Introduction to Metabolism Lecture 2 A/Prof Gareth Denyer

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

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

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”

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 Adenine base ribose

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

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

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

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

Fuel Molecules CH 3 CH 2 COOH Fatty acids – long hydrocarbon chain with carboxylic acid group on the end Carbons in the fatty acid are generally -(CH 2 ) n - Contains a lot of hydrogens! Most of the fatty acids that we experience are even numbered with 2-26 carbon atoms Carbohydrates – also contain lots of hydrogens But this time, each carbon atom is -(CHOH )- ‘hydrated carbon’! And one carbon has an aldehyde/ketone group

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 –CH 2 - or –CHOH- to CO 2 is OXIDATION

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

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