Enzymes + proteins

Question 1

3 principles of enzymatic catalysis

Answer 1

enzymes are catalysts
cannot alter equilibrium of a reaction
accelerate chemical reactions

Question 2

What is Km?

Answer 2

substrate concentration at which the velocity is half the maximum velocity
measure for stability of enzyme-substrate complex
large Km = (~10-3 M) = low substrate-enzyme affinity
small Km = (~10-5 M) = high substrate-enzyme affinity

Question 3

What are co-enzymes?

Answer 3

helper molecules that play an essential role in enzyme-catalysed reactions

co-enzyme as co-substrate = co-enzyme reversibly bound to enzyme
co-enzyme as prosthetic group = co-enzyme covalently linked with enzyme

Question 4

Types of co-enzymes

Answer 4

energy-transferring coenzymes (eg. ATP transfers phosphate groups)

electron and oxygen transferring coenzymes (eg. NADH transfers electrons)

group-transferring coenzymes (eg. lipoic acid transfers acyl groups)

Question 5

What is irreversible inhibition?

Answer 5

when enzyme reacts with irreversible inhibitor, enzyme will die
eg. penicillin

Question 6

What is reversible inhibition?

Answer 6

competitive or non-competitive inhibition

Question 7

Describe competitive inhibition

Answer 7

enzyme can bind to substrate or competitive inhibitor (but not both at same time)
can overcome competitive inhibition by increasing substrate concentration
eg. sulfonamides

Question 8

Describe non-competitive inhibition

Answer 8

enzyme or enzyme-substrate complex can bind to non-competitive inhibitor (bind to different parts of enzyme)
cannot be overcome by increasing substrate concentration
enzyme usually only partially inhibited
eg. calcium channel blockers

Question 9

What is the definition of enzyme activity?

Answer 9

determined by amount of substrate converted per unit time under defined conditions (25 degrees celsius, optimal pH, substrate saturation)

Question 10

What are proteins?

Answer 10

polypeptides

Question 11

How many amino acids can be found in proteins?

Answer 11

20

Question 12

Describe the structure of an amino acid

Answer 12

basic amino group (NH3+)
hydrogen atom
side chain (variable)
acidic carboxyl group (COO-)
Central carbon atom

Question 13

Describe translation (synthesis of proteins)

Answer 13

Initiation = ribosome and initiation factors assemble at start codon (AUG)
Elongation = amino acids on tRNA bind to mRNA to deliver next amino acid to be joined to the growing polypeptide chain
Termination = any of stop codon (UAA, UAG or UGA) occur on mRNA

Question 14

What is a codon?

Answer 14

sequence of 3 amino acids that code for a specific amino acid

Question 15

What does degenerate mean?

Answer 15

more than one codon can specify a specific amino acid (some exceptions)

Question 16

Describe a peptide bond

Answer 16

chemical bond between 2 amino acids when a carboxyl group of one amino acid reacts with the amino acid group of another amino acid and a water molecule is released
peptide bond = planar, rigid + cannot rotate (double bond character)

Question 17

What is the primary structure of a protein?

Answer 17

sequence of amino acids

always written from amino end/N-terminus –> carboxyl end/C-terminus

Question 18

What is the secondary structure of a protein?

Answer 18

certain common/repeating structures
alpha helix
beta pleated sheet

Question 19

Describe alpha helix structure

Answer 19

rod-like structure
polypeptide chain coiled tightly
stabilised by hydrogen bonds
right-handed (clockwise) coiled

Question 20

Describe beta pleated sheet structure

Answer 20

zigzag structure
stabilised by hydrogen bonds
polypeptide chain runs in same direction = parallel beta pleated sheet
polypeptide chain runs in opposite direction = antiparallel beta pleated sheet

Question 21

What is the tertiary structure of a protein?

Answer 21

overall folding of backbone (3D structure)

stabilised by: ionic bonds, hydrogen bonds, disulfide bridges, hydrophobic interactions

Question 22

What is the quaternary structure of a protein?

Answer 22

only if more than one polypeptide chain
identical and/or different polypeptide chains = subunits
prosthetic groups may be present (non amino acid group part of conjugated proteins)

Question 23

What are the 4 protein types?

Answer 23

globular = compact spheres (haemoglobin, albumin)
fibrous = filamentous molecule (collagen, keratin)
soluble = dissolve in water (haemoglobin, immunoglobulins)
membrane = associated with membranes (glucose transporter)

Question 24

What is serum protein electrophoresis?

Answer 24

examines specific proteins in blood
serum proteins separated based on size and charge
separated into 5 major fractions (albumin, alpha 1, alpha 2, beta 1, beta 2, gamma)
diseases can be indicated by changes in serum proteins

Question 25

What is KI?

Answer 25

inhibitor constant
indicator for how potent an inhibitor is
concentration required to produce half the maximum inhibition