What is the role of NADH in Cell Respiration?
NADH is simply an electron carrier present in the cell and important for cell respiration, particularly aerobic cell respiration. During glycolysis and the Krebs cycle, NAD+ is reduced to NADH through the addition of electrons removed in the reactions, and these electrons (which are too unstable to move around freely by themselves) are "taxied" to the inner mitochondrial membrane, where they are useful in the electron transport chain and chemiosmosis in order to produce a significant amount of ATP.
What is the rate-limiting step in cholesterol biosynthesis and why is it important?
The rate-limiting step in cholesterol biosynthesis is the conversion of mevalonate from HMG CoA via the enzyme HMG CoA reductase. It's important to know this because HMG CoA reductase is the enzyme targeted by the cholesterol-reducing medications known as statins
Can you explain to me why a protein must be in tertiary structure to be considered functional?
Sure! So before we get to that, let's recap what the different structures of a protein, or polypeptide, are. First, we have primary structure. This is just a linear sequence of amino acids in a chain, each bound to its neighbors via a peptide bond. Hence the name "polypeptide" that is synonymous with protein. However, for it to be functional, a protein must be in some sort of three-dimensional arrangement. This is reached when the protein is in the tertiary structure. Think of secondary structure as the "intermediate" between primary and tertiary. In this stage, the protein is beginning to fold a little bit in two ways: one is helical (called alpha helices) and the second is in a zig-zag formation (called beta-pleated sheets). These occur due to temporary hydrogen bonds between functional groups of nearby amino acids that interact with each other. So we're not linear anymore, but we're not completely and permanently three-dimensional just yet! The tertiary stage is finally reached when we have more durable covalent bonds form between amino acids within a polypeptide, mostly through strong di-sulfide bonds. The protein is now folded in a certain shape due to the bonds, and this confers a particular function on the protein. For example, an enzyme will be shaped in a way such that now it can fit with its substrate to catalyze a particular reaction! Lastly, for completion's sake, we also have quarternary structure, which is a level above tertiary. Not every protein needs to do this, but some proteins need to have more than one polypeptide chain bonding with each other in order to be functional. An example of this is hemoglobin, which requires four polypeptide chains in order to effectively function (carry oxygen in the blood).