Proteins and lipids

MDCAT Biology · Biology · Biological molecules. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Proteins are chains of amino acids. Each amino acid has an amino group, a carboxyl group, a hydrogen atom and a variable R group on the same carbon. Two amino acids join by a peptide bond (–CO–NH–), with the loss of one water molecule. The order of amino acids decides how the chain folds, and the folded shape decides what the protein does.

Lipids are not polymers. They are grouped together because they do not dissolve in water but dissolve in organic solvents. A triglyceride is glycerol joined to three fatty acids by ester bonds, and it stores energy. A phospholipid has a phosphate group in place of one fatty acid, giving a water-loving head and two water-hating tails, which is why it forms membranes.

Rules to remember

About 20 kinds of amino acid occur in proteins; glycine is the simplest (R = H). A chain of n amino acids has (n − 1) peptide bonds
Primary structure = sequence of amino acids. Secondary = α-helix or β-pleated sheet held by hydrogen bonds
Tertiary = 3-D folding of one chain (ionic bonds, hydrogen bonds, disulphide bridges between cysteines, hydrophobic interactions). Quaternary = two or more chains together (haemoglobin has four)
Fibrous proteins: long, insoluble, structural (keratin, collagen, silk). Globular proteins: compact, soluble (enzymes, antibodies, haemoglobin)
Triglyceride = 1 glycerol + 3 fatty acids, 3 ester bonds, 3 water molecules released
Saturated fatty acid: no C=C, higher melting point (fats). Unsaturated: one or more C=C, lower melting point (oils)
Other lipids: waxes (waterproofing), steroids such as cholesterol (four fused rings), terpenoids such as carotenoids (isoprene units). Fat stores about twice as much energy per gram as carbohydrate

Common mistake

Giving the number of peptide bonds as equal to the number of amino acids. In one chain it is always one less, and each separate chain loses one more.

Worked example

Haemoglobin has two α chains of 141 amino acids each and two β chains of 146 amino acids each. How many peptide bonds does one molecule contain?

Total amino acids = (2 × 141) + (2 × 146) = 282 + 292 = 574.
Each chain has one bond fewer than its amino acids, and there are 4 chains.
Peptide bonds = 574 − 4 = 570.

Answer: 570 peptide bonds.

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