What is Levels of structure? — Biology

Definition 7.1 University Biology — Year 3 · Chapter 7 — Structural Biology of Proteins

The primary structure is the sequence of residues. The secondary structure is the local regular conformation of the backbone, stabilised by hydrogen bonds between backbone carbonyls and amides: the $\alpha$ -helix, a right-handed spiral of $3.6$ residues per turn, rising $0.15\,\mathrm{nm}$ per residue ( $0.54\,\mathrm{nm}$ per turn), each carbonyl bonded to the amide four residues on; and the $\beta$ -sheet, in which extended strands lie side by side, parallel or antiparallel, bonded to their neighbours, the side chains alternating above and below the sheet. The tertiary structure is the fold of a whole chain in three dimensions; a compact, independently folding unit of $50\text{ to }250$ residues within it is a domain, and the arrangement of a domain’s secondary elements is its fold. The quaternary structure is the assembly of several chains: haemoglobin’s $\alpha_{2}\beta_{2}$ . About $1300$ folds account for all known domains, and a few dozen of them — the Rossmann fold, the TIM barrel, the immunoglobulin fold — for a large fraction of all proteins: nature reuses folds and varies their sequences.

The Ramachandran plot. The shaded regions are the sterically allowed combinations of the backbone angles and ; the right-handed -helix and the -strand each correspond to a small region, and most residues of a folded protein fall in one of them. — The Ramachandran plot. The shaded regions are the sterically allowed combinations of the backbone angles $\phi$ and $\psi$ ; the right-handed $\alpha$ -helix and the $\beta$ -strand each correspond to a small region, and most residues of a folded protein fall in one of them.

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