.1. Which of the following best describes the activity level and phosphorylation state of the listed hepatic enzymes in an individual who consumed a carbohydrate-rich meal about an hour ago? PFK-1 = phosphofructokinase1; PFK-2 = phosphofructokinase-2; P = phosphorylated.
Correct answer = C. Immediately following a meal, blood glucose levels and hepatic uptake of glucose increase. The glucose is phosphorylated to glucose 6-phosphate and used in glycolysis. In response to the rise in blood glucose, the insulin/glucagon ratio increases. As a result, the kinase domain of PFK-2 is dephosphorylated and active. Its product, fructose 2,6-bisphosphate, allosterically activates PFK-1. (PFK-1 is not covalently regulated.) Active PFK-1 produces fructose 1,6-bisphosphate that is a feedforward activator of pyruvate kinase. Hepatic pyruvate kinase is covalently regulated, and the rise in insulin favors dephosphorylation and activation.
.2. Which of the following statements is true for anabolic pathways only?
A. Their irreversible (nonequilibrium) reactions are regulated.
B. They are called cycles if they regenerate an intermediate.
C. They are convergent and generate a few simple products.
D. They are synthetic and require energy.
E. They typically require oxidized coenzymes.
Correct answer = D. Anabolic processes are synthetic and energy requiring (endergonic). Statements A and B apply to both anabolic and catabolic processes, whereas C and E apply only to catabolic processes.
.3. Compared with the resting state, vigorously contracting skeletal muscle shows:
A. decreased AMP/ATP ratio.
B. decreased levels of fructose 2,6-bisphosphate.
C. decreased NADH/NAD+ ratio.
D. increased oxygen availability.
E. increased reduction of pyruvate to lactate.
Correct answer = E. Vigorously contracting skeletal muscle shows an increase in the reduction of pyruvate to lactate compared with resting muscle. The levels of reduced nicotinamide adenine dinucleotide (NADH) increase and exceed the oxidative capacity of the electron transport chain. Consequently, the levels of adenosine monophosphate (AMP) increase. The concentration of fructose 2,6-bisphosphate is not a key regulatory factor in skeletal muscle.
.4. Glucose uptake by:
A. brain cells is through energy-requiring (active) transport.
B. intestinal mucosal cells requires insulin.
C. liver cells is through facilitated diffusion involving a glucose transporter.
D. most cells is through simple diffusion up a concentration gradient.
Correct answer = C. Glucose uptake in the liver, brain, muscle, and adipose tissue is down a concentration gradient, and the diffusion is facilitated by tissue-specific glucose transporters (GLUT). In adipose and muscle tissues, insulin is required for glucose uptake. Moving glucose against a concentration gradient requires energy and is seen with the sodium-dependent glucose cotransporter 1 (SGLT1) of intestinal mucosal cells.
.5. Given that the Km of glucokinase for glucose is 10 mM, whereas that of hexokinase is 0.1 mM, which isozyme will more closely approach Vmax at the normal blood glucose concentration of 5 mM?
Correct answer = Hexokinase. Km (Michaelis constant) is that substrate concentration that gives one half Vmax (maximal velocity). When blood glucose concentration is 5 mM, hexokinase (Km = 0.1 mM) will be saturated, but glucokinase (Km = 10 mM) will not.