Lipitor, also known as atorvastatin, is a medication used to treat high cholesterol levels. It belongs to a class of drugs called statins, which work by inhibiting the HMG-CoA reductase enzyme, thereby reducing the production of cholesterol in the body [1].

Lipitor directly affects the following proteins:

1. HMG-CoA Reductase: Lipitor inhibits HMG-CoA reductase, which is a key enzyme in the biosynthetic pathway for cholesterol. By inhibiting this enzyme, Lipitor reduces the production of cholesterol in the body, leading to a decrease in low-density lipoprotein (LDL), or "bad" cholesterol, and an increase in high-density lipoprotein (HDL), or "good" cholesterol [1][2].

2. LDL Receptors: Lipitor also increases the number of LDL receptors on the surface of liver cells. These receptors remove LDL from the bloodstream, further reducing the levels of LDL cholesterol in the body [2].

In summary, Lipitor directly affects HMG-CoA reductase and LDL receptors, thereby reducing the levels of LDL cholesterol and increasing the levels of HDL cholesterol in the body.

Sources:
[1] "Atorvastatin." MedlinePlus, U.S. National Library of Medicine, 11 Jan. 2022, medlineplus.gov/druginfo/meds/a601245.html.
[2] "Atorvastatin." DrugPatentWatch, drugpatentwatch.com/drug/atorvastatin.