Only enzymes that have allostericity/cooperativity will exhibit a sigmoidal (what I think you meant by sinusoidal) curve. An example of this is Hemoglobin. This isn't the case with carrier proteins for the most part. I would trust the question stem or passage to tell me something that would indicate non- MM enzyme kinetics but otherwise understand that the carrier protein has a receptor that can be saturated which is the reason why the curve levels off at Vmax.