrogenase in the heart, liver and kidney.

Question 23

What is the biochemical basis of the clinical conditions of galactosaemia?

Answer 23

• Due to the lack of kinase (rare) or transferase enzyme (more common), galactose is unable to be converted to glucose.
• Absence of kinase: accumulation of galactose
• Absence of transferase: accumulation galactose and galactose 1-phosphate
• Accumulation of galactose -> reduction to galactitol, reducing amounts of NADPH
• Effects of accumulated galactose/galactitol: formation of cataracts due to damaged lens of the eye (due to cross linking of lens proteins), raised intra-ocular pressure - glaucoma
• Effects of accumulated galactose 1-phosphate: damage to liver, kidney and brain

Question 24

Why is the pentose phosphate pathway is an important metabolic pathway in some tissues?

Answer 24

• Produce NADPH in cytoplasm (provides reducing power for anabolic processes, maintains free -SH groups in RBC, involved in detoxification mechanisms)
• Produce C5-sugar ribose (synthesis of nucleotides, building blocks from which DNA & RNA are made)

Question 25

What is the clinical condition of glucose-6-phosphate dehydrogenase deficiency?

Answer 25

Haemolytic anaemia: RBCs are destroyed faster than they can be made

Question 26

What is the biochemical basis for G6PD deficiency?

Answer 26

• Mutation in gene coding of glucose 6-phosphate dehydrogenase
• Reduces activity of enzyme
• Low levels of NADPH
• Insufficient NADPH to recycle glutathione back to active form
• Glutathione unable to protect cells against oxidative damage
• RBCs affected as pentose phosphate pathway is only source of NADPH
• Oxidative damage occurs, hemoglobin become cross linked by disulphide bonds
• Forms Heinz bodies which leads to premature destruction of RBCs, causing haemolysis

Question 27

What is the key role of pyruvate hydrogenase in glucose metabolism?

Answer 27

Converts pyruvate to acetyl-CoA

Question 28

What are the roles of the tricarboxylic acid cycle in metabolism?

Answer 28

• Central pathway in the catabolism of sugars, fatty acids, ketone bodies, alcohol and amino acids
• Oxidative pathway that occurs in mitrochonddia and required NAD+, FAD Zander oxaloacetate
• Main function: break C-C bond in acetate and oxidize the C-atoms to CO2
• Important to major energy requiring tissues and does not function in absence of oxygen
• Produces 32 ATP molecules per glucose molecule

Question 29

How is the TCA cycle regulated?

Answer 29

• Rate of ATP utilization
• Signals: ATP/ADP ratio and NADH/NAD+ ratio
• Inhibited by high energy signals and activated by low energy signals

Question 30

What are the key features of oxidative phosphorylation?

Answer 30

• Final stage of catabolism
• Occurs at inner mitochondrial membrane
• Involves electron transport and ATP synthesis
• Electron transport: electrons in NADH and FAD2H are transferred through carrier molecules to oxygen with release of energy, requires oxygen
• ATP synthesis: free energy released in electron transport drives ATP synthesis from ADP

Question 31

What is the process of electron transport?

Answer 31

• Electrons are transferred from NADH and FAD2H sequentially through complexes to molecular oxygen with release of free energy
• Complexes I, III and IV and transferring electrons act as proton translocations complexes
• They use energy to move protons from inside to outside of inner mitochondrial membrane
• Proton concentration outside increases (chemical bond energy of electrons converted to electro-chemical potential difference of protons = proton motive force)
• Oxygen acts as terminal electron acceptor - essential

Question 32

What is the process of ATP synthesis?

Answer 32

• Proton motive force is gives energy to drive synthesis of ATP from ADP

Question 33

How are electron transport and ATP synthesis coupled?

Answer 33

• One does not occur without the other
• Mitochondrial concentration of ATP regulates both processes
• ATP high, ADP low, ATP synthase stops, prevents protons from going back to mitochondria, H+ outside increases, prevents more proteins being pumped, electron transport stops

Question 34

How does uncoupling of electron transport and ATP synthesis occur?

Answer 34

• Uncouplers: increase permeability of membrane to protons, enables protons to re-enter mitochondrial matrix without driving ATP synthesis
• Uncoupling proteins: functions to uncouple the 2 processes to produce heat -> present in brown adipose tissue and enables mammals to survive in cold environments
• Inhibitors of electron transport: Inhibited under anaerobic conditions and by substances (CO and poisons), prevents NADH & FAD2H from being oxidized, no energy to drive pumping of protons, pmf not created, ATP synthesis fails, no heat generated, irreversible cell damage

Question 35

What are the similarities and differences between oxidative phosphorylation and substrate level phosphorylation?

Answer 35

• Oxidative requires membrane associated complexes while substrate level requires soluble enzymes
• Oxidative energy coupling occurs indirectly through generation and utilization of proton gradient, substrate level occurs directly through formation of high energy hydrolysis bond
• Oxidative cannot occur without oxygen, substrate level occurs to limit extent without oxygen
• Oxidative is a major process while substrate level is a minor process for ATP synthesis in cells that need large amounts of energy