Proteins

The **monomer (subunit)** that **proteins** are built from.

An **amino group (—NH₂)**, a **carboxyl group (—COOH)**, a **hydrogen (—H)** and a **variable R group**.

The **amino group (—NH₂)** — a nitrogen-containing group.

The **carboxyl group (—COOH)**.

The **variable side chain** of an amino acid — the only part that differs from one amino acid to the next.

**20** — because there are **20 different R groups**; the rest of the structure is identical.

The **central carbon**, the **amino group**, the **carboxyl group** and the **hydrogen** — only the R group differs.

**Carbon, hydrogen, oxygen and nitrogen** (some also contain sulfur).

From the **amino group (—NH₂)**.

A protein contains **nitrogen**; carbohydrates and lipids contain only **C, H and O**.

**No** — they contain only carbon, hydrogen and oxygen. Nitrogen is found in proteins / amino acids.

**One** — all four groups attach to this single central carbon.

**Condensation** — it forms a **peptide bond** and releases one water molecule.

The covalent bond (**CO—NH**) that joins two amino acids; it forms by condensation.

The **carboxyl group (—COOH)** of one amino acid and the **amino group (—NH₂)** of the next.

An **—OH** (from the carboxyl group) and an **—H** (from the amino group), leaving together as **one water molecule**.

**One** molecule of water (H₂O) per peptide bond.

**Two amino acids** joined by a single peptide bond.

A long chain of **many amino acids** joined by peptide bonds.

**Hydrolysis** — one water molecule is **added** across the bond, splitting the chain into amino acids.

During **digestion**, when dietary protein is broken back down into amino acids.

**n − 1** — each bond links a pair, so there is one fewer bond than amino acids.

**Total amino acids − number of chains** (each chain has one fewer bond than its amino acids).

**4** (5 − 1).

**20** — about **nine** are essential and about **eleven** are non-essential.

An amino acid the body **cannot synthesise (make)**, so it **must be obtained from the diet**.

An amino acid the body **can synthesise (make)** for itself, so it **does not have to be supplied by the diet**.

**No** — every amino acid is needed to build proteins. 'Non-essential' only means it is **not required in the diet**, because the body can make it.

To **make / build** a molecule inside the body from simpler materials.

**Essential** amino acids — the body cannot make them.

**Essential = cannot be made by the body → must be eaten**; **non-essential = can be made by the body → need not be eaten**.

They contain **all nine essential amino acids** in a single source (meat, fish, eggs, dairy).

A single plant protein often **lacks one or more essential amino acids**.

**Combine different plant proteins** (e.g. rice + beans) so that **all nine essential amino acids** are supplied.

The body has **no source** of it, so it **cannot build the proteins** that contain it (a protein deficiency).

It **cannot be synthesised by the body**, so it **must be obtained from the diet**.

Its specific **folded 3D shape**. The protein only works correctly in its normal conformation.

The **sequence (order) of its amino acids** — the primary structure.

The **sequence (order) of amino acids** in the chain, joined by peptide bonds.

Local folding into **α-helices** (coils) and **β-pleated sheets**, held by **hydrogen bonds**.

The way the **whole single chain folds** into one overall **3D shape**, held by bonds between the R-groups.

The way **two or more folded chains (subunits)** join together to make one functional protein (e.g. haemoglobin).

**Primary → secondary → tertiary → quaternary.**

**Single-chain** proteins — quaternary structure needs **two or more** chains.

**Hydrogen bonds** between parts of the polypeptide backbone.

The **loss of a protein's folded 3D shape**, so it can no longer do its job.

**High temperature** and **extreme pH** (very acidic or alkaline).

The **amino acid sequence (peptide bonds) is preserved**; the **folded 3D shape (conformation) is lost**.

Its **active site changes shape**, so the **substrate no longer fits** and the reaction is not catalysed.

Its specific **3-D shape**, which comes from the **order of its amino acids**.

Proteins, as a group, can carry out a very **wide range of different jobs** — more than any other type of molecule.

**Amylase** — it **catalyses** the breakdown of starch into sugar.

**Haemoglobin** — it **carries oxygen** in red blood cells.

**Collagen** — it strengthens **skin, tendons and bone**.

**Insulin** — it signals cells to **take up glucose** from the blood.

**Defence** — they **bind to specific pathogens** so the body can destroy them.

**Actin and myosin** — contractile proteins that generate **movement**.

**Rhodopsin** — it **absorbs light** in the rod cells of the retina (needed for vision).

A **shortage of, or fault in, a particular protein**, so the job it normally does cannot be carried out.

Name the **protein's job**, then state that this **job is lost** — so the process that relied on it **fails**.

A shortage of **rhodopsin** — it normally **absorbs light** in rod cells, so without it light is not detected and vision is impaired.

The 20 amino acids can be **ordered in countless ways**, giving countless **shapes** — each shape gives a different **function**.