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Enzymes are proteins that function as catalysts in biochemical reactions.
- Definition of Catalyst: A catalyst is a substance that increases the rate of a reaction without being consumed in the process. At the end of the reaction, the enzyme remains unchanged and can catalyze subsequent reactions.
Enzymatic reactions can occur in a series or cascade involving multiple steps and enzymes.
- Example: Starting molecule A catalyzed to product D through intermediates B and C by specific enzymes.

Metabolism: The totality of enzymatic reactions in the cell, often linked to perform efficiently to produce necessary products.
- Enzymes play crucial roles in cellular processes by converting substrates (the material an enzyme acts upon) to products.
The substrate must bind to the enzyme's active site, forming an enzyme-substrate complex.
- Definition of Active Site: The specific region on an enzyme where the substrate binds.
- Hydrolysis Reaction: In some reactions, like the breakdown of sucrose into glucose and fructose by sucrase, water is involved in breaking bonds.

Enzymes work by reducing the energy of activation needed for a reaction to occur.
- Energy of Activation: The minimum energy required for a chemical reaction to start.
Induced Fit Theory: This replaces the lock-and-key model, noting that the enzyme changes shape to accommodate substrate binding, enhancing the reaction's efficiency.

Substrates bind to the active site through non-covalent and sometimes covalent interactions, including:
- Hydrogen Bonds: Weak interactions contributing to the binding of substrates.
- Ionic Bonds: Occur between charged R groups of amino acids in the active site.
- Covalent Bonds: Can form temporarily during the reaction but must be broken afterwards.

Enzymes allow biochemical reactions to occur under conditions that minimize temperature increase to avoid cellular damage.
- E.g., 42°C can be damage for human cells; enzymes can lower the activation energy necessary for reactions.

Environmental Conditions: Temperature and pH can significantly influence enzyme function.
- Human enzymes generally function optimally at around 37°C and specific pH levels:
- Pepsin (stomach enzyme): Optimal at pH 2.
- Trypsin (intestinal enzyme): Optimal at pH 8.
Cofactors and Coenzymes: Additional non-protein molecules that assist enzyme function.
- Definition of Cofactors: Inorganic ions or organic molecules that support enzyme activity.
Inhibitors: Molecules that decrease enzyme activity by binding to them.
- Types of Inhibitors:
- Competitive Inhibitors: Bind directly to the active site, preventing substrate binding.
- Non-competitive Inhibitors: Bind to a site other than the active site, inducing a conformational change that inhibits enzyme function.

Allosteric Sites: Sites other than the active site where regulatory molecules can bind, influencing enzyme activity.
- Definition of Allosteric Regulation: The process whereby the binding of a molecule at one site affects the activity at another site of the enzyme.
- Both inhibitors and activators can bind to allosteric sites, enhancing or inhibiting enzyme activity.

Definition of Feedback Inhibition: A process where the final product of an enzymatic pathway inhibits an earlier step in the pathway, regulating substrate use.
- Example: Isoleucine binding to threonine deaminase to prevent further conversion of threonine when enough isoleucine is present.

Enzymes may function in groups, forming multi-enzyme complexes, which allow for efficiency in metabolic processes.
- Example: Citric acid cycle enzymes within the mitochondrial matrix work closely together.

Enzymes can be synthesized as inactive precursors known as proenzymes or zymogens.
- These require the removal of a specific peptide segment to become active; this often involves proteolytic cleavage by proteases.
- Example: Caspases are proenzymes that become active during apoptosis (programmed cell death).