KREB and DUMPLIN

TCA CYCLE

Today, we learn about the TCA cycle as known as the famous Krebs cycle for all our scientist out there and is sometimes called the Citric Acid cycle. TCA stands for tricarboxylic acid. So what is the purpose or the end product of the TCA cycle? Well the easy answer would be energy but as a tertiary level student, more information would be needed to express the level of knowledge and understanding that I should be at. The TCA cycle can be defined as a continuation of different chemical reactions to produce energy in the form of ATP (Adenosine Triphosphate) via the oxidation of acetate. The TCA cycle can be considered as the step following glycolysis in the breakdown of sugar towards producing energy. Be sure to remember that the cycle takes place in ALL aerobic organisms and must take place under aerobic condition to generate ATP via oxidative phosphorylation.

So if glycolysis takes place in the cytosol, where does the TCA cycle take place? The answer to that would be in the mitochondria. Remember those little things? In eukaryotic cells? I’m sure you do. Yes, so the cycle takes place on the inner cristae within the mitochondria.

We need to link glycolysis to the TCA cycle via a reaction. This reaction will be:

Pyruvate + CoA+ NAD⁺      →     acetylCoA + CO2 + NADH

The primary substrate in the TCA cycle is Pyruvate(3C) but it is immediately turned into acetylCoA(2C) which is the main product of the reaction above. The above reaction is catalysed by three enzymes and collectively, they are called the pyruvate dehydrogenase complex. Once this is produced, our eight chemical reactions can begin. For our level, we must know these 8 enzymes used in the reactions. So let’s list them.

ü  Citric synthase- Synthesises Citrate(6C) from Oxaloacetate(4C) + acetyl CoA(2C)

ü  Aconitase- produces cis-aconitate(6C) and isocitrate(6C)

ü  Iso-citrate dehydrogenase- produces α-ketoglutarate (5C)

ü  α ketoglutarate dehydrogenase- produces succinyl CoA(4C)

ü  Succinyl-Coenzyme A synthetase- produces succinate(4C)

ü  Succinate dehydrogenase- produces fumerate(4C)

ü  Fumerase- produces malate(4C)

ü  Malate dehydrogenase- produces olaloacetate(4C)

 

TIPS

• An easy way to remember these reactions is by abbreviating but I’m sure everyone will have their own special way to remember.
• Always pay close attention to the carbon balance when learning and doing the Krebs cycle
• The TCA cycle is not an easy topic so be sure to go it over in much more detail.

 REFERENCES

Wikipedia. “Citric acid cycle with aconitate 2.svg.” 2014. http://en.wikipedia.org/wiki/File:Citric_acid_cycle_with_aconitate_2.svg (accessed 23 Mar 2014).

Carbohydrates- The Bread of Life

 

Carbohydrates are hydrated carbons with the empirical formula (CH₂O)n or Cn(H₂O)n-1.

Classification Of Carbohydrates

• Monosaccharides
• Disaccharides
• Oligosaccharides
• Polysaccharides

Functions of Carbohydrates

Function	Details
Acts as a source of energy	Carbohydrates break down quickly to act as quick sources of energy. Note Polysaccharides take longer to be broken down.
Storage	In animals- glycogen ( when glucose levels are high insulin converts glucose to glycogon. When glucose levels are low Glucagon coverts glycogen to glucose)In plants- Starch
Structure	In animals- Chitin is found in some insects.In plants- Cellulose: major component of plant walls.
Pre cursor Molecule	Used as building blocks for other molecules.Example: DNA, RNA
Fat oxidation	Carbohydrates are essential for fat oxidation, without carbohydrates it would not take place.
Add flavor to the diet	They are also easy to digest.

Monosaccarides

There are 2 types:

Aldoses- these has an aldehyde group as seen in the molecular description of D-glucose below.

Ketoses- these have a keto group as seen in D- fructose.

How do differentiate a Aldoses from a Ketose?

This is done by looking at the position of the C=O bond on the molecular structure.

Chiral Carbon must have four different groups attached to it. Chiral compounds have same number of carbons however they are arranged differently.

D vs L Sugars & the Chiral Carbon

*this is based on the asymmetrical carbon (chiral Carbon) in glycerydehyde.

* To calculate if it is a D or a L sugar you first must find the Asymmetrical carbon.

*Then look at the OH group

*If the OH group is on the right it is the D form if it is on the left it is the L form.

More than one chiral carbon? No problem… Just follow these easy steps:

*Find the asymmetrical carbon furthest away from the aldehyde or keto group.

*Then determine which side the hydroxl group is on.

No There not twins they are Epimers

Two sugars that differ ONLY around one carbon atom are called epimers.

Hemiacetals OR Hemiketals

Aldotetioses and all monnosaccarides with 5 or more carbon atoms is the backbone occurs predominatly as a ring structure in which the carbonyl group formed a covalent bond with the oxygen of a hydroxl group along the chain.

The formation of these structures are a result of a general reaction between aldehydes or ketones and alcohols to formed hemiacetals or hemiketals.

The Glycosidic Bond

Disaccharides, oligosaccharides and polysaccharides are formed by joining monosaccharides together. The bonds between the sugars are known as glycosidic bonds. These are formed by the enzyme known as glycosyltransferases. Glycosidic bonds are named according to the numbers of carbon connected and the position of the anomeric hydroxyl group.

Table showing Structure and Function of the main Polysaccarides

Name Of Polysaccharides	Composition	Function
Starch-AmyloseAmylopectin	Unbranched chain of alpha glucose with 1,4 glycosidic links and branched chains of alpha glucose units with 1,4 and 1,6 glycosidic links.	Major storage carbohydrate in plants.
Cellulose	Straight chains of alpha glucose.Found only in plants	Needed for structural support
Chitin	Unbranched chain of beta acetylgucosamine units with 1,4 glycosidic links.	Major part of the exoskeleton of many arthropods.
Glycogen	Highly branched short chains of alpha glucose units.

This week in labs we did the various tests to determine the presence of carbohydrates in a sample. The following flowchart is a basic skeletal representation of the various tests that were done.

Flowchart showing the classification of an unknown carbohydrate

Did You Know!!!!!!!!

• The sole energy source for the brain is glucose
• Lactose intolerance is caused by a lack of lactase
• Honey is considered a food  high in carbohydrates. (It is high in natural sugars)
• Principal sites for dietary carbohydrate digestion are the mouth and intestinal lumen.
• 

• Haworth perspective formula commonly used to show stereochemistry of the ring  forms of monosaccharide.
• Carbohydrates are more easily digestible than lipids.
• Some dissolvable stitches are made from chitin.

References

Information:

Pamela C. Champ, Richard A. Havery, Denise R. Ferrier, Lippincott’s Illustrated Reviews,(1987-2005), 83

Flowchart:

ANALYSIS OF CARBOHYDRATES.” 7. ANALYSIS OF CARBOHYDRATES. http://people.umass.edu/~mcclemen/581Carbohydrates.html (accessed February 6, 2014).

Pictures:

Carbohydrate Cartoons and Comics.” – funny pictures from CartoonStock. http://www.cartoonstock.com/directory/c/carbohydrate.asp (accessed February 1, 2014).

“D-glucose – Google Search.” D-glucose – Google Search. N.p., n.d. Web. 07 Feb. 2014.