carb metabolism Flashcards
Km
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
Vmax
maximum rate of reaction catalyzed by enzyme
Describe the common features that make a particular step in a linked enzyme pathway a “key step”.
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
Primary function of glycolysis
generation of energy and useful chemical intermediates from the breakdown of glucose
key steps in glycolysis
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
Net result of glycolysis
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
compare the fate of pyruvate in aerobic vs anaerobic environment
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
Primary function of TCA/electron transport system
produces energy from pyruvate
key steps in TCA cycle
pyruvate > Acetyl CoA > oxidized to CO2 with release of energy stored in GTP, NADH and FADH2
compare TCA cycle in energy surplus vs energy deficit
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.
key steps in electron transport chain
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)
primary function of gluconeogenesis
During fasting, the liver (and to lesser extent the kidney) produce glucose
What are the substrates used for gluconeogenesis
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.
key steps in gluconeogenesis
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
Name the organs that can produce/export glucose into the circulation
liver and kidney only b/c they can undergo gluconeogenesis
when does glycolysis vs gluconeogenesis occur?
When glucose is abundant in a liver cell, glycolysis predominates. When glucose is in short supply, gluconeogenesis predominates
What compound is an important node in glucose metabolism
glucose-6-phosphate: it can go down pathway of glycolysis to be oxidized for energy or go towards storage in process of glycogen synthesis
Function of glyogen synthesis
Forms a rapidly available source of glucose for acute oxidative needs
key steps in glycogen synthesis
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.
When is glycogen synthesized vs broken down?
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.
structure of glycogen
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
function of pentose phosphate pathway
when glucose is abundant it is used for generation of NADPH and for generation of 5-carbon sugars used for nucleotide synthesis
Pentose phosphate pathway key steps
oxidation of glucose-6-phosphate to 6-phosphogluconolactone by Glucose-6-Phosphate Dehydrogenase (G6PD) is the key regulated step in this pathway.
function of NADPH
source of energy for a variety of synthetic reactions. These include fatty acid biosynthesis, cholesterol biosynthesis, defense against oxidative stress and white cell function
what factors determine flux through a pathway
amount of substrate available, amount of key enzymes available, allosteric regulation and covalent modification of a key enzyme (ie. phosphorylation)
Things that can affect levels of key enzymes available in gluconeogenesis
PEPCK is a key enzyme in gluconeogenesis and isnulin can decrease transcription of this enzyme, resulting in reduced flux through the pathway
what is allosteric regulation
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.
Give example of allosteric regulation in pentose phosphate pathway
NADPH/NADP+ (present when the pentose phosphate pathway has been active) reduces the activity of G6PD (the key regulated step in the pathway)
describe hormonal regulation of carbohydrate metabolism
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.
