Review Reaction mechanisms 2. Reducing sugars 3. Amino acid mutations and their effects 4. Lipids

Review Reaction mechanisms Draw the catalytic amino acid side chains with an eye for geometry Remember the substrate and products Use arrows to move to another panel when you need to For this class, focus on Attack! Let’s go through the serine protease mechanism again.

Review Carbohydrates and Reducing sugars What does a reducing sugar have? What makes non-reducing sugar? When dealing with disaccharides or oligosaccharides, draw the monomers first and then connect them appropriately

Review Amino acid mutations What are the effect(s) of the mutation? Which kinetic parameters change? What does that tell you? Can you relate it to what we already know about kinetics and inhibition? You must think about the relationship between the actual mutation, the physical difference between the wild type and the mutant as well as the effect of this difference

Review Lipids As usual, FIND THE FUNCTIONAL GROUPS Think about the role unsaturation plays in the behaviour of the molecule Where/when would you want to change the saturation of the acyl chain? What other types of linkage could you have in acylglycerides? What effect(s) would the different linkage have?

Electron Transfer Types of biological redox reactions Direct electron transfer: 2. H atom transfer 3. H:- (hydride) ion transfer 4. Direct reaction with O2 in some form

Electron Transfer Direct electron transfer: Fe+2 + Cu+3 --> Fe+3 + Cu+2 Oxidation Half Reaction: Fe+2 --> Fe+3 + 1e- Reduction Half Reaction: Cu+3 + 1e- --> Cu2+ The 2 half reactions make a RedOx couple We can combine 2 half reactions from the previous table and a reaction will occur as long as the E°’ is POSITIVE

Electron Transfer 2. Hydrogen atom transfer A Hydrogen atom has a single electron: AH2  A + 2e- + 2H+ AH2 is the hydrogen/electron donor This is not an acid/base reaction, the H+ comes from the removal of a hydrogen atom with its electron, not just the proton AH2 and A together constitute a conjugate redox pair that can reduce another compound, B, or redox pair (B/BH2) by transfer of hydrogen atoms: AH2 + B  A + BH2

Electron Transfer 3. H:- (hydride) ion transfer A hydride ion is a hydrogen atom with 2 electrons Hydrides are transferred to NAD+ and FADH2 We’ll look at these in just a bit…

Electron Transfer 4. Direct combination with oxygen Oxygen combines with an organic reductant and is covalently incorporated into the product Example: Oxidation of a hydrocarbon to an alcohol R-CH3 + 1/2 O2 --> R-CH2OH The hydrocarbon is the electron donor and the oxygen atom is the electron acceptor

Electron Carriers in Biological Systems In Many biological reactions, electrons are transferred as hydrides to a Carrier Molecule Nicotinamide adenine dinucleotide (NAD+) and Flavin Adenine Dinucleotide (FAD2+) are the 2 primary electron carrier molecules in cells

Nicotinamide adenine dinucleotide NAD+, Nicotinamide Adenine Dinucleotide, is an electron acceptor in catabolic pathways. The nicotinamide ring, derived from the vitamin niacin, accepts 2 e- & 1 H+ (a hydride) in going to the reduced state, NADH. NADP+/NADPH is similar except for Pi. NADPH is e- donor in synthetic pathways.

NAD+/NADH The electron transfer reaction may be summarized as : NAD+ + 2e- + H+  NADH It may also be written as: NAD+ + 2e- + 2H+  NADH + H+

FAD (Flavin Adenine Dinucleotide), derived from the vitamin riboflavin, functions as an e- acceptor. The dimethylisoalloxazine ring undergoes reduction/oxidation. FAD accepts 2 e- + 2 H+ in going to its reduced state: FAD + 2 e- + 2 H+  FADH2

Enzymes involved in RedOx Reactions Enzymes that catalyze RedOx reactions are generally called Oxidoreductases This includes : Oxidases, Dehydrogenases, Hydroperoxidases and Oxygenases. Oxidases use oxygen as an electron acceptor Dehydrogenases can’t use as an electron acceptor Hydroperoxidases use H2O2 as a substrate Oxygenases catalyse the direct transfer of O2 into the substrate Oxidases & dehydrogenases are involved in respiration; hydroperoxidases neutralize free radicals & oxygenases are involved in biotransformation

Oxidases Catalyze the removal of hydrogen from a substrate with the involvement of oxygen as a Hydrogen acceptor Exist in two different forms : some of them are copper containing as, Cytochrome oxidase - the terminal component of ETC which transfer the e - to O2. Other are flavoproteins such L – aminoacid oxidase, xanthine oxidase

Dehydrogenases Perform 2 main functions: Transfer hydrogen from one substrate to another in a coupled RedOx reaction As components of Electron transport chain Dehydrogenases use coenzymes – nicotinamides & riboflavin to carry hydrogens

Haloperoxidases Includes 2 sets of enzymes : Catalase and Peroxidases Peroxidases reduce H2O2 at the expense of several other substances H2O2 + AH2  2H2O + A Catalase uses H2O2 as electron acceptor & electron donor 2H2O2  2H2O Peroxisomes are rich in oxidases and catalases

Oxygenases Catalyse the incorporation of O2 into subtrates in 2 steps Oxygen is bound to the active site of the enzyme The O2 is then reduced or transferred to the substrate Consists of two sets of enzymes Dioxygenases : incorporate both atoms of oxygen into the substrate A + O2  AO2 Monooxygenases : incorporates one atom of oxygen into the substrate & the other is reduced to water AH + O2 + ZH2  AOH + H2O + Z