2.3 Proteins

Amino acids are the monomers of proteins. Every amino acid has the same basic arrangement around a central carbon: an amine group, a carboxyl group, a hydrogen atom and a variable R group. The R group changes the amino acid’s properties, but it is not the part that forms the peptide link.

1. Position the carboxyl group of one amino acid beside the amine group of another.
2. Remove the hydroxyl group from the carboxyl group and a hydrogen from the amine group; together they form one water molecule.
3. Join the remaining carbonyl carbon to the nitrogen, creating a covalent peptide bond and a dipeptide.
4. Repeat the condensation reaction to join more amino acids and form a polypeptide chain.
5. In hydrolysis, add water across a peptide bond to break the chain back into smaller peptides or amino acids.

The sequence, number and types of amino acids in a polypeptide determine which interactions can occur as it folds. Peptide-bond formation therefore creates the covalent chain that later supports protein structure and function; hydrolysis reverses the joining process.

A polypeptide is a chain of amino acids joined by peptide bonds, but it is not automatically a functional protein until it folds or assembles correctly. When locating a peptide bond, look for the carbon–nitrogen link next to the carbonyl carbon; do not mistake an R group for the peptide bond.

The four levels describe increasing organisation of a protein, not four separate molecules. A protein’s amino-acid sequence can fold locally and globally, and some proteins then assemble from multiple polypeptide subunits.

The sequence is the starting information: changing it can change later folding and function. Secondary structure is local backbone folding; tertiary structure is the complete shape of one chain; quaternary structure exists only when multiple chains work together. Haemoglobin is a useful example because its four globin subunits give it quaternary structure.

Do not say that every protein has quaternary structure, or that each level is a new molecule. A single-chain protein can have primary, secondary and tertiary structure without a quaternary level; also distinguish backbone hydrogen bonds in secondary structure from R-group interactions in tertiary structure.

A polypeptide’s tertiary structure is its overall three-dimensional shape. Interactions between R groups help hold that shape, and interactions between separate polypeptide chains can contribute when a protein has quaternary structure.

• Hydrophobic interactions: non-polar R groups tend to cluster away from water, helping form the protein’s interior.
• Hydrogen bonds: polar groups can form many relatively weak attractions that help stabilise particular folds.
• Ionic bonds: oppositely charged R groups attract and can support the folded arrangement.
• Disulfide bonds: two cysteine R groups can form a strong covalent bridge that stabilises the chain.
• Quaternary assembly: the same kinds of interactions can help hold multiple folded subunits together in a functional protein.

The exact R-group sequence determines which interactions are possible and where they occur. The resulting shape creates specific surfaces, pockets or binding sites, so changing the fold can change the protein’s function even when the peptide sequence itself has not been cut apart.

These are not all the same kind of link or the same strength: disulfide bonds are covalent, while hydrophobic interactions and many hydrogen-bond/ionic attractions are non-covalent. Do not list an interaction without identifying the relevant R groups, and do not treat a protein’s shape as independent of its sequence.

Globular proteins are compact, roughly spherical proteins that are generally soluble in water. Their folded shape places many non-polar hydrophobic R groups inside and exposes polar hydrophilic R groups to the surrounding water.

• Folding: R-group interactions fold the polypeptide into a compact tertiary structure rather than a long open chain.
• Solubility: hydrophobic R groups are mostly shielded inside, while hydrophilic R groups interact with water at the surface, helping the protein remain dispersed.
• Specific shape: the fold creates a particular three-dimensional surface, pocket or binding site.
• Physiological roles: solubility allows transport in body fluids and access to aqueous reactions; the specific shape allows roles such as enzyme catalysis, antibody recognition or transport.

Haemoglobin is a globular protein whose folded subunits remain soluble in blood and position haem groups for oxygen transport. Enzymes are another example: their globular folds create active sites that bind particular substrates. These examples show different functions arising from the same broad globular class, not one universal globular-protein job.

“Globular” describes typical shape and solubility, not a single function or a guarantee that every protein is perfectly spherical. A change in conditions or sequence can alter the interactions that maintain the fold, reducing the shape-dependent function or solubility.

Globular and fibrous proteins are broad structural and functional classes, not mutually exclusive chemical categories. Globular proteins are generally compact and soluble; fibrous proteins are generally long, organised and insoluble. Their shape and organisation help explain their typical roles.

Globular folding tends to bury hydrophobic R groups and expose hydrophilic R groups, helping the protein remain dispersed in water and form specific working sites. Fibrous proteins use long, repetitive or aligned organisation with stabilising interactions to form strong structures, so insolubility is compatible with their structural role.

Use “generally” rather than treating the classes as absolute rules: globular describes a typical compact shape and solubility pattern, while fibrous describes a typical extended structural pattern. “Fibrous” does not mean unstructured, and a protein’s class is not a separate chemical type defined by one bond.

Haemoglobin is a globular protein with quaternary structure because four polypeptide subunits assemble into one protein: two α-globin and two β-globin chains.

• Subunit organisation: The four globin chains form a compact, roughly spherical assembly. Interactions between subunits, including disulfide bonds, help hold the quaternary structure together.
• Within each subunit: Each globin chain contains one prosthetic haem group. Therefore one haemoglobin molecule has four haem groups.
• Iron position: Each haem group contains an Fe²⁺ ion. The iron is located in the haem prosthetic group rather than being part of the polypeptide backbone.
• Shape and solubility: Hydrophobic R groups tend to face inwards and hydrophilic R groups outwards, helping maintain the folded, water-compatible protein.

The four-subunit arrangement positions four haem groups within the globin assembly, while the Fe²⁺ ion in each haem provides the site associated with reversible oxygen binding. Thus the quaternary structure and prosthetic groups create the structural basis for haemoglobin’s oxygen-carrying role; detailed oxygen-transport behaviour is treated separately.

Do not describe haemoglobin as one single polypeptide or as having only one haem group. The structure card establishes four globin subunits and four haem groups; it does not replace the separate card on haemoglobin function.

Haemoglobin transports oxygen by binding it reversibly to the Fe²⁺ ion in each prosthetic haem group. The haem group provides the oxygen-binding site; the globin protein provides the soluble, organised structure that presents these sites.

• Binding site: Each haem group contains an Fe²⁺ ion that can reversibly combine with one oxygen molecule, forming oxyhaemoglobin.
• Loading and release: Reversible binding allows oxygen to be picked up where it is available and released where it is needed, rather than making oxygen permanently part of the pigment.
• Capacity: A haemoglobin molecule has four haem groups, so it can bind four oxygen molecules when all sites are occupied.
• Why the protein matters: Haemoglobin is soluble in blood, and its globin subunits position the haem groups so oxygen can be carried efficiently.

The haemoglobin structure therefore links composition to function: Fe²⁺ supplies the reversible oxygen-binding site, while the globin assembly carries those sites in a soluble protein. Oxygen binding can also alter the protein’s quaternary arrangement, helping subsequent oxygen molecules bind more readily; this is a structural basis for cooperative loading, not a claim that oxygen is covalently fixed.

The oxygen-binding site is the Fe²⁺-containing haem group, not an arbitrary amino acid in the globin chain. “Reversible” means oxygen can bind and be released; it does not mean the haem group is absent or that haemoglobin permanently stores oxygen.

Collagen is a fibrous protein built from three polypeptide chains that wind together as a triple helix, also called a tropocollagen molecule. Its repeated sequence and layered stabilisation make the structure strong and organised.

• Three-chain assembly: Each polypeptide chain has a helix shape, and three chains are held closely together by hydrogen bonds to form the triple helix.
• Repetitive sequence: Glycine occurs at about every third position. Its small R group allows the three chains to pack closely inside the helix.
• Within the helix: Many hydrogen bonds between the chains stabilise the triple-helix arrangement.
• Between molecules: Parallel triple helices form covalent cross-links between amino-acid R groups, holding collagen molecules together into fibrils.

The sequence supports close packing, close packing enables the three-chain helix, and hydrogen bonds stabilise that helix. Covalent cross-links then connect neighbouring triple helices into larger fibrils, so stability is built at more than one structural level.

Keep the levels distinct: three polypeptide chains form one triple-helix collagen molecule; cross-links between parallel molecules help form fibrils. This card explains collagen structure and stabilisation, while the next card handles the full structure-to-tissue-function account.

Collagen is suited to support because its triple-helix molecules assemble into parallel fibrils and then larger collagen fibres. The organised fibres have high tensile strength: they resist being pulled apart along their length.

• Molecular links: Covalent cross-links between neighbouring, parallel triple-helix molecules hold them together in fibrils.
• Fibre organisation: Many fibrils combine to form collagen fibres, with molecules arranged in a staggered pattern that adds strength.
• Force alignment: Collagen fibres can be lined up with the forces they must withstand, so the load is shared along the fibre direction.
• Functional result: The triple-helix stability, cross-links and parallel alignment together produce a strong supporting material suitable for tissues such as tendons and ligaments.

Triple helices provide stable collagen molecules; cross-links join neighbouring molecules; fibrils bundle into fibres; and aligned fibres transmit pulling forces. This structure-to-function chain explains why collagen is a fibrous support protein rather than a compact, soluble transport protein.

Do not reduce collagen strength to a single bond or to the triple helix alone: cross-linking, staggered molecular arrangement and fibre alignment also matter. This card explains the structural basis for support; it does not add unsupported tissue physiology.