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TYPES OF BIOCHEMICAL REACTIONS
PRESENTED BY
GROUP 1
BIOCHEMICAL REACTION
Definition Chemical reactions taking place in
biological systems are called biochemical reactions
Eg: photosynthesis metabolism
TYPES OF BIOCHEMICAL REACTIONS Five major types: Redox reaction Hydrolysis Substitution Addition and elimination Isomerisation
ENZYMES
Characteristics: Biocatalyst Stereospecific Types Synthesis(anabolic) eg:protein synthesis degradation(catabolic) eg:digestion
E + S ---> ES ---> P + Unaltered E E= Enzyme , S = Substrate,
ES = Enzyme /Substrate complex , P = Product(s).
Enzyme action is determined by, Shape of enzyme Shape is determined by, temperature pH
REDOX REACTION
Both oxidation and reduction Oxidation: addition of oxygen or loss of electrons Reduction: removal of oxygen or addition of
electrons Redox reaction: transfer of electrons between donor
and receptor.
EXAMPLES
Eg of redox reaction: Burning of natural gas CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2O(g) +
energy Burning of carbohydrates C 6H 12O 6(aq) + 6O 2(g) → 6CO 2(g) + 6H 2O(l)
Ethanol and oxygen-> etanoic acid ,ethanol oxidised and oxygen get reducedCH3CH2OH + [O] ---> CH3COOH
SUBSTITUTION REACTION
In a substitution reaction, a functional group in a particular chemical compound is replaced by another group.
These reactions can be distinguished by the type of substituting species into a nucleophilic, electrophilic or radical substitution.
NUCLEOPHILIC SUBSTITUTION
A nucleophile, an atom or molecule with an excess of electrons(negative charge).
The electron pair from the nucleophile attacks the substrate forming a new bond, while the leaving group departs with an electron pair.
Examples of nucleophiles are hydroxide ion, alkoxides, amines and halides.
ELECTROPHILIC SUBSTITUTION
The attacking atom or molecule is an electrophile, has low electron density and thus a positive charge
Typical electrophiles are the carbon atom of carbonyl groups, carbocations or sulfur or nitronium cations.
takes place in aromatic hydrocarbons called electrophilic aromatic substitution.
RADICAL SUBSTITUTION
The attacking particle is a radical. This process usually takes the form of a
chain reaction, for example in the reaction of alkanes with halogens
Reaction proceeds until two radicals meet and recombine.
X + R-H -> X-H + R R + X2 -> R-X + X
SN1 SUBSTITUTION
"SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular.
Eg: hydrolysis of tert-butyl bromide with water forming tert-butyl alcohol.
SN1 MECHANISM
1) Formation of a tert-butyl carbocation by separation of a leaving group (a bromide anion) from the carbon atom: this step is slow and reversible.
2) Nucleophilic attack: the carbocation reacts with the nucleophile. When the solvent is water, the intermediate is an oxonium ion. This reaction step is fast.
3) Deprotonation: Removal of a proton on the protonated nucleophile by water acting as a base forming the alcohol and a hydronium ion. This reaction step is fast.
SN2 SUBSTITUTION
Occurs at an aliphatic sp3 carbon center with an electronegative, leaving group attached to it - 'X' . The breaking of the C-X bond and the formation of the new C-Nu bond occur simultaneously to form a transition state.
SN2 reaction of bromoethane with hydroxide ion
Usually occurs at an unhindered primary carbon centre
If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an SN1 rather than an SN2 mechanism
ADDITION REACTION
An organic reaction where two or more molecules combine to form a larger one.
Takes place in multiple bonded atoms, such as molecules with carbon-carbon double bonds, i.e., alkenes, or with triple bonds, i.e., alkynes
.
TYPES OF ADDITION
Electrophilic addition - addition to alkene
Nucleophilic addition-addition to carbonyl
Free radical addition- halide to alkene
ELIMINATION REACTION
An elimination reaction is a reaction whereby a multiple covalent bond is formed in a molecule by the removal of another, usually smaller molecule.
Eg:
E1 AND E2 ELIMINATION
Loss of the LG to form a carbocation, removal of H+ and formation of C=C bond : E1 reaction
Simultaneous H+ removal, C=C bond formation and loss of the LG : E2 reaction
HYDROLYSIS
Hydrolysis means reaction with water. It is a chemical process in which a
molecule is cleaved into two parts by the addition of a molecule of water.
One parent molecule gains a H + from the water molecule
The other group collects the remaining OH − .
X-(aq) + H2O(l) <--> HX(aq) + OH-
(aq)
EXAMPLES
Hydrolysis of weak acid or weak base sodium acetate + water-
>NaOH+CH3COOH Saponification triglyceride(fatty acid)+aqueous NaOH -> soap + glycerol Hydrolysis of ATP to form ADP and AMP. Hydrolysis of proteins ,fats and
carbohydrates.
HYDROLYSING ESTER
Hydrolysed by water or by dilute acids
Hydrolysing by alkali
ISOMERISATION
Isomerizations can involve either the interconversion of constitutional isomers, in which bond connectivity is altered, or of stereoisomers, where the stereochemical configuration is changed.
Enzymes which interconvert constitutional isomers are called isomerases
EXAMPLE
In the phosphoglucose isomerase reactionin glycotic pathway,glucose-6-phosphate (an aldehyde sugar) and fructose-6-phosphate (a ketone sugar) are interconverted in a very similar fashion.
REFERENCE: L. G. Wade, Jr., Organic Chemistry, 6th
ed., New Jersey, USA, 2005 J. March, Advanced Organic
Chemistry, 4th ed., Wiley, New Yor
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