# Coenzymes, Cofactors & Prosthetic Groups ## Introduction to Cofactors - There are substances **other than** substrates and inhibitors that interact with enzymes - Some enzymes can only function properly if another **non-protein substance** is present - For example, some enzymes are inactive until they combine with a non-protein substance that changes their **tertiary structure** - This allows the active site to bind correctly with the substrate - These substances are broadly known as **cofactors**

Cofactors are non-protein substances that enzymes require in order to function properly

## Inorganic Cofactors - Some enzymes require **inorganic ions** to function properly - Particular inorganic ions may help to **stabilise the structure** of the enzyme or may actually **take part in the reaction** at the active site - For example, **chloride ions** act as a cofactor for amylase - This means that in order for amylase to be able to digest starch into maltose, chloride ions must be present - The inorganic ions that an enzyme requires in order to function are known as **inorganic cofactors** ## Coenzymes - Larger **organic** (carbon-containing) cofactors are known as **coenzymes** - Some coenzymes are **permanently bound** to the enzyme they assist, often in or near the active site - Some coenzymes only **bind temporarily** during the reaction - Coenzymes are involved in **carrying electrons or chemical groups between enzymes**, aiding in catalysis - Coenzymes **link different enzyme-catalysed reactions into a sequence** during metabolic processes, such as photosynthesis and respiration

Coenzymes are organic non-protein cofactors that carry electrons or chemical groups between enzymes

### Vitamins as Sources of Coenzymes - **Vitamins** are an important source of coenzymes - Many vitamins in the **B vitamin group** are used in the production of important coenzymes, including: - **Pantothenic acid**, a key component of coenzyme A (a coenzyme required for the oxidation of pyruvate during the link reaction that occurs between the glycolysis and Krebs cycle stages of respiration) - **Nicotinic acid**, used to produce the coenzymes NAD and NADP (coenzymes required in many different metabolic reactions, including many of the reactions that take place during photosynthesis and respiration) - **Vitamin B₁** (riboflavin), used to produce the coenzyme FAD (a coenzyme required in the Krebs cycle during respiration)

Vitamins of the B group are used in the production of several important coenzymes

## Examples of Coenzyme Functions ### Hydrogen Transfer in Respiration and Photosynthesis - During many of the reactions in **respiration**, the coenzymes **NAD and FAD** are alternately reduced and oxidised - These coenzymes **transfer energy in the form of hydrogen ions** - The coenzyme **NADP** fulfils this same role in chloroplasts during **photosynthesis** ### Chemical Group Transfer - The coenzymes **ATP** and **coenzyme A** act in a different way, by **transferring chemical groups** - For example: - **ATP** is responsible for the transfer of **phosphate groups** between respiration and energy-consuming processes in cells - **Coenzyme A** is responsible for the transfer of an **acetyl group** ($-\text{CH}_3\text{CO}$) from fatty acids and glucose during respiration ### How Coenzymes Work - A non-protein cofactor (coenzyme) enables the active enzyme to bind correctly with the substrate - Once the coenzyme binds to the enzyme, the active site changes shape - The substrate (which does not fit into the active site initially) can now bind - The active enzyme converts substrate to products - The coenzyme may remain permanently attached or detach after the reaction ## Summary | **Feature** | **Cofactors** | **Coenzymes** | |-------------|---------------|---------------| | **Type of substance** | Inorganic ions | Organic molecules | | **Examples** | Chloride ions (Cl⁻) for amylase | NAD, FAD, NADP, ATP, Coenzyme A | | **Source** | Minerals from diet | Derived from vitamins (especially B vitamins) | | **Binding** | May stabilise enzyme structure or participate in reaction | May be permanently bound or bind temporarily | | **Function** | Help enzyme function or take part in reaction at active site | Carry electrons or chemical groups between enzymes |

Cofactors are non-protein substances, meaning they are not made from amino acids

## Key Distinctions - **Cofactors** are the broad term for all non-protein substances that enzymes require to function - **Inorganic cofactors** are mineral ions needed for enzyme function - **Coenzymes** are organic non-protein cofactors - Coenzymes contribute to enzyme-catalysed reactions by **accepting or donating hydrogen ions** or **chemical groups** (e.g. phosphate groups) - Coenzymes link different enzyme-catalysed reactions into a sequence during metabolic processes ## The Role of Coenzymes in Metabolic Pathways - Metabolic pathways consist of sequences of enzyme-catalysed reactions - Coenzymes act as **carriers** that transfer molecules or groups from one enzyme to another - For instance, during cellular respiration: - NAD accepts hydrogen from one reaction - Carries it as reduced NAD (NADH) - Donates it to another enzyme in a different reaction - This links the reactions together in a coordinated pathway - Without coenzymes, many metabolic pathways would not function efficiently or at all