Question

2. Lactose is a disaccharide found in milk. Many adults throughout the world get sick from drinking milk because they cannot digest lactose. Lactose intolerance varies markedly among various human populations. For example, only about 3% of people of Danish descent are lactose intolerant, compared with 97% of people of Thai descent. When someone who is lactose intolerant ingests milk, the lactose accumulates in the lumen of the small intestine because there is no mechanism for uptake of the disaccharide. This causes abdominal distension, cramping, and watery diarrhea. Adults who can drink milk can do so because of the enzyme lactase which is located on the outer surface of epithelial cells lining the small intestine. Lactase hydrolyzes lactose into its two component monosaccharides, glucose and galactose. Both glucose and galactose can cross the epithelial cells, and therefore do not cause illness.

a) You decide to study lactase further, and see whether it can also cleave other common disaccharides, such as maltose. (Maltose = glucose + glucose.) You find that maltose is NOT cleaved by lactase, and furthermore, maltose appears to have some kind of inhibitory effect on lactase's ability to cleave lactose. Is maltose a more likely candidate for competitive or noncompetitive inhibition of lactase? Why do you think so?

b) In order to confirm your hypothesis in part (c), you quantitatively study the kinetics of lactase with lactose alone, and in the presence of both lactose and maltose. You measure the initial velocity of the reaction (rate at which lactose is cleaved) at varying concentrations of substrate. The data are given below.

[Lactose] moles/L	Velocity (mol/min)
	lactose only	with maltose
0.3 x 10-5	10.4	4.1
0.5 x 10-5	14.5	6.4
1.0 x 10-5	22.5	11.3
3.0 x 10-5	33.8	22.6
9.0 x 10-5	40.5	33

Do these data support the model that maltose is a competitive or noncompetitive inhibitor? (You may need to graph 1/V vs. 1/[S] for lactase in the presence and absence of maltose).

Answer 1

Answer a)

1. Both the disaccharides namely maltose and lactose acts as substrate for enzymatic activity.
2. Hence, both will be converted into monosaccharide. But, lactase cleaves lactose into glucose and galactose however maltase cleaves maltose into two molecules of glucose.
3. So, both the substrates are having glucose although lactose has a galactose which resembles glucose structure. So, it can be stated that lactose can resemble maltose at structural level. The molecular formula for both of them is C₁₂H₂₂O_11.
4. This indicates that they are structural isomers in which the molecular formula is same however the structural formula differ. Due to this, both can have affinity towards active site of the enzyme Lactase.
5. However, enzyme activity is to specifically cleave the bonds. In lactose it is the beta (1-4) glycosidic bond and in maltose it is alpha (1-4) linkages. So, this ensures specificity towards substrate digestion. But, it does not alter binding affinity of the enzyme towards both substrate and hence it can be considered as COMPETITIVE INHIBITION.