C1.1 Enzymes and metabolism

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Enzymes

Organic catalysts, speed up cell reactions which maintain life functions

Metabolism

All the chemical reactions occurring in one organism, can be independent or interact

Each reaction controlled by an enzyme

Anabolic reactions

Build complex organic molecules (condensation reactions)

Require energy

E.g photosynthesis, protein synthesis, glycogen formation

Catabolic reactions

Break down complex molecules into monomers

Release energy

E.g digestion, oxidation of substrates in respiration

Types of energy

• Kinetic

• Chemical

• Thermal (measured in kcals)

• Electric current

• Mechanical

• Sound

• Potential

• Radioactivity

ATP

Energy current of the cell

Protein enzymes

Long chains of amino acids bent into 3D globular shape

Active site

Composed of a few amino acids, matching the substrate

Interaction between amino acids give it necessary properties to carry out catalysis

If shape is changed, enzyme is denatured and no longer works

Induced fit model

Developed from lock-key model

Many enzymes undergo changes in shape when combined with substrates, causing stress on chemical bonds of substrate which become destabilised and increase reaction rate

Activation energy

Energy necessary to destabilise bonds of reactant so reaction can proceed

Substrate needs certain of amount of kinetic energy when entering active site to provide it

Ways to increase reaction rate

Increase energy of reacting molecules and rate of collisions, usually by adding heat

Lower activation energy to break bonds more easily, usually with enzymes (cannot force a reaction, only makes it more likely)

Reversible reactions

Some require different enzymes to catalyse the reverse reaction

Substrate-active site collisions

In order to react, this needs to happen so need energy for it

Often the substrate or enzyme is anchored to a membrane to make it more efficient

Mechanism of enzyme action

Surface of substrate makes contact with enzyme’s action site

Change shape to provide fit

Temporary complex called the enzyme-substrate complex forms

Activation energy is lowered, substrate altered by rearrangement of existing atoms

Transformed substrate is released

Unchanged enzyme is then free to repeat

Factors affecting enzyme-catalysed reactions

Temperature

pH

Substrate concentration

Effect of temperature

Increases molecular motion, and thus reaction rate

But enzymes have an upper limit- too high will denature the enzyme

Denaturation can be temporary (intra-molecular bonds will reestablish) but if it’s too high, it is permanent

Effect of pH

Amino acids in enzymes have areas with pos./neg. charge, the substrate must match the charged amino acids

If solution is acidic, H+ conc rises and bonds with negative charges- prevent matching

If solution is too basic, OH- conc rises and bonds with positive charges- prevent matching

Enzymes have different optimums, generally neutral is best but there are exceptions (pepsin active in acidic stomach)

Effect of substrate concentration

Collision theory→ increase here means there are molecules to react and collide, increasing reaction rate

But there is a limit as enzymes have max rate at which they work and only one active site- if they are all occupied then increasing substrate concentration won’t do anything

Measuring enzyme-catalysed reactions

Measure rate at which substrate is used or product is produced