Enzymes

What are enzymes?

• large biological molecules which catalyse chemical reactions in living organisms
• responsible for the metabolic processes which sustain life
• enzymes can be proteins or ribonucleic acid (RNA)

Properties of enzymes:

• lower the minimum amount of energy needed to start a reaction (activation energy)
• has active site of specific shape, which is complementary to its substrate 
• does not alter properties of end products of reaction
• highly efficient in small amounts
• denatured easily due to high heat

How do enzymes work?

1. After an effective collision between a substrate and an enzyme (where the substrate binds to the active site of the enzyme), an enzyme-substrate complex is formed. 
2. The substrate molecule is held in the active site by interactions such as hydrogen and ionic bonds between the R groups of the amino acids and the substrate molecule. 
3. Enzyme catalyses conversion of substrate to the end product. 
4. The alteration of chemical conformation results in the product being released from the active site since it is no longer complementary to the structure of the active site. 
5. The active site is then available for other substrates to bind to it.

"Lock and Key" Hypothesis

• there is an exact fit between the substrate and the active site of the enzyme
• the enzyme (lock) has a unique shape complementary to the substrate (key)
• enzymes are very specific; only substrates that are exactly complementary to its active site are able to bind with it

image taken from: http://katysstudynotes.files.wordpress.com/2010/11/enzymes.png

Induced Fit model

• active site of enzyme is flexible 
• active site is able to mould itself around the substrate to make the fit better
• active site returns to original shape after the products are released 

Illustration of the induced fit model

image taken from: http://zebrafish.umdnj.edu/Pre-Enrollment/Resources/Biochemistry/Enzymes%20and%20Metabolism/Images/enzyme_fit_diagram.png

Food Tests

Benedict's Test for Reducing Sugars

• Pour 2cm³ of solution sample solution into a test tube and add an equal volume of Benedict's solution. 
• Shake the mixture.
• Heat test tube in a boiling water bath for 5 minutes.
• Record observation after 5 minutes.

Colour of precipitate and the corresponding amount of reducing sugar present

http://brilliantbiologystudent.weebly.com/uploads/1/4/8/3/14836282/44628.jpg?513

Biuret's Test for Proteins

• Add 2cm³ of sample solution to a clean test tube and add 2cm³ of dilute sodium hydroxide solution.
• Shake the mixture.
• Add 1% copper (II) sulphate solution, drop by drop. Shake mixture after every drop and observe the colour change. 
• If mixture remains blue, no protein is present.
• If mixture turns violet/purple, protein is present. 

Iodine Test for Starch

• Add 2cm³ of sample solution to a clean test-tube, followed by a few drops of iodine solution.
• Observe the colour change.
• If mixture remains brown, no starch is present.
• If mixture turns blue black, starch is present.

Ethanol Emulsion Test for Fats

• Add 2cm³ of sample solution to a clean test tube and add 2cm³ of ethanol.
• Shake mixture thoroughly.
• Add 2cm³ of water into the test tube and shake. 
• Record the observation.
• If mixture remains clear, no lipid is present.
• If a white emulsion was obtained, lipid is present. 

Condensation and Hydrolysis

Condensation and hydrolysis are reverse processes of each other.

Condensation (or dehydration synthesis) is a chemical process which by 2 molecules are combined to make a larger molecule through the loss of water. It is the process which by biomolecules are formed.

Hydrolysis is a chemical process which by a macromolecule is broken down into smaller molecules by the addition of water. Often, enzymes are needed to carry out hydrolysis, as water alone is insufficient.

image taken from: http://classconnection.s3.amazonaws.com/311/flashcards/1406311/jpg/polymers1334535675092.jpg

Biomolecules — Lipids

What are lipids?

• lipids are insoluble in water
• contain C, H and O
• contain less oxygen than carbohydrates
• made up of fatty acids and glycerol

What are fatty acids?

• contain hydrophobic "tail" of a long hydrocarbon chain, and hydrophilic "head" which is a carboxyl group
• can be saturated (no double bonds, no kinks) or unsaturated (double bonds and kinks in fatty acid tail)

Classification of lipids

• simple lipids
• compound lipids
• steroids and sterols

Simple Lipids

• formed by joining fatty acid with an alcohol (eg. glycerol)
• fats (eg. triglyceride) are simple lipids
• triglyceride structure: glycerol + 3 fatty acid tails

Compound Lipids (eg. phospholipids)

• found in phospholipid bilayer of cell membrane
• phospholipid structure: phosphate head + 2 fatty acid tails

Biomolecules — Carbohydrates

Carbohydrates are the most abundant biomolecule in nature. 

Function of carbohydrates

• building blocks for larger molecules 
• energy storage — starch (plant) and glycogen (animal)
• structural — cellulose cell wall (plant) and chitin (insects, crabs, shrimps)

The bond between monomers of carbohydrates is known as glycosidic bond.

Monosaccharides

• monomers of carbohydrates
• eg. glucose, fructose, galactose
• all monosaccharides are reducing sugars

Disaccharides

• made up of two monosaccharides

http://i.imgur.com/2Kx8x.jpg

Polysaccharides

• made up of many monosaccharides

What is starch? 

• polymer of glucose molecules
• has two components; amylose and amylopectin
• both fit together to form a complex 3-dimensional structure which is insoluble in water
• amylose helices are entangled in the branches of amylopectin molecules
• each amylose chain is coiled into a helix, with six glucose residues for every complete turn of the helix — compact shape making it a complex structure for storage
• amylopectin have many branches

What is glycogen?

• animal equivalent of starch
• found in liver and skeletal muscles of vertebrate animals 

Starch and glycogen (energy stores)

• their molecules have many side branches where glucose molecules can be removed from their tips (by enzymes)
• their insolubility stops them interfering with osmosis
• their compactness provides an efficient way to store lots of glucose for future cellular respiration

Cellulose (long, unbranched chain)

• most abundant organic molecule on Earth
• major component of cell wall in plants
• made from long, straight unbranched chains of glucose
• chains cross-linked by H-bonds which holds them tightly together (excludes water)
• chemically very inert and insoluble 
• many molecules form strong fibrils
• only some bacteria, fungi and a very small number of animals can secrete cellulase enzymes