Lesson 6 : Proteins

Question 1

Proteins (_____________) are that are _____________ made up of ________________ arranged in a _______________.

Answer 1

Polypeptides
biochemical compounds
amino acids
linear chain

Question 2

The amino acids in a polymer are joined together by the _____________ between the ____________ and the amino group of ___________________.

Answer 2

peptide bonds
carboxyl group
adjoining amino acids

Question 3

Structures of Proteins

Answer 3

Primary
Secondary
Tertiary
Quaternary

Question 4

What are Amino Acids

Answer 4

the monomer units of proteins, are molecules containing an amino group, a carboxyl group and a side chain.

Question 5

Amino acids can be differentiated by their ___________________.

Answer 5

variations in their side chains

Question 6

In amino acids, the carboxyl group is _______________ charged, and the amino group is _____________ charged. Amino acids are classified based on their charges. At a particular pH, amino acids have ____________ charge. This pH is known as the _______________. When amino acids have no overall charge, they are known as _______________.

Answer 6

negatively
positively
no overall
isoelectric point
zwitterions

Question 7

Amino acids are vital to human existence. They serve as ____________________ and as ____________________.

Answer 7

building blocks of proteins
intermediates in metabolism

Question 8

Purpose of Qualitative Tests for Proteins

Biuret Test
Ninhydrin Test
Xanthoproteic Test
Millon’s Test
Sakaguchi Test
Hopkins-Cole Test
Lead Acetate Test

Answer 8

Biuret Test : identifies the presence of proteins and peptide bonds

Ninhydrin Test : used to check whether a given analyte contains amines or a-amino acids.

Xanthoproteic Test : determine the amount of protein soluble in a solution, using Nitric acid.

Millon’s Test : presence of phenol-containing protein

Sakaguchi Test : detect presence of arginine or a guanidinium compound in proteins

Hopkins-Cole Test : indole ring containing amino acid in proteins; tryptophan-containing proteins

Lead Acetate Test : used to detect sulfur-containing amino acids (cystine or cysteine)

Question 9

Biurets Test Procedure

Answer 9

1. Add 1 mL of 10% NaOH to 3 ml of each of the protein suspensions (albumin, casein, gelatin - in 3 separate test tubes), and mix.
2. Add a drop of 0.01 M of CuSO4, to each and mix if no definite color develops, add another drop or two of the CuSO4 solution.

Question 10

Ninhydrin Test Procedure

Answer 10

1. Add 5 drops of 0.1% Ninhydrin solution to 2 mL each of the 3 protein suspensions.
2. Heat in boiling water for 10 minutes. Observe and record the results.

Question 11

Xanthoproteic Test Procedure

Answer 11

1. Slowly add 1 mL of concentrated HNO3 to 3 mL of each of the protein suspensions.
2. Place in water bath for about 30 seconds and place each test tube in a rack to cool the solutions.
3. Add slowly, drop by drop, saturated NaOH to each test tube until the solutions are alkaline (or basic)

Question 12

Millon’s Test Procedure

Answer 12

1. Add 5 drops of fresh Millon’s reagent to 3 mL of each protein suspension.
2. Carefully heat each mixture in boiling water for 5 minutes.
3. Cool the test tubes and note the colors formed.

Question 13

Sakaguchi Test Procedure

Answer 13

1. Add 1 mL of 10% NaOH and 1mL of 0.02% alphanaphthol solution to 3mL of **each of the protein suspensions. **
2. After** 3 minutes,** add **2-4 mL of bromine water to each. A strong red color can be stabilized by adding urea **to destroy the excess hypobromite.

Question 14

Hopkins-Cole Test Procedure

Answer 14

1. Add 2 mL of Hopkins-Cole reagent to 3 mL of each of the 3 protein suspensions.
2. Mix. Incline each test tube and with a dropper, add slowly about 1 mL conc. H2SO4 down the side of the tube so that two layers will form. Do not stir.
3. Let stand for 1 to 2 minutes and note the color formed in-between the layers.

Question 15

Lead Acetate Test Procedure

Answer 15

1. Add 5 mL of 5% NaOH and a few crystals of Pb(Ac)2 (lead (II) acetate) to 3 mL of each protein suspension.
2. Heat in boiling water for 5-10 minutes with occasional mixing of the contents of the tube. Describe the color change.

Question 16

The Biuret test (also known as _____________) positively identifies the presence of ____________ and ___________. The reaction in this test involves the complex formation of the ___________________ in a _______________.

Answer 16

Piotrowski’s test
proteins
peptide bonds
proteins with Cu2+ ions
strongly alkaline solution

Question 17

Despite the name of the test, the reagent used to perform the procedure does not contain Biuret, a chemical derived from _______. It is named so because it also gives a positive reaction to the ___________ in the Biuret molecule.

Answer 17

Urea
peptide-like bonds

Question 18

Biuret test Expected Reaction

Answer 18

POSITIVE: Purple colored solution
NEGATIVE: Blue colored solution

Question 19

Ninhydrin Test Expected Reaction

Answer 19

For ammonia, primary/secondary amines, and amino acids, deep purple colour is obtained.

For hydroxyproline and proline, a yellow colour is obtained.

For asparagine, brown colour is obtained.

If no colour change is observed, the analyte does not contain amino acids, amines, or ammonia.

Question 20

The ninhydrin test is a chemical test which is used to check whether a given analyte contains ___________ or ____________.

Answer 20

amines or a-amino acids

Question 21

In this test, ninhydrin (a chemical compound with the formula ____________; IUPAC name: ____________) is added to a test solution of the analyte.

Answer 21

C9H604
2,2-dihydroxyindane-1,3-dione

Question 22

In the ninhydrin test, the development of a ___________color indicates the presence of _____________, _________________, or ________________ in the analyte.

Answer 22

deep blue
ammonia
primary/secondary amines
amino acids

Question 23

Xanthoproteic test is a _________________ to determine the ____________________ in a solution, using ______________.

Answer 23

qualitative test
amount of protein soluble
Nitric acid

Question 24

The test is named after the ____________ substance produced after the reaction - Xanthoproteic acid.

Answer 24

yellow color

Question 25

Xanthoproteic Test Expected Reaction

Answer 25

POSITIVE: Formation of yellow to orange color
NEGATIVE: No color change

Question 26

In Millons test, if too much reagent is used, the color may ________________

Answer 26

disappear in boiling

Question 27

Millon’s test is specific to ____________ containing structures. Millon’s reagent is ______________, in which ____________ is dissolved. As a result of the reaction a _______________ or a _____________ is considered a ___________ test. A ___________ precipitate of is not a positive reaction but usually indicates that the solution is _____________.

Answer 27

phenol
concentrated HNO3
mercury
red precipitate
red solution
positive
yellow
Mercuric (II) oxide (HgO)
too alkaline

Question 28

Who developed the Millons Test

Answer 28

The test was developed by a French chemist, Auguste Nicolas Eugene Millon

Question 29

Millons Test Expected Reaction

Answer 29

POSITIVE: Brick red colored solution or red precipitate
NEGATIVE: No red color

Question 30

Sakaguchi Test Expected Reaction

Answer 30

Positive result: Formation of red color. This indicates the presence of an arginine or guanidinium compound.

Negative result: Absence of red color. This indicates an absence of arginine or a guanidinium compound.

Question 31

Sakaguchi test is based on the principle of reaction between _____________ and the ________________ in ___________, in the presence of an _______________. The exact mechanism of the reaction is not yet known; however, the reaction results in the formation of a ______________ due to the formation of an ____________structure

Answer 31

1-naphthol
guanidinium groups
arginine
oxidizing agent
red-colored complex
indole-like

Question 32

Lead sulfide test (or Lead acetate test) is a biochemical test for the detection of amino acids like ______________ and _______________. The test is a specific test for the detection of amino acids containing ________,_________ in __________, and ______________ in __________. The test is also called a lead acetate test as the reagent for the test is lead acetate. Even though the test is specific for the detection of sulfur-containing amino acids, ___________ doesn’t give a positive result in this test.

Answer 32

cysteine and cystine
sulfur, S-S group ; cysteine
S-H group in cysteine
methionine

Question 32

Hopkin-Cole’s test is a chemical test for the determination of the presence of _______________ of ___________________.

Answer 32

heterocyclic side chain (indole group)
tryptophan in proteins

Question 32

Hopkins-Cole test Expected Reaction

Answer 32

POSITIVE: Purple color ring at the interface
NEGATIVE: No formation of purple color ring

Question 33

The Hopkins-Cole test is named after ____________ and ___________ for their work on ________________.

Answer 33

Frederick Gowland Hopkins and Sydney W. Cole
tryptophan isolation

Question 34

Lead Acetate Expected Reaction

Answer 34

Positive test: Formation of black precipitate at the bottom of the test tube. This indicates the presence of cysteine or cystine in the solution.

Negative test: Absence of black residue in the test tube. This indicates the absence of cysteine or cystine.