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Lecture 4

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NUCLEIC ACIDS1- They are the most important macromolecules in the cells of all

microorganisms2- They are carriers of genetic information3-There have Two forms (RNA and DNA)

Composition and structure of nucleic acidsNucleic acid molecule is a polymers of monomeric unit called

nucleotides. These nucleotide monomer unitare joined by the formation of phosphodiester bond

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NUCLEIC ACIDSPhosphodiester Bond- Phosphate linkage that connects two sugars by ester linkage- Diester bond is one which involves two ester bond- Phosphodiester bond will be formed between any two adjacent nucleotides

(bet. The 5’ phosphate of one nucleotide and the 3’ hydroxyl of another)

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Nucleotide

Nucleotides composed of:- Pentose (C5) sugar, either ribose (RNA) or deoxyribose (DNA)-Nitrogen bases which belong to two chemical classes

- Purine bases (adenine and guanine): contain two fused heterocyclic ringsPyrimidine bases (thymine, cytosine, and uracil): contain a single six-membered

heterocyclic ring-Phosphate group

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Nucleotide

Nucleoside: nitrogen base bonded to its C5 sugarNucleotide: nitrogen base attached to C5 sugar by glycosidiclinkage and bonded to a phosphate

Major components of nucleic acidsKey forms of chemical energy (e.g., ATP)Carriers of sugars in biosynthesis of polysaccharidesRegulatory molecules for certain enzymes or metabolic events

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Nucleotide

PO4

Nucleoside

O- P O

O-

O-

Ribose

Nitrogen base

Pentose Sugar

Deoxyribose

Purines

DNA

RNA

(A)

(G)Pyrimidines

(T)

(U)

(C)

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Primary Structure of nucleic acid

Is a Sequence of nucleotides in a DNA or RNA molecule

- oligonucleotide: small polymer contains only a few nucleotides- polynucleotide: very large polymer contains thousand or

millions of nucleotides

- the polynucleotide chain (strand) has a sense of direction withone end of the chain terminating in a 5’ phosphate group and the other in a 3’ hydroxyl group of growing Chain

- the sequence of bases in RNA and DNA is written in the 5’ to 3’direction

- Two polynucleotide strands wrap around each other to form a DNA double helix

- The two strands are associated because particular bases always hydrogen bond to one another A pairs with T, and C pairs withG, producing base pairs

- RNA is usually a single polynucleotide strand

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Base –pairing of nitrogen bases

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Types of nucleic acid

DNADouble-stranded molecules consisting of two complementary polynucleotide chains running in opposite direction

Strands held together by hydrogen bonds

Basepair

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Types of nucleic acid

RNATypically single-strandedDemonstrates secondary structure (folding back upon itself)Four classes: mRNAs, tRNAs, rRNAs, and small RNAs

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Gene

A particular nucleotide sequence that can instruct the formation of a polypeptide is called a gene

- Most DNA molecules consist of millions of base pairs and, consequently, many genes

- These genes, many of which are unique to the species, determine the structure of proteins and, thus, life’s structures and functions

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Enzymes

Enzymes are specialized macromolecules that speed up chemical reactions in cellsThey are characterized by the following fundamental properties:1- They are Catalytic proteins( biological catalysts) that accelerate the rate of biological reactions by reducing the activation energy (energy required to reach the substrate to thetransition state (high energy state)2- enzymes can be denatured and precipitated with salts, solvents and other reagents.3- Many enzymes require the presence of other compounds -cofactors - before their catalytic activity can be exerted. This entire active complex is referred to as the holoenzyme; i.e., apoenzyme (protein portion) plus the cofactor (coenzyme,

prosthetic group or metal-ion-activator) is called the holoenzyme.

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Apoenzyme + Cofactor = Holoenzyme

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Enzymes

4- increase the rate of chemical reaction without themselves beingconsuming or permanently altered by the reaction

5- increase reaction rates without altering the chemical equilibrium between reactants and products Therefore : the enzyme must accelerate both forward and reverse reaction equally

6- Enzymes are usually very specific as to which reactions they catalyze and the substrates that are involved in these reactions.

Cofactors and coenzymessome enzymes do not need any additional components to show full activity.others require non-protein molecules called cofactors to be bound for activity

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Cofactors

Cofactors can be either:- inorganic (e.g., metal ions and iron-sulfur clusters) - organic compounds (e.g., flavin and heme). which can be

either:- prosthetic groups, which are tightly bound to an enzyme

and play critical function roles (e.g : heme, Zinc)- coenzymes, which are released from the enzyme's active

site during the reaction, they called coenzymes because they work together with enzymes to enhance reaction rate. Coenzymes include NADH, NADPH and adenosine triphosphate. These molecules transfer chemical groups between enzymes.

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Coenzymes

Coenzymes are small organic molecules that transport chemical groups from one enzyme to another. The chemical groups carried include the hydride ion (H-) carried by NAD or NADP+, the acetyl group carried by coenzyme ACoenzymes are usually regenerated and their concentrations maintained at a steady level inside the cell

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Enzyme classification

EC 1 Oxidoreductases: catalyze oxidation/reductionreactions EC 2 Transferases: transfer a functional group (e.g. a methyl or phosphate group) EC 3 Hydrolases: catalyze the hydrolysis of various bonds EC 4 Lyases: cleave various bonds by means other than hydrolysis and oxidation EC 5 Isomerases: catalyze isomerization changes within a single molecule EC 6 Ligases: join two molecules with covalent bonds

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many factors influence enzyme function

pH Enzymes in your stomach may prefer an acidic environment with a low pH while enzymes elsewhere may not .

higher temperatures speed reactions -- to a point. Above 104 degrees fahreinheit, enzymes become denatured and can no longer catalyze reactions.

Higher concentrations of substrate also speed reactions until the solution reaches a saturation point .Past that point, the addition of substrate will not affect the velocity of the reaction .

Irreversible inhibition occurs when the inhibitor added denatures or destroys the enzyme.

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RememberRemember

MonomersSmall molecules that are the building blocks of larger moleculesChemical elements bond in different combinations to form monomers

PolymersLarger molecules composed of bonded monomers

MacromoleculesLarger molecules composed of covalently bonded polymers

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References

Gupta, K, (2008) Cell and Molecular biology (Third edition. Rastogi publication, Meerut, New Delhi. ISBN: 61-7133-617-8.

Lodish, H., Berk, A., Zipursky, S. L., Matsudaire, P., Baltimore, D. and Darnell, J. (1999) Molecular cell biology (Fourth edition). Freeman company. ISBN: 0-7167-3136-3.

Cooper, G.M.and Hausman, R.E. (2004) The cell a molecular approach (Third edition). Sinauer publication

Campbell, Reece, Taylor , Simon, Dickey . BIOLOGY – Concepts & Connections, Sixth Edition. 2009