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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