The Biochemistry Reviewer Part I

Biochemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and metabolism. This is the biochemistry reviewer as part of the Biological Science reviewer compiled by LET Review University.

1. Nonenzymatic glycosylation or glycation creates glycoproteins by:

1. Chemical addition of sugars to polypeptides

2. Extracellular synthesis

3. Attaching 20 amino acids

4. Secretion into extracellular matrix

2. Which of the following is a 5-carbon sugar?

1. Glucose

2. Fructose

3. Ribose

4. Sucrose

3. The optimum pH of pepsin is:

1. 5

2. 7.1

3. 8.1

4. 2

4. A catabolic enzyme:

1. Joins two or more substances into a larger molecule

2. Cleaves a molecule into two or more parts

3. Inserts a substance between two or more existing molecules

4. Acts as a chemical messenger between amino cells

5. An anabolic enzyme:

1. Cleaves a molecule into two or more parts

2. Inserts a substance between two or more existing molecules

3. Joins two or more substances into a larger molecule

4. Acts as a chemical messenger between amino cells

6. An allosteric enzyme has which of the following properties?

1. It can only operate in an acidic environment,

2. It can only operate in an alkaline environment,

3. It becomes active only when it binds with a specific cofactor,

4. It can function either as a catabolic or anabolic enzyme.

7. A proteolytic enzyme has the following action:

1. It cleaves complex sugars into simple sugars.

2. It joins fatty acids into proteins.

3. It joins proteins to sugars to form glycoproteins.

4. It cleaves protein molecules into smaller units.

8. Amylase has all of the following properties except:

1. It breaks down starches into sugars.

2. It cleaves proteins into amino acids.

3. It is a component of human saliva.

4. High serum levels may indicate pancreatic inflammation.

9. The function of a protein is determined primarily by:

1. Its molecular weight

2. The number of amino acids it contains

3. Its spatial conformation

4. Its affinity for hydrocarbons

10. Which of the following is not involved in the biosynthesis of a protein molecule?

1. Codon

2. Ribosome

3. Messenger RNA

4. Amylase

11. Amino acids in a protein are joined by:

1. Zwitterions

2. Hydrogen bonds

3. Peptide bonds

4. Van der Waals bonds

12. Protein catabolism can yield which of the following:

1. Complex carbohydrates

2. Glucose

3. Free fatty acids

4. Omega-3 fatty acids

13. The primary function of the Citric Acid Cycle is to:

1. Facilitate aerobic cellular energy production

2. Oxidize cellular toxins

3. Produce anaerobic energy by fermentation

4. Synthesize vitamin D

14. Which of the following is not produced by or an intermediate of the Citric Acid Cycle?

1. NADH

2. Citrate

3. Water

4. Glucose

15. What is ATP?

1. A cellular energy transfer medium

2. A source of biological electrons

3. A purine base and sugar bound to three phosphates

4. All of the above

16. ATP is synthesized from ADP by which process?

1. Dehydration synthesis

2. Oxidative phosphorylation

3. Protein catabolism

4. Beta oxidation

17. The reduced intermediaries produced by the TCA Cycle are used to:

1. Digest carbohydrates

2. Link amino acids into polypeptides

3. Supply electrons to phosphorylate ADP into ATP

4. Cleave polypeptides into amino acids

18. Which of the following is a free fatty acid?

1. Eicosapentaenoic acid

2. Citric acid

3. Cholesterol

4. Glycerol

19. Fatty acids are esterified into mono-, di-, or triglycerides by attaching to:

1. Sterol

2. Cholesterol

3. Glycerol

4. Sodium chloride

20. Omega-3 fatty acids have which of the following?

1. At least 3 double bonds

2. A 3-carbon backbone

3. A double bond 3 carbons from the end of the chain

4. At least 3 fatty acid molecules linked by omega bonds

ANSWERS AND EXPLANATIONS

1. A

Nonenzymatic glycosylation or glycation creates glycoproteins by the chemical addition of sugars to polypeptides. Since this type of glycosylation is nonenzymatic, the time and the concentration of sugar control glycosylation. Because people with higher circulating levels of glucose have higher levels of nonenzymatic glycosylation, measurement of the glycosylated hemoglobin A1c is a diagnostic test used to monitor blood sugar levels in persons with diabetes.

2. C

Ribose is a pentose, a simple sugar (monosaccharide) that has five carbon atoms per molecule. It is synthesized in the body and obtained in small amounts from consumption of ripe fruits and vegetables. Ribose serves as an energy substrate for the resynthesis of ATP and is a key component of ribonucleic acid (RNA). Deoxyribose, a component of DNA, also is a pentose.

3. D

The optimum pH for enzymes varies for different enzymes and even enzymes with similar actions may have different optimal pH based on where they act. For example, trypsin, a digestive enzyme that acts in the small intestine has an optimal pH of 8 while pepsin, which acts in the more acidic milieu of the stomach, has an optimal pH of 2.

4. B

A catabolic enzyme engages in destructive metabolism, which involves degrading or breaking down complex molecules into simpler ones with the resulting release of energy. In the body, the breakdown of food in the gastrointestinal tract by a variety of digestive enzymes is an example of a catabolic process. The opposite of catabolism is anabolism.

5. C

Anabolism is the opposite of catabolism and is a set of metabolic pathways that serve to create, construct, or synthesize larger molecules from smaller ones, such as the synthesis of carbohydrates, proteins and fatty acids. Anabolic processes consume or require energy rather than releasing energy. Examples of anabolic processes include gluconeogenesis, glyoxylate cycle, and glycosylation.

6. C

Allosteric enzymes change their configurations when they bind with cofactors. Their catalytic activity is altered – either enhanced or reduced – by binding of specific ligands at sites other than the substrate-binding site. When catalytic activity of the enzyme is enhanced, the effector is termed an activator; when it is diminished or eliminated, it is called a deactivator or inhibitor.

7. D

Proteolytic enzymes, also known as proteases, catalyze the splitting or breakdown of proteins into smaller peptide fractions and amino acids. The process of accomplishing this breakdown is called proteolysis. Peptidases are a subgroup of proteases that that catalyze the hydrolysis of peptide linkages; they are present in plants and yeast and in the body. In the body, they are involved in digestion.

8. B

Amylase is the class of enzymes involved in the breakdown and digestion of starches – complex carbohydrates – into simple sugars to supply energy. Amylases are glycoside hydrolases and act on ?-1,4-glycosidic bonds. Plants and some bacteria make amylases. In the body, they are present in saliva and are produced by salivary glands and the pancreas.

9. C

The stable three-dimensional shape and orientation of a protein (its unique folds, termed its native configuration) determine its function and chemical reactivity. Proteins are composed of series of as many as twenty different L-?-amino acids. Proteins are distinguished by their configurations into three broad classes – globular proteins, fibrous proteins, and membrane proteins.

10. D

The biosynthesis of a protein molecule uses information encoded in genes. The amino acid sequence of the protein is delineated by the nucleotide sequence of the gene that encodes the protein. Codons, sequences of three adjacent nucleotides along a DNA or messenger RNA molecule, designate the specific amino acid to be incorporated into a polypeptide.

11. C

Peptide bonds are the primary linkages of all proteins. They are the chemical connections that form between the carboxyl group (COOH) of one amino acid and the amino group (NH2) of adjacent amino acids. A dipeptide contains two amino acids, a tripeptide three, a tetrapeptide four, and so on.

12. B

Protein catabolism is the digestion, or breakdown of macromolecules – proteins – into amino acids able to transit through cells’ plasma membranes. The products of protein catabolism are used to synthesize new amino acids or are converted to other compounds via the citric acid cycle. Protein catabolism is most often carried out by proteases.

13.A

The citric acid cycle, also known as the tricarboxylic acid cycle (TCA cycle), the Krebs cycle, or the Szent-Gyorgyi-Krebs cycle is a key component of the metabolic pathways that chemically converts proteins, carbohydrates, and fats into usable energy. It is called citric acid cycle because the citric acid is the first product and the final reactant of the metabolic pathway.

14. D

The substrate of the citric acid cycle is acetyl coenzyme A. It is derived from glycolysis by a decarboxylating dehydrogenase activity of pyruvate dehydrogenase. The cycle yields the following intermediates: citrate, cis-aconitate, iso-citrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate. At the conclusion of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle begins again.

15. D

Adenosine triphosphate (ATP) is an energy-filled nucleotide that powers the cells and stores energy. It is present in all life forms, from simple, single-celled organisms to humans. Organisms catabolize carbohydrates to create ATP. Then, the ATP is used for anabolic cellular reactions. Each molecule of glucose can be catabolized to as many as 34 molecules of ATP.

16. B

Oxidative phosphorylation is a biochemical process that takes place in cells at a plasma membrane. The final metabolic pathway of cellular respiration, it occurs after glycolysis and the citric acid cycle. As many as 26 of 30 ATP molecules generated from a single glucose molecule during cellular respiration result from the process of oxidative phosphorylation.

17. C

The reduced intermediaries of the citric acid cycle are the source for multiple biosynthetic pathways. Every round of the cycle joins an acetyl group to oxaloacetate, oxidizing two carbons off as Co₂, which leaves succinate. Succinate is converted back to oxaloacetate producing one GTP and reduced cofactors. The rate of the cycle is determined by the availability of substrates, inhibition of accumulating products and allosteric inhibition of enzymes.

18. A

Free or unesterified fatty acids are released by the hydrolysis of triglycerides within adipose tissue. Free fatty acids can be used as an immediate source of energy by multiple organs and tissue and can be converted by the liver into ketone bodies. Because free fatty acids can interact with a variety of enzyme systems, they must be rapidly sequestered in tissues to ensure that their activities are closely regulated.

19. C

An ester is formed when an alcohol combines with an acid in a reaction to form an organic compound. While fatty acids occur in nature in their free (unesterified) state, they are most often found as esters linked to glycerol, cholesterol, or long-chain aliphatic alcohols and as amides in sphingolipids.

20. C

Omega-3 fatty acids have a final carbon-carbon double bond in the third bond from the methyl end of the fatty acid. Omega-3 fatty acids – eicosapentaenoic acid and docosahexaenoic acid may be derived from the diet. They are present in oily or fatty fish, such as salmon, tuna, sardines, trout, and mackerel, and alpha linolenic acid is derived from plants. Flaxseed oil is a dietary source of alpha linolenic acid.