July 2009 Molecule of the Month by David Goodsell
Keywords: beta-amyloid metabolic process, aspartic-type endopeptidase activity, amyloid precursor protein secretases, Alzheimer disease, amyloid beta-protein
Many of our proteins need to be shaped, folded and trimmed after they are made, to coax them into their proper functional form. A variety of specialized chaperones and proteases perform these tasks. Occasionally, however, these chaperones and proteases make mistakes that can have life-threatening consequences.
Beta-secretase, also known as BACE1 or memapsin-2, is a protease that makes specific cuts during the maturation of some protein chains. It is normally found in the endoplasmic reticulum and the Golgi, where it trims a few proteins that are particularly important in neural function. These include neuregulin, a protein that helps control the formation of myelin sheaths around nerve axons, and voltage-gated sodium channels, which are important for the transmission of nerve signals.
Beta-secretase is very similar to the digestive protease pepsin. Like pepsin, it has a deep active site cleft that grips protein chains, and a pair of aspartate amino acids that make the cut. Beta-secretase is different, however, in that it has a long tail that tethers the enzyme to the membrane surface. This tail localizes the enzyme in the proper place, so that it doesn't float freely through the cell and wreak havoc on other proteins. Two structures in the PDB show two portions of the enzyme: 1sgz (at the top) is the catalytic domain that cleaves proteins, and 1py1 contains a small portion of the chain on the other side of the membrane (at the bottom) which is important for regulating the activity of the enzyme and directing it towards the proper site in the cell.
Performing a Job Too Well
Unfortunately, beta-secretase has a dark side as well. It makes a specific cut in the amyloid-beta precursor protein, breaking the protein chain and releasing a large portion of it. Then, a second protease makes another cut, and a tiny but dangerous peptide is formed. In small amounts, this peptide is important for normal synaptic function. However, if beta-secretase is overactive, the peptide builds up and can aggregate into tangled fibers. If these fibers form in nerve cells, they block nerve transmission and may lead to Alzheimer's disease. This connection to Alzheimer's disease has made beta-secretase into one of the most studied enzymes in the fight against the disease.