carb metabolism

Question 1

Km

Answer 1

Thesubstrate concentration at which the reaction is half maximal. If a reaction has a low Km it suggests that the substrates have a strong affinity for the enzyme and the reaction will go at low substrate concentrations

Question 2

Vmax

Answer 2

maximum rate of reaction catalyzed by enzyme

Question 3

Describe the common features that make a particular step in a linked enzyme pathway a “key step”.

Answer 3

Key steps may include one where a molecule changes location (ie. entering cell), where body invests energy in transition from on molecular state to another to activate precursor, and/or any rate limiting steps

Question 4

Primary function of glycolysis

Answer 4

generation of energy and useful chemical intermediates from the breakdown of glucose

Question 5

key steps in glycolysis

Answer 5

Glucose enters cell through glucose transporters > glucose converted to glucose-6-phosphate and is trapped in cell b/c charge (requires ATP) > G-6-P transformed into fructose 1,6 bisphosphate by phospho-fructo-kinase (PFK- rate limiting) > final product is pyruvate

Question 6

Net result of glycolysis

Answer 6

net production of energy in the form of ATP and NADH which can be produced in the absence of oxygen and mitochondria Also, pyruvate, the final product, has a lot of stored energy

Question 7

compare the fate of pyruvate in aerobic vs anaerobic environment

Answer 7

aerobic: pyruvate enters TCA cycle and is oxidized into CO2 and water (or synthesized into fatty acids). Anaerobic: ie. hypoxia, intense exercise, or in RBC which lacks mitochondria- pyruvate will be converted to lactate and exported from the cell

Question 8

Primary function of TCA/electron transport system

Answer 8

produces energy from pyruvate

Question 9

key steps in TCA cycle

Answer 9

pyruvate > Acetyl CoA > oxidized to CO2 with release of energy stored in GTP, NADH and FADH2

Question 10

compare TCA cycle in energy surplus vs energy deficit

Answer 10

Surplus: Acetyl-CoA from glycolysis can enter the TCA cycle and then leave without being oxidized to be used in fatty acid synthesis. Deficit: acetyl CoA, can continue around the TCA cycle to release energy through the conversion of NAD and FAD to NADH and FADH2 with the subsequent generation of ATP.

Question 11

key steps in electron transport chain

Answer 11

NADH and FADH2 from TCA cycle enter the electron transport system on the inner membrane of the mitochondria in a process that couples oxygen consumption with ATP generation. Results in conversion of O2 to H2O and ADP to ATP (oxidative phosphorylation)

Question 12

primary function of gluconeogenesis

Answer 12

During fasting, the liver (and to lesser extent the kidney) produce glucose

Question 13

What are the substrates used for gluconeogenesis

Answer 13

lactate produced by glycolysis in muscle or red blood cells, amino acids derived from protein breakdown in muscle, or glycerol from triglyceride breakdown in adipose tissue.

Question 14

key steps in gluconeogenesis

Answer 14

pyruvate converted to oxaloacetate then phosphoenol pyruvate via enzymes Pyruvate Carboxylase and Phospho-Enol-Pyruvate Carboxy-Kinase (PEPCK) > fructose 1,6 bis phosphonate converted to fructose-6-P by fructose 1,6 bisphosphatase > glucose-6-phosphate converted to free glucose by glucose-6-phosphatase

Question 15

Name the organs that can produce/export glucose into the circulation

Answer 15

liver and kidney only b/c they can undergo gluconeogenesis

Question 16

when does glycolysis vs gluconeogenesis occur?

Answer 16

When glucose is abundant in a liver cell, glycolysis predominates. When glucose is in short supply, gluconeogenesis predominates

Question 17

What compound is an important node in glucose metabolism

Answer 17

glucose-6-phosphate: it can go down pathway of glycolysis to be oxidized for energy or go towards storage in process of glycogen synthesis

Question 18

Function of glyogen synthesis

Answer 18

Forms a rapidly available source of glucose for acute oxidative needs

Question 19

key steps in glycogen synthesis

Answer 19

glucose-6-P converted to glucose-1-P then to UDP-glucose (this is the committed step) > glycogen synthase adds UDP-glucose to growing glycogen molecule > glycogen phosphorylase can remove glucose from glycogen.

Question 20

When is glycogen synthesized vs broken down?

Answer 20

glycogen synthesis is stimulated when the liver/skeletal muscle is in positive energy balance, and glycogen breakdown is activated when the body is in negative energy balance.

Question 21

structure of glycogen

Answer 21

Highly branched polymer- this allows rapid release of many glucose molecules when needed. This also means that an enzyme is required to create branch points when building glycogen

Question 22

function of pentose phosphate pathway

Answer 22

when glucose is abundant it is used for generation of NADPH and for generation of 5-carbon sugars used for nucleotide synthesis

Question 23

Pentose phosphate pathway key steps

Answer 23

oxidation of glucose-6-phosphate to 6-phosphogluconolactone by Glucose-6-Phosphate Dehydrogenase (G6PD) is the key regulated step in this pathway.

Question 24

function of NADPH

Answer 24

source of energy for a variety of synthetic reactions. These include fatty acid biosynthesis, cholesterol biosynthesis, defense against oxidative stress and white cell function

Question 25

what factors determine flux through a pathway

Answer 25

amount of substrate available, amount of key enzymes available, allosteric regulation and covalent modification of a key enzyme (ie. phosphorylation)

Question 26

Things that can affect levels of key enzymes available in gluconeogenesis

Answer 26

PEPCK is a key enzyme in gluconeogenesis and isnulin can decrease transcription of this enzyme, resulting in reduced flux through the pathway

Question 27

what is allosteric regulation

Answer 27

This is where another molecule (sometimes a substrate for or product of the reaction or an intermediate in a different pathway) alters the activity of a key enzyme in a pathway by altering the Km or Vmax.

Question 28

Give example of allosteric regulation in pentose phosphate pathway

Answer 28

NADPH/NADP+ (present when the pentose phosphate pathway has been active) reduces the activity of G6PD (the key regulated step in the pathway)

Question 29

describe hormonal regulation of carbohydrate metabolism

Answer 29

In fed state, insulin is high and glucagon/ catecholamines/ growth hormone/ cortisol are low resulting in glycolysis and glycogen synthesis. In fasting state, insulin is low and counterhormones are high, reuslting in gluconeogenesis and glycogen breakdown.