Proteins: MDCAT Biology notes
Proteins MDCAT notes: amino acids, peptide bonds, four levels of protein structure, fibrous vs globular proteins, functions and denaturation.
Amino acids
Proteins are polymers of amino acids. Each amino acid has a central α-carbon carrying an amino group ($\mathrm{-NH_2}$), a carboxyl group ($\mathrm{-COOH}$), a hydrogen atom and a variable R group. The R group is the additional atom or group of atoms that makes each amino acid different. In glycine, the simplest amino acid, R is just a hydrogen atom. In alanine, R is a methyl group.
- About 20 types of amino acids make up most proteins.
- Nine are essential for adults. The body cannot make them, so they must come from the diet.
Peptide bond
Two amino acids join by condensation: the carboxyl group of one reacts with the amino group of the next, water is removed, and a peptide bond (–CO–NH–) forms. Many amino acids joined this way form a polypeptide. Breaking proteins into amino acids is hydrolysis.
A chain of $n$ amino acids has $n-1$ peptide bonds. Insulin has two chains of 21 and 30 amino acids, so it has $20 + 29 = 49$ peptide bonds. Two different amino acids can form $2^2 = 4$ different dipeptides (for glycine and alanine: Gly-Gly, Gly-Ala, Ala-Gly and Ala-Ala), because order matters.
Levels of protein structure
| Level | What it is | Bonds | Example |
|---|---|---|---|
| Primary | Number and sequence of amino acids | Peptide bonds | Insulin sequence (Sanger) |
| Secondary | Regular coiling into an α-helix, or folding into a β-pleated sheet | Hydrogen bonds | Keratin |
| Tertiary | Unique 3-D folding of one whole polypeptide into a globular shape | Hydrogen, ionic and disulphide bonds, plus hydrophobic interactions | Enzymes, hormones such as insulin |
| Quaternary | Two or more polypeptides combined | The same bonds that hold tertiary structure | Haemoglobin (4 chains) |
An α-helix has 3.6 amino acids per turn, so 360 amino acids make 100 turns. The primary sequence decides how the chain folds. Changing a single amino acid (glutamic acid replaced by valine in the β chain of haemoglobin) causes sickle cell anaemia. The tertiary shape decides the protein's biological function.
Fibrous and globular proteins
| Fibrous | Globular |
|---|---|
| Long strands or fibrils; mostly secondary structure | Spherical, folded; tertiary or quaternary structure |
| Insoluble in water, structural and often elastic | Soluble in water, and can be crystallized |
| Keratin (hair, nails), collagen (bone and cartilage matrix), elastin, fibrin | Enzymes, antibodies, haemoglobin, albumin, hormones |
Keratin is rich in disulphide bridges. Hair straightening and rebonding treatments break and re-form these bonds.
Functions of proteins
- Structural (collagen, keratin) and functional, since most cell work is done by proteins.
- Enzymes catalyse reactions. Hormones such as insulin and thyroxine carry messages.
- Transport: haemoglobin carries $\mathrm{O_2}$, and ceruloplasmin carries copper in plasma.
- Defence: antibodies (immunoglobulins) give immunity.
- Plasma proteins made in the liver, especially albumin, maintain blood osmotic pressure.
- Muscle contraction: actin and myosin.
Insulation against heat loss is a function of fat, not protein. Denaturation is the loss of 3-D shape, and with it the loss of function. Heat, pH changes and heavy-metal ions destabilize proteins in this way.
Common MDCAT traps
- Primary structure is held by peptide bonds; the α-helix is held by hydrogen bonds.
- Haemoglobin is quaternary. A single enzyme chain folded into a globule is tertiary.
- Collagen and keratin are fibrous, not globular.
- Glycosidic bonds belong to carbohydrates and play no part in protein structure.
- To count peptide bonds, subtract the number of chains from the number of amino acids.
Quick revision
- Glycine's R group is H.
- An α-helix has 3.6 residues per turn.
- Heating first disrupts the α-helix, which is secondary structure.
- Collagen forms the matrix of bone and cartilage.
- Sickle cell anaemia comes from a single amino acid substitution.