Amino acids as compounds with both amine and carboxylic acid groups and their behaviour as zwitterions. Formation of proteins by condensation of amino acids and their hydrolysis. The structure of DNA nucleotides, base pairing by hydrogen bonding, and the action of cisplatin. Enzymes as biological catalysts with stereospecific active sites.

What this dot point is asking

AQA wants you to describe amino acid structure and zwitterion behaviour, explain protein formation by condensation and hydrolysis, describe enzymes and their stereospecific active sites, and describe DNA nucleotides, base pairing by hydrogen bonding, and the action of cisplatin.

Amino acids and zwitterions

The general formula of a 2-amino acid is $RCH(NH_2)COOH$. Because they contain both an acidic and a basic group, amino acids are amphoteric.

In acid the amino acid gains $H^+$ (becomes positive, $-NH_3^+$ and $-COOH$); in alkali it loses $H^+$ (becomes negative, $-NH_2$ and $-COO^-$).

Proteins

Amino acids join by condensation, forming a peptide (amide) bond $-CONH-$ and releasing water. The sequence of amino acids is the primary structure. Proteins are broken down by hydrolysis (reflux with $6 \text{ mol dm}^{-3}$ HCl) back into their amino acids, which can be separated by chromatography.

Enzymes

Enzymes are biological catalysts (globular proteins). They have an active site with a specific 3D shape, complementary to one substrate. Because the active site is stereospecific, it binds only one stereoisomer (enantiomer) of a substrate, lowering the activation energy. Drug molecules are designed to fit (and block) specific active sites, which is why so many drugs must be made as a single enantiomer rather than a racemate, since the wrong enantiomer either does nothing or causes harm. The same chirality runs through the whole of biochemistry: naturally occurring amino acids are nearly all the same single enantiomer, so the proteins and active sites they build are themselves chiral.

DNA

A nucleotide is made by condensation of a phosphate group, the sugar deoxyribose, and an organic (nitrogenous) base (adenine, thymine, cytosine or guanine). Nucleotides polymerise into two strands forming a double helix.

Cisplatin

Cisplatin is a platinum(II) complex used to treat cancer. It binds to DNA in the cancer cell, forming a coordinate bond between platinum and a nitrogen of guanine. This prevents the DNA from unwinding and replicating, so the cell cannot divide and undergoes cell death. A side effect is harm to healthy fast-dividing cells, so the dose must be controlled.

Try this

Q1. Draw or describe the zwitterion of glycine, $H_2NCH_2COOH$. [1 mark]

• Cue. $H_3N^+CH_2COO^-$.

Q2. State how many hydrogen bonds form between cytosine and guanine. [1 mark]

• Cue. Three.