Week 2 Enzymes Flashcards
Recap: Enzymes are biological catalysts
What does a catalyst do?
Increases the rate of a reaction, without being consumed by the reaction.
How does it do it?
By lowering the activation energy.
Burning” glucose for fuel: Cellular respiration
Enzymes in the body
Lots of digestive enzymes break down food.
Releases nutrients that make up our bodies and energy we need to exercise.
Enzymes catalyse just about every reaction that happens in cells.
Enzymes in industry and agriculture
Rennet is a mix of enzymes used in cheesemaking: Pepsin breaks down proteins, Chymosin curdles milk, Lipases break down fats
Invertase breaks down hard sugars into soft sugars – used to make soft-centred chocolates
Phytases added to animal feed make minerals more bioavailable.
What are enzymes and why are they necessary?
Enzymes are usually proteins and their activity depends on the integrity of their three-dimensional structure.
A denatured or dissociated enzyme usually won’t work.
Some enzymes require nothing other than their amino acids to function, but many enzymes require cofactors and/or coenzymes.
Uncatalysed reactions in biology tend to be slow—most biological molecules are quite stable in the neutral-pH, mild-temperature, aqueous environment inside cells.
Many biological reactions are chemically unfavourable so need enzymes.
Enzymes are usually much better (specific and efficient) than catalysts in chemistry.
If human enzymes don’t work properly it can lead to disease.
Ribozymes: RNA enzymes
RNA molecules can have complex tertiary structures and so can be catalytically active.
Example: Hammerhead ribozymes promote site- specific cleavage of RNA molecules.
The RNA world hypothesis: very early in the evolutionary history of life RNA acted as both a storage molecule for genetic information and a catalyst for reactions.
Not all enzymes exclusively use amino acid functional groups
Amino acids, with their various R-groups, can take part in lots of chemical reactions.
But some enzymes need extra molecules to function, these are often coenzymes and cofactors.
Cofactors: metal ions
Cofactors are extra molecules or ions that are needed for the activity of some enzymes.
Very commonly these are metal ions like Fe2+ or Zn2+.
They are often found in enzymes that catalyse redox reactions (recall that Oxidation is loss of electrons, Reduction is gain of electrons). They also affect bonding, and can have a structural role.
More complex organic cofactors are called coenzymes.
The Zn2+ cofactor in carboxypeptidase A stabilises negatively charged groups in the amino acids surrounding it.
Many metal ions are used as enzyme cofactors
Coenzymes
Coenzymes are complex organic molecules needed for enzyme function.
They are often derived from vitamins.
They often act as transient carriers for functional groups.
Nicotinamide Adenine Dinucleotide (NAD) is very commonly used in redox reactions: it carries a hydride ion (:H-) and exists in reduced (NADH) and oxidised (NAD+) forms.
Many types of coenzymes exist
Some enzymes need both a coenzyme and a metal cofactor
Example: Alcohol dehydrogenase
Prosthetic groups
Prosthetic groups (like haem) bind tightly or covalently to a protein, and act as structural elements. What we commonly call coenzymes bind more loosely. There are some inconsistencies in usage of the terms.
Cofactors-coenzymes. - prosthetic groups, cosubstrates
- metal ions.
Holoenzymes and apoenzymes
Apoenzyme. Coezyme =. Substrate
(Protein portion) + Cofactor Holoenzyme
Inactive (Nonprotein portion). Whole enzyme
Activator Active
Classifying enzymes
Many enzymes’ names end in –ase. E.g. urease breaks down urea.
Others have names derived from their roles. E.g. pepsin comes from the Greek word for digestion.
Some have several names or ambiguous names, so an international agreement has been made to sort enzymes into classes and sub-classes.