B1.2 Proteins

Every amino acid has an amino group, a carboxyl group and a variable R-group attached to a central carbon.

The amino and carboxyl groups provide the chemistry for peptide-bond formation, while the R-group changes charge, polarity, size or reactivity. That shared backbone lets amino acids join; the variable side chain lets proteins differ.

Identify the structure by locating:

• the central carbon
• amino and carboxyl groups
• the R-group that gives the amino acid its distinctive chemistry

In a diagram, two amino acids may have the same backbone but different R-groups. They can still join in the same type of condensation reaction, yet interact differently after joining.

The R-group is not an optional decoration: ignoring it loses the reason different amino acids produce different protein shapes and functions.

A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of another and water is released.

The reaction removes OH from the carboxyl group and H from the amino group, leaving a covalent C–N link. Repeating this condensation builds a polypeptide whose sequence can later fold into a functional shape.

Read a synthesis diagram in this order:

• find carboxyl and amino groups
• remove H and OH as water
• mark the new peptide bond

Joining two amino acids releases one water molecule and gives a dipeptide with one peptide bond; adding a third amino acid creates a second bond and a tripeptide.

A peptide bond is not a hydrogen bond. It is the covalent link in the chain; hydrogen bonds help stabilize folded regions later.

Essential amino acids must be obtained from food because the body cannot synthesize enough of them; non-essential amino acids can be made by the body.

Protein synthesis needs a complete set of amino-acid building blocks. If one essential amino acid is missing, making a particular protein can stop even when other amino acids are available.

For a dietary claim, separate:

• essential: required from diet
• non-essential: synthesized in the body
• protein synthesis: still needs the full required set

A meal can provide plenty of amino acids but still limit synthesis of a protein if it supplies too little of one essential amino acid.

‘Non-essential’ does not mean unimportant; it means the body can synthesize it. Dietary need and biological function are different questions.

A polypeptide’s amino-acid sequence can vary enormously because many positions can contain different amino acids in different orders.

Each position has multiple possible choices, so the number of possible sequences grows rapidly with chain length. Sequence controls which R-groups meet during folding and therefore helps determine final structure and function.

Explain sequence variety by checking:

• chain length
• the number of amino-acid choices per position
• order, not just the total amino-acid count

Two chains containing the same numbers of alanine and glycine can fold differently if those residues occur in different positions.

A different sequence does not automatically mean a different function; the change matters when it alters folding, stability or an active site.

Protein shape is maintained by weak interactions that can be disrupted by extreme pH or temperature, causing denaturation and loss of function.

Heating increases molecular motion and extreme pH changes charges on R-groups. These changes disturb hydrogen bonds, ionic attractions and other interactions holding the folded chain in its working shape.

Predict a condition effect by asking:

• which interaction is disturbed
• whether the chain unfolds or changes active-site shape
• whether the change is reversible under the conditions

An enzyme may work faster as temperature rises to its optimum, then lose activity sharply when heating disrupts the shape of its active site.

Denaturation changes conformation, not necessarily the amino-acid sequence. Do not treat every loss of activity as peptide-bond hydrolysis.

Amino-acid R-groups differ in charge, polarity, size and chemical reactivity, giving polypeptides a wide range of folding interactions.

R-groups can attract or repel one another, form hydrogen bonds, avoid water or create covalent links. Their chemistry makes a sequence more than a string of identical beads: it creates a three-dimensional interaction pattern.

Classify an R-group by considering:

• charge at the relevant pH
• polarity and water interaction
• size or special reactivity

A charged side chain on a protein surface can interact with water, while a non-polar side chain is more likely to be buried in the folded interior.

R-group category is condition-dependent: charge can change with pH. Do not label every side chain as permanently positive, negative or neutral.

Primary structure is the exact amino-acid sequence of a polypeptide, and that sequence constrains every later level of folding.

The order places particular R-groups at particular positions. A substitution can create or remove an interaction, alter a bend or change an active site, so sequence is the starting information for conformation.

Trace a sequence change by checking:

• which residue changed
• what chemistry the new R-group adds or removes
• which later interaction or function could shift

Replacing one non-polar residue with a charged residue in a buried region can destabilize folding because the new charge is poorly suited to the hydrophobic interior.

Primary structure means sequence, not the first stage in time only. It remains part of the molecule even after secondary and tertiary folding occur.

Secondary structure is local folding of the polypeptide backbone into patterns such as alpha helices and beta-pleated sheets, stabilized mainly by backbone hydrogen bonds.

Hydrogen bonds form between backbone C=O and N–H groups at regular positions. Their repeated geometry produces a helix or aligns strands into a sheet without requiring the R-groups to form the main stabilizing bonds.

Recognize secondary structure by checking:

• repeated backbone hydrogen bonds
• local helix or sheet geometry
• R-groups projecting away from the backbone pattern

A stretch of chain can coil into an alpha helix when backbone hydrogen bonds repeat along the segment, even though the amino-acid sequence itself remains unchanged.

Secondary structure is not the whole folded protein. Interactions among distant regions and R-groups belong mainly to tertiary structure.

Tertiary structure is the overall three-dimensional shape of one polypeptide, produced by interactions among its R-groups and with the surrounding water.

Non-polar groups tend to be buried, while charged and polar groups can remain exposed or attract one another. Hydrogen bonds, ionic attractions, disulfide links and hydrophobic interactions stabilize the final fold.

Explain a tertiary interaction by naming:

• the two groups involved
• the type of interaction
• how it changes the chain’s shape or stability

A disulfide link between two cysteine R-groups can hold distant parts of a polypeptide together, making the folded shape more resistant to change.

Tertiary structure is not simply ‘all bonds in the protein’. Peptide bonds define the chain; tertiary interactions fold that chain.

The positions of polar and non-polar R-groups influence where parts of a protein sit: non-polar groups are often buried from water while polar or charged groups are often exposed.

Water makes the hydrophobic effect energetically favourable, while polar and charged groups can interact with water or each other. Folding therefore rearranges the chain to match each R-group’s chemistry.

Predict placement using:

• non-polar group: usually shielded from water
• polar/charged group: often solvent-exposed or paired
• exception: active sites can place unusual groups together

Moving a charged residue from the surface into a non-polar core can destabilize a protein unless another interaction compensates for the buried charge.

‘Polar outside, non-polar inside’ is a useful tendency, not an absolute rule. Function can require an exception at a binding site or channel.

Quaternary structure is the arrangement and interaction of two or more polypeptide subunits in one functional protein.

Each subunit has its own primary, secondary and tertiary structure, but their contact surfaces can create a stable complex or allow cooperation. The whole complex can therefore behave differently from an isolated chain.

Describe a quaternary protein by stating:

• how many subunits are present
• how the subunits contact each other
• what combined function the complex provides

A multi-subunit transport protein can bind a molecule at one subunit and change the shape or affinity of another, creating coordinated function.

A protein with only one polypeptide has no quaternary structure, even if that chain contains many secondary and tertiary features.

Globular proteins fold into compact shapes suited to soluble, catalytic or transport roles, whereas fibrous proteins form elongated structures suited to support and strength.

A globular fold brings functional groups together and often leaves a water-compatible surface. Repeating fibrous arrangements align chains or subunits so forces can be distributed along a tissue.

Compare the two forms by checking:

• compact versus extended shape
• local active or binding sites versus repeated strength pattern
• soluble/mobile versus structural role

An enzyme is typically globular so its active site can form in a compact fold; collagen is fibrous so repeated structure can provide tensile support.

Shape suggests function but does not prove it alone. Use the protein’s interactions, location and role rather than classifying from appearance only.