GRE Subject Test: Chemistry : Biochemistry

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #1 : Peptide Bonds

How many water molecules are lost from the condensation of 100 amino acids into a polypeptide?

Possible Answers:

Correct answer:

Explanation:

A peptide bond is formed via the condensation of one amino acid's alpha-carboxy group with the alpha-amino group of another amino acid. Thus, the joining together of two amino acids results in the loss of one water molecule. Likewise, joining three amino acids together results in the loss of two water molecules. Following this pattern, we can conclude that the number of water molecules lost is equal to the number of amino acids joined together, minus 1. Therefore, the joining together of 100 amino acids results in the loss of 99 water molecules.

Example Question #1 : Proteins

 

An enzyme that cleaves disulfide bridges would most disrupt a protein containing which amino acid sequence?

Possible Answers:

Tyr–Cys–Val–Val–Leu–Thr

Tyr–Cys–Cys–Thr–Val–Leu

All of the answers would be equally affected

Val–Leu–Leu–Cys–Tyr–Thr

Cys–Leu–Val–Tyr–Tyr–Thr

Correct answer:

Tyr–Cys–Cys–Thr–Val–Leu

Explanation:

Disulfide bridges are made between two cysteine amino acids. An enzyme that cleaves disulfide bonds would disrupt a protein containing the most cysteine residues; therefore, Tyr–Cys–Cys–Thr–Val–Leu is the correct answer.

Example Question #1 : Nucleic Acids

Of the following groups of nitrogenous bases, which does not contain a purine?

Possible Answers:

Cytosine, thymine, guanine

Cytosine, thymine, uracil

Thymine, adenine, cytosine

Guanine, uracil, adenine

Adenine, guanine, thymine

Correct answer:

Cytosine, thymine, uracil

Explanation:

We can use the mnemonic "PurAG" to remember that the purines are adenine and guanine. The only choice that does not contain a purine, therefore, is "cytosine, thymine, and uracil." Remember, pyrimidines contain a single ring, while purines have two.

Example Question #1 : Lipids

Which of the following molecules is amphipathic?

Possible Answers:

Triglyceride

Maltose

Phospholipid

Glutamate

Correct answer:

Phospholipid

Explanation:

Amphipathic molecules contain both polar and nonpolar regions, making them an extremely diverse class with an array of functions. For example, bile is an amphipathic molecule whose nonpolar region interacts with fats and whose polar region interacts with the aqueous environment of the small intestine.

Most lipids are entirely nonpolar and hydrophobic. Phospholipids, however, are formed from a glycerol molecule bound to two hydrophobic fatty acid tails and a hydrophilic phosphate head. This structure allows phospholipids amphipathic properties. Most notably, phospholipids are able to interact with the aqueous environments in the cell cytosol and extracellular environment, while maintaining the hydrophobic region of the cell membrane that acts as a semipermeable barrier.

Triglycerides are considered nonpolar. Glutamate is an acidic amino acid with highly polar properties. Maltose is a six-carbon sugar (carbohydrate) and is highly polar.

Example Question #1 : Properties Of Lipids

Which of the following will you most likely find in a steroid molecule?

Possible Answers:

Nitrogenous base

Cyclohexane ring

Phosphate group

Pentose sugar

Correct answer:

Cyclohexane ring

Explanation:

Steroids are a type of lipid that are characterized by their four-ring molecular structure. The four rings consist of three six-membered rings and one five-membered ring. Recall that six-membered rings are called cyclohexanes and five-membered rings are called cyclopentanes; therefore, you will most likely find a cyclohexane in a steroid.

Phosphate groups, pentose sugars, and nitrogenous bases are found in nucleotides, which are monomers that make up nucleic acids. Steroids are a type of lipid; therefore, you will most likely not find these substances in a steroid.

Example Question #3 : Carbohydrates

Which type of bond is created between carbohydrates and the sidechain amine of select asparagine residues in proteins?

Possible Answers:

N-glycosidic

O-glycosidic

Both are correct

Neither is correct

Correct answer:

N-glycosidic

Explanation:

A glycosidic bond covalently joins a carbohydrate molecule to another molecule. An O-glycosidic bond is a covalent linkage between a carbohydrate and a protein, joining a serine or threonine hydroxyl side chain and a sugar (oxygen in the bond yields "O"). An N-glycosidic linkage involves bonding of a carbohydrate and a protein, joining an asparagine side chain amide and a sugar (nitrogen in the bond yields "N"). Thus, N-glycosidic is the correct answer.

Example Question #1 : Polymerization

Cellulose is a polymer that is composed of what monomer?

Possible Answers:

Fructose

Glycerol

Glucose

Glycogen

Galactose

Correct answer:

Glucose

Explanation:

Cellulose is a polysaccharide (a sugar). Glucose is the monosaccharide used to generate the structure of cellulose via  linkages. Cellulose is composed of several hundred glucose molecules bound in this chain. Due to the nature of the beta linkage, humans cannot digest cellulose.

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