Biochem - metabolism and glycolysis

What are the main three functions of metabolism?

• Conversion of food energy to energy available to run cellular processes

• Conversion of food chemicals to building blocks of proteins, lipids, nucleic acids and some carbohydrates

• Elimination of metabolic waste

What are two types of reactions used in metabolism?

• Catabolism

• Anabolism

What does catabolism in metabolic pathways do?

• Turns a wide variety of energy-rich nutrients into few energy-poor end products - often through oxidation

• Generates energy

• Carbohydrates/fats/proteins —> CO2/H2O/NH3

What does anabolism do in metabolic pathways?

• Turns a small number of precursor molecules into a wide variety of complex molecules - often through reduction

• Require energy

• Amino acids/sugars/fatty acids —> proteins/lipids/nucleic acids

What is ATP?

• Adenosine Triphosphate

• Universal currency of free energy in biological systems

• Primary cellular energy carrier

• Free energy donor in most energy requiring processes

How does ATP provide free energy to systems?

• New bonds are made and interactions with water occur

• Increase in entropy of the products relative to the reactants

• Produces ADP, an inorganic phosphate, H+ and free energy

• Reaction liberates ~30.5kJ per mol of free energy

• ATP has a high phosphoryl moving potential

• ATP is constantly consumed and produced

What are the electron carriers in metabolism?

• NADH and NADPH are main reducing powers in biological systems

• NADH is produced during the oxidation of fuel molecules

• NADPH is used during reductive biosynthesis

• Forward reactions (producing) are reduction

• Backward reactions (using) are oxidation

• Each NADH/NADPH molecule is a carrier of 2 electrons

How can glucose be metabolised?

• Aerobic metabolism

• Anaerobic metabolism

What is produced from aerobic metabolism?

• Stored as glycogen

• Converted to nucleotides and fatty acids

• Generation of ATP

What is produced from anaerobic metabolism?

• Stored as glycogen

• Converted to nucleotides

• Generates less ATP - forms lactate that causes muscle fatigue and cramps

• Utilised in yeast cells for brewing and breadmaking

What is the overall reaction for glycolysis?

glucose + 2Pi + 2ADP + 2NAD+ —> 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H2O

What are the net outcomes for glycolysis?

• 1 glucose —> 2 pyruvate

• 2ADP —> 2ATP

• 2NAD+ —> 2NADH

What are the key stages in glycolysis?

• Investment stage

• Cleavage stage

• Energy generation stage

What happens overall in the investment stage?

• Two phosphorylation reactions and one isomerisation reaction

• ATP is consumed

• Glucose is ‘trapped’

What happens overall in the cleavage stage?

• Cleavage of one 6-carbon molecule into 2 3-carbon molecules

• Each is an isomer of the other

What overall happens in the energy generation stage?

• Reactions occur twice

• Steps result in the transfer of phosphate groups to ADP and reduction of NAD+

What happens in the first reaction of the energy-investment stage of glycolysis?

• Glucose (+ATP) produces glucose-6-phosphate (+ADP)

• Uses a molecule of ATP

• Irreversible - traps glucose

• Catalysed by hexokinase

What happens during the second reaction in the energy-investment stage of glycolysis?

• Glucose-6-phosphate is isomerised to fructose-6-phosphate

• Catalysed by phosphoglucose isomerase

What happens during the third reaction in the energy-investment stage of glycolysis?

• Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate

• Rate limiting step

• Uses a molecule of ATP

• Irreversible reaction catalysed by phosphofructokinase

• Inhibited by high levels of ATP - allosteric inhibitor

• Fructose-6-phosphate (+ATP) —> fructose-1,6-bisphosphate (+ADP)

What happens during the first reaction of the cleavage stage in glycolysis?

• Fructose-1,6-bisphosphate is cleaved into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate

• Catalysed by aldolase

• Fructose-1,6-bisphosphate —> glyceraldehyde 3-phosphate + dihydroxyacetone phosphate

What occurs during the second reaction in the cleavage stage of glycolysis?

• Isomerisation of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate

• Catalysed by triose phosphate isomerase

• Reversible reaction

What occurs during the first reaction of the energy generation stage of glycolysis?

• Oxidise the aldehyde group on glyceraldehyde-3-phosphate

• Produces phosphorylated 1,3-bisphosphoglycerate

• Glyceraldehyde-3-phosphate + NAD+ + Pi —> 1,3-bisphosphoglycerate + NADH + H+

• Generates a molecule of NADH

• Catalysed by glyceraldehyde-3-phosphate dehydrogenase

• Oxidising the aldehyde group causes NAD+ to be reduced to NADH

What happens during the second reaction of the energy-generation stage in glycolysis?

• Phosphoryl group transfer to produce 3-phosphoglycerate

• Generates a molecule of ATP

• Catalysed by phosphoglycerate kinase

What happens during the third reaction of the energy generation stage of glycolysis?

• Phosphoryl group is shifted to produce 2-phosphoglycerate

• Catalysed by phosphoglycerate mutase

What happens in the fourth reaction of the energy generation stage of glycolysis?

• Dehydration to produce phosphoenolpyruvate

• Increases phosphoryl group transfer potential

• Catalysed by enolase

What happens in the fifth reaction of the energy generation stage of glycolysis?

• Phosphoryl group transfer to produce pyruvate

• Phosphoenolpyruvate + ADP —> pyruvate + ATP

• Generates a molecule of ATP

• Irreversible reaction

• Catalysed by pyruvate kinase

• Inhibited by high levels of ATP - allosteric inhibitor

How can glycolysis be regulated within the cell?

• Can be regulated at all three points - three irreversible reactions

• Activities are regulated by reversible binding of allosteric inhibitors (milliseconds), covalent modification (seconds) or transcriptional control (hours)

• Reaction 3 (catalysed by phosphofructokinase) - inhibited by high levels of ATP

• Reaction 1 (catalysed by hexokinase) - inhibited by glucose-6-phosphate

• Reaction 10 (catalysed by pyruvate kinase) - inhibited by high levels of ATP