Proteins, nucleic acids and conjugated molecules

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 polymers of amino acids. Each amino acid has a central carbon joined to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and an R group. Only the R group differs. A peptide bond (C–N) forms between the carboxyl group of one amino acid and the amino group of the next, and one molecule of water is released.

The order of the amino acids decides how the chain coils and folds, and so decides the job of the protein.

Nucleic acids (DNA and RNA) are polymers of nucleotides. Conjugated molecules are made of two different kinds of molecule joined together.

Rules to remember

Peptide bonds in one chain = number of amino acids − 1.
Primary structure = number and sequence of amino acids; secondary = α-helix or β-pleated sheet held by hydrogen bonds; tertiary = folding of one chain into a 3D shape; quaternary = two or more chains held together.
Fibrous proteins are long strands, insoluble in water, structural (keratin, silk fibre, myosin); globular proteins are spherical and soluble (enzymes, antibodies, hormones, haemoglobin).
Insulin: 51 amino acids in two chains (21 + 30), sequence found by Sanger. Haemoglobin: 574 amino acids in four chains (two α of 141, two β of 146).
Nucleotide = pentose sugar + nitrogenous base + phosphoric acid. Purines (double ring): adenine, guanine. Pyrimidines (single ring): cytosine, thymine, uracil.
DNA (Watson and Crick): double helix; A pairs with T by 2 hydrogen bonds, G pairs with C by 3; so A = T and G = C. RNA is single stranded, has ribose, and uracil in place of thymine; rRNA is about 80% of cell RNA.
Glycolipid = carbohydrate + lipid; glycoprotein = carbohydrate + protein; lipoprotein = lipid + protein; nucleoprotein = nucleic acid + protein.

Common mistake

In base-pair questions students set A equal to G. In double-stranded DNA, A equals T and G equals C, so A + G is always 50% of the bases.

Worked example

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

Amino acids: (2 × 141) + (2 × 146) = 282 + 292 = 574.
Each chain has one peptide bond fewer than its amino acids: α chain 140, β chain 145.
Peptide bonds: (2 × 140) + (2 × 145) = 280 + 290 = 570.

Answer: 574 amino acids and 570 peptide bonds.

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Chapter 2: Biological Method

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