Enzymes

What are enzymes

They are proteins that possess tertiary or quaternary structure and catalyze chemical transformations by binding substrates at specific active sites

What is substrate?

The specific molecules acted upon by they enzyme, and converted to products

What is the Active Site?

substrate specific site for binding, this is where the chemistry happens

• attaches to active site

The enzyme process to product steps

1. Substrate binds to enzyme

2. The enzyme will change shape, creating the Enzyme-substrate complex (E-S complex)

3. A transition state forms, the substrate is being converted to product

4. Product is released from active site

• binding dependent on non covalent forces

What is the function of enzymes?

• enzymes bind substrates with high specificity

• active site fits one or few related molecules

• binding often induces structural change

• enzyme activity is tightly regulated

• lower activation energy; enzyme is not consumed

Catalytic power

the ration of the enzyme-catalyzed rate to the uncatalyzed rate

Specificity

the selectivity of enzymes for their substrates (what substrates can bind

Regulation

mechanisms that balance the rate of metabolic reactions for maintenance of cellular requirements

Catalytic power

The ration of the enzyme-catalyzed rate of a reaction to the uncatalyzed rate

• enzymes can accelerate reactions as much as 10²⁶ over uncatalyzed rates

• Catalyzed rate → 3×10⁴/sec Uncatalyzed Rate → 3×10^-10/sec

• Ratio(catalytic power) is (3×10⁴)/(3×10^-10) = 1×10¹⁴

Enzyme Substrate Mechanisms

• enzymes selectively recognize proper substrates over other molecules

• enzymes produce products in very high yields → often greater than 95%

• Specificity is controlled by structure - the unique fit of the substrate with enzyme controls the selectivity for substrate and the product that’s formed

The “Lock and Key” hypothesis

First explanation for specificity

• The active site of an enzyme is rigid and shaped to fit specific substrate, like a key into a lock

• Once the substrate binds, the enzyme facilitates the reaction and releases products

The “induced fit” hypothesis

More accurate description of specificity

• The active site of an enzyme is flexible and changes to fit a substrate

• The induced fit model explains experimental data and observations better

• Induced fit favors formation of the transition state

Induced Fit in action

• specificity and reactivity are linked

• substrate binding induces conformational change

• Hexokinase closes around glucose

• Domain closure forms the catalytic site

Enzyme Classification Class 1 Oxireductase

enzymes that catalyze redox reactions involving transfer of electrons/hydrogen atom/oxygen atom

• peroxidases, catalases

Enzyme Classification Class 2 Transferases

Enzymes which catalyze transfer of an atom or a functional group between molecules

Enzyme Classification Class Hydrolases

Enzymes which catalyze hydrolytic reactions and their reversal

Enzyme Classification Class 4 Lyases/Synthases

Enzymes involved in elimination reactions in the absence of water, leading to formation of double bonds or addition across a double bond

Enzyme Classification Class 5 Isomerases

Enzymes which catalyze isomerisation and racemization reactions

Enzyme Classification Class 6 Ligases/Synthetases

Enzymes which catalyze synthesis of a C-X bond while utilizing ATP

Coenzymes

organic molecules that help enzymes carry chemical groups or electrons

• always organic

• loosely bound

• transfer electrons or groups

• Modified during reaction

• NAD⁺ FAD, CoA

Cofactors

Non-protein helpers required for enzyme activity

• either inorganic or organic

• loosely or tightly bound

• stabilize structure

• typically unchanged

• Mg⁺, Zn²⁺, Fe²⁺

Apoenzyme

The protein portion of an enzyme (inactive without cofactor)

• Lactate dehydrogenase without NAD⁺ → inactive

Holoenzyme

The complete, catalytically active enzyme

• Holoenzyme: Lactate dehydrogenase + NAD⁺ → active and catalyzes pyruvate ⇌ lactate