Question

Protein kinases can A) Inactivate enzymes by dephosphorylating them B) Activate enzymes by phosphorylating them C) Cleave proteins after Tyr, Ser, or Thr residues D) Contain a catalytic triad to cut peptide bonds Peanut oil contains a high percentage of monounsaturated fatty acids whereas vegetable oil contains a high percentage of polyunsaturated fatty acids. During a cold spell, the peanut oil freezes whereas the vegetable oil remains liquid. Explain why. Predict which one of the following organisms will have the highest percentage of unsaturated fatty acid chains in their membranes. Explain your answer. A) Antarctic fish B) Desert iguana C) Hunan being D) Polar Bear E) Thermophilic bacterium For the following problems (13 - 15) there may be more than one correct answer: 13. Chemical features of all membrane lipids include: A) a polar portion B) a glycerol backbone C) sugar constituents D) a hydrophobic domain E) a phosphate 14. Phospholipids in a fluid, pure phospholipid bilayer frequently undergo: A) lateral diffusion B) rotational movement around their long axis C) spontaneous flip flop from one leaflet to the other D) movement from the membrane to the aqueous phase E) swapping positions with neighboring lipids 15. A membrane-embedded segment of an integral membrane protein A) is typically an alpha helical segment long enough, about 20 or more amino acids, to span the phospholipid bilayer B) typically has polar C=O and NH groups in contact with the fatty acyl chains of phospholipids C) often contains hydrophobic amino acids, flanked by positively charged residues that interact with phospholipid head groups D) is always alpha helical E) can often be predicted by analyzing hydropathy plots

Answer 1

Ans. #10. Protein kinases are enzymes that phosphorylate (add a phosphate group) other proteins and modulate their activities.

Correct option. B. Activate enzymes by phosphorylating them. Kinases are characterized by their ability to phosphorylate enzymes or other proteins.

Option A. Incorrect. Kinases phosphorylate, but NOT dephosphorylate, their substrate.

Option C and D. Incorrect. Kinases don’t cleave proteins and don’t have catalytic triad either.

#11. Fatty acids with unsaturated bonds don’t pack uniformly and tightly because of kinks (bent-form due to cis-double bonds) and greater kinetic movement. Poor packing (less densely) of unsaturated molecule also provides them greater degree of freedom. Therefore, they require relatively low temperature to attain the solid phase in which the position of all the molecules is relatively fixed in space.

Saturated fatty acids are roughly linear molecules and pack well. They have relatively lesser kinetic energy and degree of freedom in space. So, they attain the solid phase at a temperature relatively higher than the freezing point of a monounsaturated fatty acid of same chain length.

#12. A cell residing in colder environment may have larger number of cholesterol and unsaturated fatty acid chains to maintain the membrane fluidity- similar to fluidity of another cell living in hotter environment.

Desert iguana, Human beings and thermophilic bacteria live in relatively warm climates. So, unsaturated fatty acid content in them in lesser than Antarctic fish and polar bear which thrive much colder environment.

Polar bear has body hair and can also maintain its body temperature to relatively stable level through thermoregulation. So, under normal conditions, the body temperature of a polar bear is maintained at around 37⁰C.

Fish don’t have mechanism of thermoregulation. The temperature of their body is nearly equal to that of water. So, Antarctic fish is directly exposed to very low temperature in water. To retain membrane fluidity of its cell, the fish must have relatively highest content of unsaturated fatty acids in its membrane.

Correct option. A. Antarctic fish

#13. The membrane lipid includes phospholipids, glycolipids and cholesterol.

The correct options may include-

Option B. a glycerol backbone. All lipids have a glycerol backbone.

Option A. a polar group. Almost all membrane lipids have polar groups like –OH groups.

Option C, D and E are not absolutely fit for “all” membrane lipids. For example, cholesterol, a membrane lipid, does not contain a sugar moiety or phosphate group.

The term “domain” species “An independently stable segment of protein that may undergo conformational change as a single entity within the protein is called a domain.” Lipids have a hydrophobic region but don’t have a hydrophobic domain.   

#14. The phospholipid molecules moves laterally, rotate, shake their hydrophobic tails, or bobble in each leaflet independently or even flip from one layer to another and are responsible for the fluidity of the plasma membrane. Unlike other modes of displacement in phospholipids, flipping is least frequent and mediated by flippase enzyme. The sterols (cholesterol) in plasma membrane randomly move through the hydrophobic core and flip between the bilayers and provide additional fluidity to the membrane. The presence of proteins on the outer surface of the leaflet gives a ‘mosaic’ appearance of the plasma membrane.

Correct options are- A, B, E

Options C and D are incorrect. Flipping is NOT spontaneous, it’s mediated by flippase enzyme. Phospholipids DON’T move from membrane to aqueous phase, and vice-versa.

#15. Correct options- A, E. The membrane- embedded segment or transmembrane domain of an integral protein has relative abundance of hydrophobic residues, which can be predicted by hydropathy plot.

Option B. Incorrect. The C- and N-terminal may be embedded in protein towards cytoplasm or extracellular matrix. Or, they may interact with other proteins or modulators. However, they DON’T contact with fatty acid chains of phospholipids.

Option C. Presence of positively charged residues in transmembrane domain in NOT feasible for its interaction with polar heads a t surface.

Option D. It may also contain beta-sheets, too.