Introduction to Metabolism Lecture 2

1. Introduction to MetabolismLecture 2 A/Prof Gareth Denyer gareth@usyd.edu.au

2. Metabolism • The chemical reactions of life • The reactions which happen inside our cells • Divided into: • Anabolism • the reactions that build things up • Catabolism • the reactions that break things down

3. MACROMOLECULES Proteins, DNA, fats, polysaccharides BUILDING BLOCKS CATABOLIC ANABOLIC Amino acids, sugars, nucleotides, fatty acids SIMPLE INORGANICS Water, carbon dioxide, ammonia

4. Linking Anabolism and Catabolism • Anabolic reactions require an input of energy • Energy is produced by the breakdown of building blocks • The catabolic reactions! • The linkage is ATP • “The Energy Currency of Life”

5. Adenosine Triphosphate • A tri-phosphonucleotide • Energy released when terminal phosphates are hydrolysed • ATP  ADP releases energy • Energy released as heat… UNLESS we can trap it do do something useful • Like anabolic reactions! • But also ANYTHING that requires the input of ‘energy’ PPP ribose Adenine base

6. ATP facts I • The “Energy Intermediary” of our cells • Made by catabolic reactions (but not macromolecular conversion to building blocks) • Used by anabolic reactions (and any ‘work’) • We don’t store much of it • Cell [ATP] is about 5 mM (well, 4.8 mM ) • Cellular [ATP] must not dip below 3 mM • Or the cell will die.. • ATP is continually being used for lots of essential ‘housekeeping’ roles (eg, maintaining ion gradients) • In context: 5 mM = 5 umol/g tissue • and exercising muscle can use ATP at 3 umol/s/g • So there’s only 1-2 seconds ‘store’ in muscle

7. ATP facts II • TURNOVER is massive in comparison to store size • Total amount of ATP in whole body is about 50 grams • Total production and destruction of ATP is about 65 kg/day (about 1g/kg body weight/day) • We don’t have a ‘store’ of ATP • We can only store it ‘indirectly’ as fuels like fat and carbohydrate • Can’t ‘make’ ATP and store it somewhere for later • ATP levels are pretty tight balancing act! Wallet analogy! • Imagine trying to ‘balance’ a wallet that was only allowed to contain $30-50 at any given time • But with a throughput of money of $65,000 per day! • Imagine how tightly you’d have to regulate the flow in/out of the wallet • The rate of ATP generation must be ABSOLUTELY matched to the rate of ATP consumption

8. ATP facts III • ATP is quite stable • Will not spontaneously hydrolyse • Hydrolysis is under enzyme control • Makes sense for an energy currency to be under some sort of control • Don’t want something explosive! • ATP is not THE most energy-containing molecule in metabolism • Something has to MAKE it, of course! • ATP is intermediate in energy content – perfect for its role as ‘mediator’ between the arms of anabolism and catabolism

9. Strategies for making ATP • Fuel molecules are the ‘building blocks’ • Fatty acids and sugars • Will be broken down by catabolic reactions • To water and carbon dioxide • These are OXIDATION reactions

10. Fuel Molecules Fatty acids – long hydrocarbon chain with carboxylic acid group on the end CH3 CH2 CH2 CH2 CH2 CH2 CH2 COOH Most of the fatty acids that we experience are even numbered with 2-26 carbon atoms Carbons in the fatty acid are generally -(CH2 )n- Contains a lot of hydrogens! Carbohydrates – also contain lots of hydrogens But this time, each carbon atom is -(CHOH )- ‘hydrated carbon’! And one carbon has an aldehyde/ketone group

11. Biochemists Redox! • Oxidation • Addition of oxygens • Removal of hydrogens • Removal of electrons • Reduction • Addition of hydrogens • Addition of electrons • Removal of oxygens • Under those definitions • Carbon dioxide is the most oxidised form of carbon • Conversion of –CH2- or –CHOH- to CO2 is OXIDATION

12. Fuel Oxidation • Oxidising fuels releases energy • But the trick is to TRAP the energy as chemical energy as ATP • And the ATP can later be used to do something useful • Compare with the lack of trapping when fuels just burn on a fire • All the energy is lost as heat and light

13. Oxidation of Fatty Acids A large charged molecule Coenzyme A Fatty acids Simple diffusion* Trapping involves a small investment of energy (use of ATP) Fatty acyl-CoA Fatty acids mitochondria Need to be trapped to stop them getting back out blood carnitine carnitine cytoplasm CoA Fatty acyl-carnitine Swapping the CoA for carnitine allows the FA to get into the mitochondrial matrix FA-carnitine Inside the matrix, the CoA is put back onto the FA, thus trapping the FA in the matrix and releasing the carnitine which can go back into the cytoplasm matrix Fatty acyl-CoA Beta-oxidation!! Note – there is an intra-mitochondrial pool of CoA