PREGUNTAS DE TIPO USLME/ IFOM /OSMOSIS Flashcards
GLYCOLYIS (Glucólisis/glicólisis)
An investigator is studying two human isoenzymes capable of phosphorylating glucose, the first step of glycolysis. This step results in a negatively charged product, glucose-6-phosphate, which is then trapped on the intracellular side of the phospholipid bilayer. One form of this isoenzyme is only found in hepatocytes and beta-cells of the pancreas and is induced by insulin ; the other, tissue isoform, is found elsewhere throughout the body, is not inducible by insulin and is feedback inhibited by glucose-6-phosphate. The investigator is trying to characterize these isoforms on the basis of their kinematics. Which of the following observations correctly describes these two isoforms? (image)
Major takeaway
Hexokinase, the enzyme which mediates the**first step **in glycolysis , has two major isoforms: Glucokinase, the hepatic /pancreatic isoform, has a low affinity (high Km) for glucose and and a high maximum velocity (Vmax). Hexokinase, the tissue isoform, as a high affinity (low Km) and a low maximum velocity.
Main explanation
The correct answer is C. This vignette describes the biochemistry of hexokinase and glucokinase, kinases that phosphorylate hexoses, like glucose. Glucokinase is the isoform of hexokinase found in the liver and beta cells of the pancreas. These two enzymes have opposite kinetics that serve their biochemical roles within our metabolism . Hexokinase, the tissue isoform, has a high affinity (low Km) for glucose and a low maximum velocity (Vmax). These kinetics allow the various tissues like the brain and muscle to readily sequester whatever glucose is around while leaving as much for other tissues, since its easily saturated (low Vmax). Glucokinase, on the other hand, has a low affinity(high Km) but a high high Vmax, befitting its role in the liver as it leaves glucose to be preferentially taken up by other (high affinity/low Km hexokinase) tissues; However, when glucose levels are high, glucokinase won’t be saturated, since it has a high Vmax and isn’t feedback inhibited by its products, and excess glucose can be taken up by the liver and diverted to other biochemical pathways like glycogenesis, or lipogenesis via products like Acetyl-CoA. In the pancreas , glucokinase’s kinetics allow it function as a glucose sensor in the regulation of insulin production.
Citric acid cycle (Ciclo de Krebs)
The citric acid cycle takes place in the …….. of the mitochondria.
Matrix
Citric acid cycle (Ciclo de Krebs)
The enzyme in the citric acid cycle that converts alpha-ketoglutarate to succinyl-CoA is …………………..
alpha-ketoglutarate dehydrogenase.
Citric acid cycle (Ciclo de Krebs)
The major type of regulation of the citric acid cycle is ………. regulation
Allosteric
Citric acid cycle (Ciclo de Krebs)
High cellular concentrations of acetyl-CoA, ATP, or ….. would inhibit the entry of pyruvate into the citric acid cycle
.
NADH
Citric acid cycle (Ciclo de Krebs)
(#) molecules of acetyl CoA are produced from one molecule of glucose for participation in the citric acid cycle.
2
Citric acid cycle (Ciclo de Krebs)
In the third step of the citric acid cycle, the isomer of citrate, …….. , is oxidized to produce alpha-ketoglutarate, carbon dioxide, and two electrons, which reduce NAD+ to NADH.
isocitrate
Citric acid cycle (Ciclo de Krebs)
The last step in the citric acid cycle regenerates ……… by oxidizing malate, and another molecule of NADH is produced.
oxaloacetate
Citric acid cycle (Ciclo de Krebs)
The enzyme in the citric acid cycle that converts isocitrate to alpha-ketoglutarate is …….
isocitrate dehydrogenase.
Citric acid cycle (Ciclo de Krebs)
The enzyme to catalyze acetyl CoA and oxaloacetate to form citrate in the first step of the citric acid cycle is called
citrate synthase.
Citric acid cycle (Ciclo de Krebs)
Coenzyme A and pyruvate are essential reactants for the formation of ……
acetyl CoA to enter the citric acid cycle.
Citric acid cycle (Ciclo de Krebs)
Each turn of the ……. yields one GTP, three NADH molecules, and one FADH2 molecule, to be used in further steps of cellular respiration.
citric acid cycle
