2.4.5 the effect of pH on enzyme activity

Question 1

what is pH

Answer 1

indicates if substance is alkaline (8-14), acidic (0-6) or neutral (7)

Question 2

which acids dissociate into protons & negatively charged ions
- include equation

Answer 2

hydrochloric acid
HCl –> H+ + Cl-
sulfuric acid
H2SO4 –> H+ + HSO4-

Question 3

which organic acids produce donors

Answer 3

• lactic acid dissociates into H+ & lactate
• pyruvic acid dissociates into H+ & pyruvate

Question 4

what is a buffer

Answer 4

something that resists changes in pH

Question 5

how do chemicals in the blood act as buffers

Answer 5

• donate or accept hydrogen ions
• some proteins (eg. haemoglobin) can also donate/accept protons & act as buffers

Question 6

how do you use buffer solutions in laboratory investigations

Answer 6

use buffer solutions to maintain desired pH for investigating enzyme action at different pH values or keep pH constant whilst investigating another factor

Question 7

how does changes in pH affect bonds within molecules

Answer 7

• hydrogen ions (proton) have positive charges so are attracted to negatively charged ions/molecules/parts of molecules –> h bonds & ionic forces between amino acids holds the tertiary structure of an enzyme molecule (esp. active site) in correct shape for substrate
• excess hydrogen ions interfere with these hydrogen bonds/ionic forces & active site of enzyme molecule changes –> if substrate doesn’t fit well, the rate of reaction is lowered (catalysing enzyme)
• increasing conc of hydrogen ions alters charges on active site of enzymes (more protons cluster around negatively charged groups) in active site –> interferes with binding of substrate

Question 8

how big is the range of pH enzymes need

Answer 8

narrow

Question 9

what effect does small change in pH have on enzymes

Answer 9

(either side of optimum) slows rate of reaction as shape of active sites disrupted

Question 10

what effect does it have on enzymes if optimum pH is restored from a small change

Answer 10

hydrogen bonds can reform & active sites shape is restored

Question 11

what effect does extreme changes in pH have on the enzyme

Answer 11

active site may be permanently changed & enzyme becomes denatured so cannot catalyse reaction

Question 12

what pH do enzymes have that work intracellularly

Answer 12

close to pH 7

Question 13

example of pH that extracellular enzymes work at

Answer 13

may have optimum pH different from 7
–> eg. digestion: foods taken into mouth & amylase digest starch to maltose - work best at pH 6.8

Question 14

explain what pH the protease enzyme, pepsin, (in stomach) works best at and why

Answer 14

• hydrochloric acid is secreted providing very low pH –> kill bacteria & other pathogens in food
• pepsin works best at very low pH (between 1-2) as digests large protein molecules into smaller peptide molecules

Question 15

explain what pH the protein-digesting enzymes (trypsin & enterokinase) work best at & why

Answer 15

• partly digested food moves into smaller intestine & salts (made in the liver neutralise it, raising pH to around 7.8
• optimal for the protein-digesting enzymes which catalyse further digestion of peptides to amino acids in the small intestine