Test: Enzyme Kinetics - 1 - MCAT MCQ

In most electrophoresis experiments, we attempt to separate out one component from the others. Because we are attempting to isolate protein A only, a pH that causes protein A to be negative while proteins B and C are neutral or positive will be best. pH 5.5 accomplishes this goal; proteins B and C will be positively charged. A pH of 4.5, choice (C), would make protein A neutral, and it would thus not migrate across the gel. Any neutral impurities would also remain in the well with protein A, making this pH not the best choice.

From the given choices, all of them are involved in cell movement with the exception of choice (D). Centrioles are composed of microtubules, but are involved in mitosis, not cell motility.

For a ligand present in low quantities to have a strong action, we expect it to initiate a second messenger cascade system. Second messenger systems amplify signals because enzymes can catalyze a reaction more than once while they are active, and often activate other enzymes. Both enzyme-linked receptors and G protein-coupled receptors use second messenger systems, while ion channels do not.

Antibodies are specific to a single antigen. Each B-cell produces a single type of antibody with a constant region that is specific to the host and a variable region that is specific to an antigen.

All trimeric G proteins have α, β, and γ subunits—choices (A), (B), and (C), respectively. G_s, G_i, and G_q are subtypes of the G_α subunit of the trimeric G protein and differ depending on the G protein-coupled receptor’s function.

The overall pI of a protein is determined by the relative number of acidic and basic amino acids. The basic amino acids arginine, lysine, and histidine, and the acidic amino acids aspartic acid and glutamic acid will therefore contribute most significantly. Glycine’s side chain is a hydrogen atom, so it will have the least contribution of all the amino acids.

UV spectroscopy is best used with conjugated systems of double bonds. While the double bond in the peptide bond does display resonance, this is not adequate for UV absorption. However, aromatic systems are conjugated, and phenylalanine, tyrosine, and tryptophan all contain aromatic ring structures.

The selective cleavage of proteins by digestive enzymes allows fragments of different lengths with known amino acid endpoints to be created. By cleaving the protein with several different enzymes, a basic outline of the amino acid sequence can be created.

Sodium dodecyl sulfate is a detergent and will digest proteins to form micelles with uniform negative charges. Because the protein is sequestered within the micelle, other factors such as charge of the protein and shape have minimal roles during separation. In essence, the protein micelles can be modeled as being spheres, dependent only on size.

The most prevalent extracellular proteins are keratin, elastin, and collagen. Tubulin and actin are the primary cytoskeletal proteins, while myosin is a motor protein.