Amino acids, proteins and DNA

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Amino acids, proteins and DNA

11.1 Amino acids

bifunctional molecules containing -NH2 and -COOH

all natural occurring α-amino acids exist as optical isomers and occur as the ι-isomer

naturally occurring 2-amino acids have similar structures, only R groups are different

Acid-base properties:

-COOH is weakly acidic

-NH2 is weakly basic

amino acids can act as both weak acids and weak bases

zwitterion: existing simultaneously as an anion and cation

isoelectric pH: the unique pH value at which a specific zwitterion exists

relatively high mps: electrostatic attraction between oppositely charged parts of the ion

11.2 Proteins

α-amino acid: an amino acid in which both -NH2 and -COOH groups are attached to the same C

2° structure: α-helix or β-pleated sheet: 2 possible structures of a protein

3° structure: determined by overall 3D shape in which 2° structure arranges itself

disulphide bond: one of the strongest types of bond in proteins - occur between 2 cysteine amino acids

peptide: compound formed from amino acids joined by peptide bonds (di=2, poly=many)

one or more polypeptide form a protein

peptide bond: formed when 2 amino acids join by condensation -CONH-

11.4 DNA

made up of 2 polymer strands that are held together by H bonding (leading to double helix shape)

the monomers are nucleotides which consist of a phosphate, sugar and base

the 4 bases are: thymine, cytosine, adenine and guanine

DNA is made by nucleotides linking together via the phosphate

possible pairs: AT and CG (they fit exactly in terms of shape and size)

the AT pair has 2x H bonds and the CG has 3

cell replication:

double helix unwinds -> complementary nucleotides bind to 2 unwound chains (=2 copies)

11.5 Action of anti-cancer drugs - chemotherapy

drugs that affect cell division do so by binding to DNA to stop replication

cisplatin works by:

a Cl- is replaced by water

then by an N from a guanine base

this creates a Pt-N bond

this fixes drug to one strand of DNA

a 2nd Cl- is lost from Pt, allowing a 2nd guanine base to bind

this distorts shape of DNA, preventing replication

side effects of cisplatin:

-kidney damage

-damage to intestinal cells

-affect action of other molecules (enzyme action)

-damage to hair follicles

-nerve damage

-damage to bone marrow

-nausea

therefore, chemo is given:

-in small quantities

-at regular intervals

this is to allow the body to recover