Practice Questions

With 20 different amino acids as monomers, the number of possible sequences and lengths for a polypeptide is astronomically large. Furthermore, the diverse chemical properties of the 20 R-groups (charged, polar, non-polar, etc.) enable a protein to fold into an immense variety of complex 3D shapes.

Bile salts are amphipathic cholesterol derivatives secreted from the liver. Their hydrophobic side associates with lipid droplets, and their hydrophilic side faces the aqueous intestinal fluid. This coating breaks large globules into smaller ones (micelles), vastly increasing the surface area for lipase action.

Many enzymes require metal ions (like Mg²⁺, Ca²⁺) as activators or cofactors. EDTA chelates (binds tightly to) these divalent cations, making them unavailable to the enzyme. This effectively inhibits the enzyme's activity. Adding back an excess of the metal ion reverses the inhibition.

The 2'-OH group in the ribose sugar of RNA is reactive. It can act as a nucleophile and attack the adjacent phosphodiester bond under alkaline conditions, leading to the self-hydrolysis (cleavage) of the RNA strand. DNA, lacking this 2'-OH, is far more chemically stable.

Metabolism is the sum of all cellular reactions. Catabolism is the breakdown of complex molecules into simpler ones, releasing energy. Anabolism is the synthesis of complex molecules from simpler ones, consuming energy. Their regulation is central to life.

When [S] is saturating (>> Km), all enzyme active sites are occupied, and the reaction rate is solely a function of how fast the enzyme can process substrate (Vmax). Vmax is proportional to the total enzyme concentration, so doubling the enzyme doubles V0 under these conditions.

tRNA acts as an adaptor, carrying a specific amino acid at its 3' end and recognizing a specific three-nucleotide codon on the mRNA through its complementary anticodon loop. This bridges the genetic code and the amino acid sequence of a protein.

On a double-reciprocal plot, a non-competitive inhibitor produces a line that intersects the control line at the x-axis (Km is unchanged), but has a steeper slope and a higher y-intercept (Vmax is decreased). It reduces the number of functional enzyme molecules.

When glucose forms a ring, carbon 1 becomes an asymmetric carbon (the anomeric carbon). In the α-anomer, the -OH on C1 is below the plane of the ring (trans to the CH2OH at C5). In the β-anomer, the -OH is above the plane of the ring (cis to the CH2OH).

Saponification is the base-catalyzed hydrolysis of the ester bonds in a fat or oil. This reaction cleaves the triglyceride, producing glycerol and the salts of the fatty acids (soaps). Lipases perform an analogous enzymatic hydrolysis.