Enzymes are, as reviewed, typically proteins that speed up biochemical reactions and are highly specific for particular reactions.
In glycolysis, there are 10 different reactions each with a distinct enzyme name, illustrating enzyme specificity.
Example: glucose to glucose-6-phosphate is catalyzed by hexokinase; the substrate is consumed and converted, while the enzyme remains unchanged and is not consumed in the reaction.
Key concepts and terminology
Substrate (S): the reactant in an enzyme-catalyzed reaction; a substrate is a starting material for that specific reaction. It is a type of reactant, but not every reactant is a substrate.
Product (P): what the substrate is transformed into during the reaction.
Enzyme (E): the catalyst, usually a protein, that speeds up the reaction without being consumed. The enzyme itself is left unchanged after the reaction.
Enzyme-Substrate complex (ES): the transient complex formed when the enzyme binds to its substrate at the active site.
Active site: the region of the enzyme where the substrate binds; the site has a shape complementary to the substrate to form the ES complex.
Substrate vs reactant distinction: substrate is the starting material in an enzyme-catalyzed step; a reactant is any starting material in any chemical reaction, but only some reactants are substrates in enzyme-catalyzed processes.
Holoenzyme: the complete, active form of an enzyme consisting of a protein plus a non-protein accessory component.
Cofactor: the non-protein component required for some enzymes to function; cofactors are not substrates or products.
Cofactors can be inorganic (metals) or organic (coenzymes).
Cofactor examples (inorganic): Fe, Cu, Ni, Mg, Mn, Zn, etc. These metals often serve as cofactors in various enzymes.
Organic cofactors (coenzymes): organic molecules that assist enzymes; one example is NAD/NADH, which functions as an electron carrier. NADH has one more hydrogen than NAD.
Ribozymes: enzymes made of RNA rather than protein (rare in biology).
Enzyme activity: a measure of how fast an enzyme speeds up a chemical reaction.
If enzyme activity is zero, the enzyme is not active and does not speed up the reaction.
If enzyme activity is high, the enzyme is very active and accelerates the reaction more rapidly.
Enzyme-catalyzed reactions: the mechanism
Long-form representation: E + S ⇌ ES → E + P
E + S forms the ES complex (binding is temporary).
ES leads to the formation of the product P, while the enzyme E is regenerated (not consumed).
The substrate is converted into product; the enzyme remains unchanged.
Short-form representation (common in textbooks): Substrate gets converted into product; the enzyme name is written above the arrow.
Example in glycolysis: Glucose (substrate) → Glucose-6-phosphate (product) with hexokinase (enzyme) acting above the arrow, illustrating the catalyzed step.
Example: sucrase-catalyzed reaction (from the figure)
Substrate: sucrose (a sugar)
Enzyme: sucrase (note the -ase suffix, common for enzymes)
Enzyme-Substrate complex: ES forms when sucrose binds to sucrase at the active site.
Products: glucose + fructose
Active site: the region on sucrase that binds sucrose; the substrate fits into this site, forming ES.
Induced fit concept: the substrate appears to fit like a puzzle piece, but the fit is slightly stressed; the bond is adjusted to facilitate the reaction. This destabilizes the substrate bonds, lowering the activation energy and increasing the reaction rate.
Key takeaway: E remains unchanged through the reaction; S is converted to P; ES is a transient intermediate.
Visualization note: the bond depicted as a straight line (in the substrate) becomes curved upon binding due to induced fit, illustrating the conformational change.
Active site and induced fit
Active site is the physical location on the enzyme where the substrate binds to form ES.
Induced fit: binding causes conformational adjustments; the fit is not perfectly static; the induced strain helps destabilize bonds and lowers the activation energy, speeding up the reaction.
Activation energy concept (referenced from earlier in the course): the energy barrier that must be overcome for a reaction to proceed; enzymes lower this barrier via ES formation and induced fit.
Enzyme structure: holoenzyme, cofactors, and ribozymes
Not all enzymes are pure proteins; some require non-protein components to be functional.
Holoenzyme: complete enzyme consisting of the protein component plus its non-protein cofactor
Cofactor: non-protein component essential for enzyme activity; not a substrate or product
Organic cofactors: organic molecules; often called coenzymes; example NAD/NADH as a coenzyme
NAD/NADH example: organic coenzyme; NADH has one more hydrogen than NAD, enabling electron transfer in metabolic reactions
Coenzyme vs cofactor terminology: coenzymes are a subset of cofactors (organic cofactors); not all cofactors are organic
Ribozymes: enzymes composed of RNA rather than protein (rare in biology)
Relationship to glycolysis and real-world relevance
In glycolysis, each of the 10 steps has a distinct enzyme name, illustrating enzyme specificity and the concept of enzyme-catalyzed reaction sequences.
Enzyme specificity ensures that only the intended reaction occurs at each step, enabling tight regulation of metabolism.
The concept of active sites, ES complexes, and induced fit underpins how enzymes recognize substrates and facilitate chemical transformations efficiently.
Summary of core points to remember
E + S ⇌ ES → E + P represents the catalytic cycle: binding to form ES, catalytic transformation to P, and release of P with E regenerated.
Substrate (S) is the starting material for an enzyme-catalyzed reaction; substrate is a specific type of reactant.
The active site is the binding region on the enzyme that provides specificity for the substrate.
Induced fit explains how binding strains substrate bonds to lower the activation energy and speed up the reaction.
Enzymes can be pure proteins or holoenzymes (protein + non-protein cofactors).
Cofactors are inorganic or organic; coenzymes are organic cofactors such as NAD/NADH.
Some enzymes are ribozymes (RNA enzymes).
Enzyme activity is a measure of how fast an enzyme speeds up a reaction; zero activity means no catalysis, high activity means rapid catalysis.
Quick references and formulas
Enzyme-catalyzed reaction representation:
E+S⇌ES→E+P
SEP (overall shorthand: substrate becomes product with enzyme on the arrow)
Conceptual statements to memorize:
The substrate is converted into the product; the enzyme is not consumed or permanently altered.
The active site is the binding location for the substrate on the enzyme.
Induced fit can destabilize bonds to lower the activation energy and accelerate the reaction.
Next steps mentioned in the lecture
The upcoming discussion will cover factors that influence enzyme activity (e.g., temperature, pH, inhibitors, activators) and how these factors regulate metabolic pathways.