Enzyme Function, Cellulose Breakdown, and Biofuel Production

Biological catalysts (usually proteins) that speed up chemical reactions by lowering activation energy without being consumed in the reaction.

Enzymes

The molecule or molecules that an enzyme binds to and converts into product.

Substrate

The specific pocket or cleft on an enzyme where the substrate binds and the chemical reaction takes place.

Active site

The minimum amount of energy required to start a chemical reaction; enzymes lower this to speed up reactions.

Activation energy

The unstable, highest-energy intermediate structure that forms during a chemical reaction; enzymes stabilize this to lower activation energy.

Transition state

The process by which an enzyme loses its functional 3-D shape due to extreme heat, pH, or other conditions, rendering it unable to bind substrate.

Denaturation

The maximum rate at which an enzyme can catalyze a reaction when all active sites are fully saturated with substrate.

Vmax

The substrate concentration at which the reaction proceeds at half the maximum rate; a measure of enzyme-substrate affinity.

Michaelis constant (Km)

The mathematical relationship describing how initial reaction rate depends on substrate concentration, Vmax, and Km.

Michaelis-Menten equation

A linear graph of 1/V₀ vs. 1/[S] used to determine Vmax and Km from enzyme kinetics data.

Lineweaver-Burk plot (double reciprocal plot)

A disaccharide made of two glucose molecules joined by a β 1-4 linkage; the natural substrate of cellobiase.

Cellobiose

The enzyme that breaks down cellobiose into two glucose molecules; catalyzes the final step in converting cellulose to glucose.

Cellobiase

A family of enzymes that break down cellulose into glucose; produced by certain fungi, bacteria, and protozoans.

Cellulases

The enzyme that cleaves internal bonds within long cellulose chains.

Endocellulase

The enzyme that cleaves glucose units from the ends of cellulose chains, releasing cellobiose.

Exocellulase

The structural polysaccharide that makes up plant cell walls, composed of long chains of glucose molecules.

Cellulose

A complex aromatic macromolecule found in secondary cell walls that must be removed before enzymatic cellulose breakdown because it inhibits cellulases.

Lignin

A polysaccharide found in secondary cell walls that must be cleaved from cellulose before enzymatic hydrolysis can occur.

Hemicellulose

Cell walls found in living plant cells, made of cellulose microfibrils embedded in polysaccharides and protein; thick and relatively flexible.

Primary cell wall

More rigid cell walls that develop in dead plant cells before they die, containing cellulose, hemicellulose, and lignin.

Secondary cell wall

Bundled strands (60-80) of individual cellulose molecules held together by hydrogen bonds.

Cellulose microfibrils

Fuel produced by breaking cellulose into glucose via enzymatic hydrolysis, then fermenting the glucose with microorganisms.

Cellulosic ethanol

A process considered carbon neutral because the CO₂ released when burning the fuel was originally absorbed from the atmosphere by the same plants used to make it.

Carbon neutral biofuel cycle

An artificial substrate used to measure cellobiase activity; when cleaved by cellobiase, it releases p-nitrophenol, which turns yellow in basic solution.

p-nitrophenyl glucopyranoside

A colorless molecule released when cellobiase cleaves p-nitrophenyl glucopyranoside; turns yellow in basic conditions, allowing quantitative measurement of enzyme activity.

p-nitrophenol

An instrument that measures how much light of a specific wavelength is absorbed by a solution, used to quantify enzyme activity via color intensity.

Spectrophotometer

A graph plotting absorbance values of known concentrations of a substance, used to convert experimental absorbance readings into concentration values.

Standard curve

The process of converting sugars to ethanol using microorganisms such as yeast.

Microbial fermentation

Plants such as corn, sugar cane, and switchgrass that photosynthesize with high efficiency, producing sugar and cellulose rapidly.

C4 plants

Fuels produced from recently living biological sources, in contrast to fossil fuels derived from ancient biological material.

Biofuels