Enzymes

Activation energy

Initial energy input in a reaction

Enzyme

• Proteins

• 3D shape

• intricate chemical structure

  • depends on weak interactions between amino acids of the protein

• Biological catalysts

• Substrates > Enzymes > Products

Exergonic reaction

Occur when the products of a chemical reaction have less energy than the reactant, tend to be catabolic

Endergonic reaction

Occurs when the products of a chemical reaction have more energy than the reactants - tend to be anabolic

Metabolism

• Complex network of interdependent and interacting chemical reactions in living organisms

  • most are intercellular

• form pathways wherein 1 type of molecule is transformed into another by small steps

Substrate

Starting substance in an enzyme-catalyzed reaction

Catalyst

Increases the rate of a chemical reaction, however is not changed by a chemical reaction

• don’t get used up by chemical reactions, so can catalyze many reactions

__________________ increases the chance a collision between a substrate and active site will occur - catalysis

Higher kinetic energy

Extracellular enzymes

Enzymes exported from cells placed in vesicles and secreted by endocytosis to work externally

Intercellular enzymes

Non-secreted enzymes

Enzyme Specificity (& its benefits)

• each one catalyzes a specific reaction, or a specific group of reactions

• Because of enzyme specificity, organisms have to make large numbers of different organisms

BENEFITS:

• allows organisms to control metabolism

  • a cell produces a particular enzyme which causes a specific reaction

• the rate of the reaction can be controlled by producing more or less of an enzyme

• mechanisms do exist for stopping certain enzymes from working if they aren’t needed

Anabolic Reaction

• Small molecules > large molecules

• require energy

• e.g. photosynthesis

  • uses Co2, H2O and more to make larger molecules with energy from light

• macromolecules can be formed from monomers using ATP

• this is a condensation reaction

EXAMPLES:

• protein synthesis (translation) by ribosomes

• DNA synthesis (replication)

• Synthesis of complex carbohydrates like starch, cellulose, and glycogen

Catabolic reaction

• Large > Small

• release energy

  • that energy can be captured by coupling the catabolic reaction to the synthesis of ATP

EXAMPLES:

• digestion of food

• cell respiration

• digestion of complex carbon compounds

  • e.g. decomposers

Enzyme-substrate binding

• For catalysis, substrates must bind to the active site

  • the shape and chemical properties of the substrate and active site match

  • therefore a certain substrate can bond while others cannot

• active sites vary in size depending on substrate size

• usually a few amino acids at the active site are essential to create chemical conditions that change substrates enough to convert them to products

• the amino acids which form the active site are brought together by folding polypeptides

Steps to enzyme-substrate reaction

1. Substrate approaches active site

• until it nears the enzyme, its direction of movement is random

• once it’s close enough, the chemical properties of the enzyme surface attract the substrate towards the active site

2. Substrate binds to the active site

• 3D molecular shapes change (induced-fit binding)

3. If there is a second substrate, it approaches and binds to another part of the active site

• the substrate and active site cause changes in each other to allow for binding

• changes to substrates make it easier for bonds to break and new bonds to form

4. Products (once converted) detach, active site returns to its original state and the catalytic cycle repeats

Molecular motion in liquids

• molecules are packed closely together but are free to move

• direction of each molecule changes rapidly and at random

• if a liquid has substrates and enzymes, they can come together

• this rate will be higher if:

  • there is a high concentration of substrates or enzymes

  • or if temperature increases, leading to faster molecular motion

Effects of temperature on rate of enzyme activity

• when liquid is heated, particles gain kinetic energy, and therefore move more quickly, and the chance of a substrate colliding with an active site increases, increasing enzyme activity

• when liquid is too heated, bonds vibrate more and are more prone to breakage, so the structure of enzymes change and they are often denatured

Effects of pH on enzyme activity

• enzymes are sensitive to chemical environment

• most enzymes have an optimum pH which is where they have the most activity

• if pH increases or decreases, bonds between amino acids are altered, changing the structure so that they can’t bind with substrates

• beyond a certain pH, they are irreversibly denatured

Effects of substrate on enzyme activity

• when the substrate concentration increases, substrate-active site collisions occur more frequently and enzyme catalysis reaction rates will increase

• however, as substrate conversation increases, more and more active sites are occupied, meaning that a greater proportion of substrate-active sites are blocked

  • the increases in enzyme catalase reaction rates get smaller

    

Independent, dependent and control variables (enzyme activity)

INDEPENDENT:

• temperature

• substrate concentration

• enzyme concentration

• pH

DEPENDENT:

• reaction rate

• enzyme activity

CONTROL:

• all factors that affect enzyme activity other than independent

Reaction Rate:

speed at which substrates are converted to products

How to measure enzyme activity

• Allow reaction to occur for a fixed time, and then count the amount of substrate that is used up, and the amount of product that is formed

• start with a known amount of substrate and allow the reaction to continue until all substrates are converted to products

Exothermic reaction

a reaction without an enzyme that has a net RELEASE of energy