Carbohydrates Flashcards
What are the main monosaccharides and why are they called hexoses?
Glucose
Galactose
Made of 6C
What are the main disaccharides?
Maltose
Lactose
Sucrose
What bond joins together sugar monomers?
Glycosidic
What do glycosidic bonds form between?
An OH group of one monomer and the anomeric carbon of another
Why are maltose and lactose classed as reducing sugars?
They have a free anomeric carbon 1 which can be oxidised
Why is sucrose not a reducing sugar?
Has no free anomeric carbon 1
What are the two types of polysaccharides?
Hetero and Homo
What is a heteropolysaccharide?
A polysaccharide made from two or more monomer species
What is a homopolysaccharide?
Polysaccharide made from one monomer species
Example of a polysaccharide?
Starch or Glycogen
What makes up starch?
Two glucose polymers - amylose and amyopectin
Difference between amylose and amylopectin?
Amylose is not branched and had alpha1–>4 bonds
Amylopectin is branched and has alpha1–>4 and alpha1–>6 bonds.
Is glycogen more or less branched than starch?
More
Why would we store glucose as glycogen?
Compactness
Glycogen has many reducing ends making it easily utilised to form glucose
Glycogen is not osmotically active yet glucose is - easier to keep glycogen in a cell than it is glucose.
Why is glycogen osmotically inactive?
Polymers form hydrated gels
Proteins with a carbohydrate attached are called?
Glycoproteins
Benefits of a glycoprotein?
Increased solubility
Influences conformation and folding
Protects it from degradation
Acts as a cell to cell communicator
Where are the 3 locations of carb digestion?
Mouth
Duodenum
Jujunum
What happens in mouth?
Salivary amylase breaks down alpha1–>4 bonds
What happens in the duodenum?
Pancreatic amylase acts same as in mouth
What happens in the jujunum?
Final digestion by 4 enzymes;
Isomaltase - breaks down alpha1–>6 bonds
Glucoamylase - removes glucose from non reducing ends of carb
Sucrase and Lactase - hydrolyses their sugars
What is the end product of carb digestion?
Glucose, Galactose and Fructose
How is glucose absorbed after digestion?
It binds to Sodium and enters the cell against its own gradient but along sodiums gradient.
How does galactose get absorbed?
Similar to glucose
How is fructose absorbed?
Binds to GLUT5 channel protein and moves down its concentration gradient
What breaks down cellulose and hemicellulose?
Not us, gut bacteria do releasing CH4 anf H2 gas
Example of a disaccharide deficiency?
Lactose intolerance
Explain lactose intolerance?
Lactase is deficient
Lactose not digested - osmotically active brings water into gut causing diarrhoea
Lactose broken down by gut bacteria - gas build up and irritant acids
Function of hexokinase and glucokinase ?
To phosphorylate glucose to G-6-P
Hexokinase and glucokinase are isoenzymes, what does this mean?
They have the same function, yet different structure, Vmax and Km
What is glucokinase for?
Has a high Km and a High Vmax, allowing to to grab lots of glucose fast and trap it quickly, only working when there is a high concentration of glucose due to high Km.
Found in hepatocytes
What is hexokinase used for?
Used in tissues - has a low Km meaning it can grab glucose more effectivly at low conc.
Has a Low Vmax meaning tissues won’t over grab and phosphorylate glucose
What is G-6-P used for?
Can be turned back to glucose and used, sent to make pentoses or stored as glycogen
How is glycogen synthesised?
Glycogenin covalently bonds glucose to UDP forming chains 8 residues long.
Glycogen synthase then takes over and extends the glucose chains.
Glycogen branching enzyme then breaks the chain and reattaches it as branches via alpha1–>6 bonds.
How is glycogen degraded?
- Glucose monomers are removed one at a time as G-1-P from non reducing ends. Phosphorylated by glycogen phosphorylase.
- Remaining glycogen moecules is debrached by the transferase activity of a de-branchin enzyme which removes 3 of the last 4 glucose residues on a branch and reattaches it to the nearest non-reducing of main chain via alpha1–> 4 bonds.
- Last branched glucose is removed by glucosiadase leaving behind an unbranched glycogen chain which glycogen phosphorylase can act upon as needed.
What is glycolysis?
A catbolic pathway that forms ATP and pyruvate from glucose anerobically via substrate-level phosphorylation.
Occurs in cytosol so no complex organelles needed.
How many phases does glycolysis have?
2 - preparatory and payoff phases each with 5 steps.
How many ATP are used and made in glycolysis?
Per glucose;
2 ATP needed for preparatory phase
4 made in payoff phase
Why is glycolysis not reversible?
It has overall -ve change in gibbs free energy making it spontaneous. It has 3 very spontanous steps which make this irreversible
What happens at the end of the prep phase?
Glucose makes G-3-P
Step 3 of this phase is irreversable and first commited phase.
What happens end of payoff phase?
2 G-3-P molecules end up as 2 ATP and 2 NADH
Why does NAD+ need to be regenerated?
For use in metabolic pathways to reduce various intermediate metabolites
Example of NAD+ regeneration?
During anerboic respiration, Pyruvate changed to lactate using NADH–>NAD+ which can go off and be used as needed.
What is the use and regeneration of NAD+ termed as?
Redox balance
How is pyruvate converted to lactate?
Via fermentation - is reduced to lactate using lactate dehydrogenase and is driven by oxidation of NADH to NAD+ - redox balance.
How is lactase later changed to glucose?
In the liver via gluconeogenisis.
What is the cori cycle?
Interaction between the liver and muscles
How is pyruvate converted to acetyl CoA?
Aerobic respiration - pyruvate is moved to the mitochondria and enzyme pyruvate dehydrogenase turns it to Acetyl-CoA and CO2. This also involves the reduction of NAD+ to NADH
What is gluconeogenisis?
Process of making glucose from non carb molecules when body is starving/fasting
Since glycolysis is irreversible, how does gluconeogenisis occur?
Uses bypass reactions to get passed the 3 irreversible steps
What are bypass reactions A and B?
Occur in mitochondria - Lactate to pyruvtae via lactatse dehydrogenase and NADH to NAD+.
Pyruvate undergoes reaction A - pyruvate to oxaloacetate.
Reaction B - oxaloactetae to PEP.
Reaction C?
F-1,6-bisP to F-6-P
Reaction D?
Final step in gluconeogenisis
G-6-P to Glucose
What is the fate of fructose?
Joins glycolysis using ATP to form F-1-P and then glyceraldehyde and using 1 more ATP forming glyceraldehyde-3-phosphate.
Fate of galactose?
Joins glycolysis through conversion of glucose-1-phosphate by UDP.
What does the pentose phosphate pathway do?
Produces NADPH and pentose sugars
Also metabolises pentose sugars in diet
What are pentose sugars needed for?
Nucleic acid synthsis
2 phases of pentose phosphate cycle?
Oxidative irreversible -
G-P-6 to pentose phosphate
AND
Makes NADPH
Non oxidative reversible - Interconverts G-P-6 and pentose phosphate to form lots of 3 to 7 carbon sugars
Difference between NAD+ and NADP+?
NAD+ - used in metabolism
NADP+ - used in anabolism
What does alcohol inhibit in liver which slows gluconeogenisis?
NAD+ is used to break ethanol down, not enough for gluconeogenisis.
