Does A Coenzyme Change The Enzymes Chemical Makeup

26.11: Enzymes and Coenzymes

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Objectives

After completing this section, you should exist able to

1. describe the catalytic function of an enzyme in a biochemical reaction.
2. requite an example of one fat‑soluble and i water‑soluble vitamin.

Key Terms

Make sure that you tin define, and utilise in context, the key term beneath.

• coenzyme
• cofactor
• enzyme
• substrate
• vitamin

Written report Notes

You should have a general noesis of the function of enzymes, merely you demand not memorize specific names or the classification system.

A catalyst is whatever substance that increases the rate or speed of a chemic reaction without being changed or consumed in the reaction. Enzymes are biological catalysts, and nearly all of them are proteins. In addition, enzymes are highly specific in their action; that is, each enzyme catalyzes only one blazon of reaction in only one compound or a grouping of structurally related compounds. The compound or compounds on which an enzyme acts are known as its substrates . Enzymes are classified past reaction type into vi categories show in Tabular array \(\PageIndex{1}\).

Table \(\PageIndex{one}\): Classes of Enzymes

Class	Type of Reaction Catalyzed	Examples
oxidoreductases	oxidation-reduction reactions	Dehydrogenases catalyze oxidation-reduction reactions involving hydrogen and reductases catalyze reactions in which a substrate is reduced.
transferases	transfer reactions of groups, such as methyl, amino, and acetyl	Transaminases catalyze the transfer of amino group, and kinases catalyze the transfer of a phosphate group.
hydrolases	hydrolysis reactions	Lipases catalyze the hydrolysis of lipids, and proteases catalyze the hydrolysis of proteins
lyases	reactions in which groups are removed without hydrolysis or addition of groups to a double bond	Decarboxylases catalyze the removal of carboxyl groups.
isomerases	reactions in which a compound is converted to its isomer	Isomerases may catalyze the conversion of an aldose to a ketose, and mutases catalyze reactions in which a functional group is transferred from i atom in a substrate to another.
ligases	reactions in which new bonds are formed betwixt carbon and another atom; energy is required	Synthetases catalyze reactions in which 2 smaller molecules are linked to form a larger one.

Enzyme-catalyzed reactions occur in at least two steps. In the start stride, an enzyme molecule (E) and the substrate molecule or molecules (Southward) collide and react to form an intermediate chemical compound chosen the enzyme-substrate (Eastward–S) complex (Equation \(\ref{step1}\)). This stride is reversible considering the complex can suspension apart into the original substrate or substrates and the free enzyme. One time the Due east–S complex forms, the enzyme is able to catalyze the formation of production (P), which is then released from the enzyme surface (Equation \(\ref{step2}\)):

\[S + E \rightleftharpoons Due east–Southward \label{step1}\]

\[East–S → P + E \label{step2}\]

Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the complex. The structural features or functional groups on the enzyme that participate in these interactions are located in a cleft or pocket on the enzyme surface. This pocket, where the enzyme combines with the substrate and transforms the substrate to production is called the active site of the enzyme (Effigy \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Substrate Binding to the Active Site of an Enzyme. The enzyme dihydrofolate reductase is shown with i of its substrates: NADP⁺ (a) unbound and (b) bound. The NADP⁺ (shown in red) binds to a pocket that is complementary to it in shape and ionic properties.

The active site possesses a unique conformation (including correctly positioned bonding groups) that is complementary to the construction of the substrate, so that the enzyme and substrate molecules fit together in much the aforementioned manner every bit a key fits into a tumbler lock. In fact, an early model describing the formation of the enzyme-substrate complex was chosen the lock-and-key model (Effigy \(\PageIndex{2}\)). This model portrayed the enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site.

Figure \(\PageIndex{2}\): The Lock-and-Key Model of Enzyme Action. (a) Because the substrate and the active site of the enzyme have complementary structures and bonding groups, they fit together as a primal fits a lock. (b) The catalytic reaction occurs while the two are bonded together in the enzyme-substrate complex.

Working out the precise iii-dimensional structures of numerous enzymes has enabled chemists to refine the original lock-and-key model of enzyme actions. They discovered that the binding of a substrate oft leads to a large conformational change in the enzyme, equally well as to changes in the structure of the substrate or substrates. The current theory, known as theinduced-fit model, says that enzymes can undergo a change in conformation when they bind substrate molecules, and the active site has a shape complementary to that of the substrate only subsequently the substrate is jump, every bit shown for hexokinase in Figure \(\PageIndex{3}\). Afterward catalysis, the enzyme resumes its original structure.

Effigy \(\PageIndex{3}\): The Induced-Fit Model of Enzyme Action. (a) The enzyme hexokinase without its substrate (glucose, shown in red) is bound to the active site. (b) The enzyme conformation changes dramatically when the substrate binds to it, resulting in additional interactions between hexokinase and glucose.

The structural changes that occur when an enzyme and a substrate join together bring specific parts of a substrate into alignment with specific parts of the enzyme's agile site. Amino acid side bondage in or about the binding site tin and so human action equally acid or base catalysts, provide binding sites for the transfer of functional groups from one substrate to another or aid in the rearrangement of a substrate. The participating amino acids, which are usually widely separated in the principal sequence of the protein, are brought close together in the active site as a result of the folding and bending of the polypeptide chain or bondage when the protein acquires its third and quaternary structure. Binding to enzymes brings reactants shut to each other and aligns them properly, which has the aforementioned consequence as increasing the concentration of the reacting compounds.

Example \(\PageIndex{1}\)

1. What type of interaction would occur between an OH group nowadays on a substrate molecule and a functional group in the active site of an enzyme?
2. Suggest an amino acid whose side concatenation might be in the active site of an enzyme and form the type of interaction yous simply identified.

Solution

1. An OH grouping would most likely appoint in hydrogen bonding with an appropriate functional group present in the active site of an enzyme.
2. Several amino acid side chains would be able to engage in hydrogen bonding with an OH group. One example would be asparagine, which has an amide functional group.

Practice \(\PageIndex{ane}\)

1. What type of interaction would occur between an COO⁻ group present on a substrate molecule and a functional grouping in the agile site of an enzyme?
2. Propose an amino acid whose side chain might be in the agile site of an enzyme and form the blazon of interaction you but identified.

Enzyme Cofactors and Vitamins

Many enzymes are simple proteins consisting entirely of ane or more than amino acrid chains. Other enzymes contain a nonprotein component called a cofactor that is necessary for the enzyme'south proper functioning. There are two types of cofactors: inorganic ions [east.g., zinc or Cu(I) ions] and organic molecules known as coenzymes. Most coenzymes are vitamins or are derived from vitamins.

Vitamins are organic compounds that are essential in very pocket-size (trace) amounts for the maintenance of normal metabolism. They generally cannot exist synthesized at adequate levels by the body and must exist obtained from the diet. The absence or shortage of a vitamin may outcome in a vitamin-deficiency disease. In the start half of the 20th century, a major focus of biochemistry was the identification, isolation, and characterization of vitamins. Despite accumulating testify that people needed more than merely carbohydrates, fats, and proteins in their diets for normal growth and health, it was not until the early on 1900s that enquiry established the need for trace nutrients in the diet.

Table \(\PageIndex{ii}\): Fat-Soluble Vitamins and Physiological Functions

Vitamin	Physiological Function	Effect of Deficiency
vitamin A (retinol)	germination of vision pigments; differentiation of epithelial cells	night blindness; continued deficiency leads to total incomprehension
vitamin D (cholecalciferol)	increases the torso'south power to absorb calcium and phosphorus	osteomalacia (softening of the bones); known equally rickets in children
vitamin E (tocopherol)	fat-soluble antioxidant	damage to cell membranes
vitamin K (phylloquinone)	formation of prothrombin, a cardinal enzyme in the blood-clotting process	increases the time required for blood to clot

Considering organisms differ in their synthetic abilities, a substance that is a vitamin for one species may not be and then for another. Over the by 100 years, scientists accept identified and isolated 13 vitamins required in the man diet and have divided them into two wide categories: the fat-soluble vitamins (Tabular array \(\PageIndex{ii}\)), which include vitamins A, D, E, and K, and the water-soluble vitamins, which are the B complex vitamins and vitamin C (Table \(\PageIndex{3}\)). All fat-soluble vitamins contain a high proportion of hydrocarbon structural components. At that place are one or ii oxygen atoms nowadays, but the compounds every bit a whole are nonpolar. In contrast, water-soluble vitamins contain large numbers of electronegative oxygen and nitrogen atoms, which can engage in hydrogen bonding with h2o. Most h2o-soluble vitamins act as coenzymes or are required for the synthesis of coenzymes. The fatty-soluble vitamins are important for a variety of physiological functions.

Table \(\PageIndex{3}\): Water-Soluble Vitamins and Physiological Functions

Vitamin	Coenzyme	Coenzyme Function	Deficiency Illness
vitamin B1 (thiamine)	thiamine pyrophosphate	decarboxylation reactions	beri-beri
vitamin B2 (riboflavin)	flavin mononucleotide or flavin adenine dinucleotide	oxidation-reduction reactions involving two hydrogen atoms	—
vitamin Bthree (niacin)	nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate	oxidation-reduction reactions involving the hydride ion (H−)	pellagra
vitamin B6 (pyridoxine)	pyridoxal phosphate	variety of reactions including the transfer of amino groups	—
vitamin B12 (cyanocobalamin)	methylcobalamin or deoxyadenoxylcobalamin	intramolecular rearrangement reactions	pernicious anemia
biotin	biotin	carboxylation reactions	—
folic acid	tetrahydrofolate	carrier of 1-carbon units such as the formyl grouping	anemia
pantothenic Acrid	coenzyme A	carrier of acyl groups	—
vitamin C (ascorbic acid)	none	antioxidant; formation of collagen, a poly peptide constitute in tendons, ligaments, and bone	scurvy

One characteristic that distinguishes an enzyme from all other types of catalysts is its substrate specificity. An inorganic acrid such as sulfuric acid tin be used to increment the reaction rates of many different reactions, such as the hydrolysis of disaccharides, polysaccharides, lipids, and proteins, with consummate impartiality. In dissimilarity, enzymes are much more than specific. Some enzymes act on a single substrate, while other enzymes act on any of a group of related molecules containing a similar functional group or chemical bail. Some enzymes even distinguish between D- and L-stereoisomers, binding one stereoisomer only non the other. Urease, for instance, is an enzyme that catalyzes the hydrolysis of a unmarried substrate—urea—but not the closely related compounds methyl urea, thiourea, or biuret. The enzyme carboxypeptidase, on the other manus, is far less specific. It catalyzes the removal of about any amino acid from the carboxyl end of any peptide or protein.

Enzyme specificity results from the uniqueness of the active site in each unlike enzyme considering of the identity, accuse, and spatial orientation of the functional groups located there. Information technology regulates cell chemistry and then that the proper reactions occur in the proper place at the proper time. Clearly, it is crucial to the proper functioning of the living cell.

Concept Review Exercises

1. Distinguish between the lock-and-central model and induced-fit model of enzyme action.

2. Which enzyme has greater specificity—urease or carboxypeptidase? Explain.

Answers

1. The lock-and-key model portrays an enzyme as conformationally rigid and able to bail only to substrates that exactly fit the active site. The induced fit model portrays the enzyme structure as more than flexible and is complementary to the substrate only subsequently the substrate is bound.

2. Urease has the greater specificity because it can bind only to a single substrate. Carboxypeptidase, on the other hand, tin can catalyze the removal of about any amino acid from the carboxyl cease of a peptide or poly peptide.

Takeaways

• A substrate binds to a specific region on an enzyme known as the active site, where the substrate tin can be converted to product.
• The substrate binds to the enzyme primarily through hydrogen bonding and other electrostatic interactions.
• The induced-fit model says that an enzyme can undergo a conformational change when binding a substrate.
• Enzymes exhibit varying degrees of substrate specificity.

Exercises

1. What type of interaction would occur between each grouping present on a substrate molecule and a functional grouping of the agile site in an enzyme?

   1. COOH
   2. NH_iii ⁺
   3. OH
   4. CH(CH₃)₂

2. What type of interaction would occur betwixt each group nowadays on a substrate molecule and a functional group of the active site in an enzyme?

   1. SH
   2. NH_two
   3. C_{half dozen}H₅
   4. COO⁻

3. For each functional group in Exercise one, suggest an amino acid whose side chain might be in the active site of an enzyme and form the blazon of interaction you identified.

4. For each functional group in Do 2, suggest an amino acrid whose side chain might be in the agile site of an enzyme and form the type of interaction you identified.

Answers

1. 1. hydrogen bonding
   2. ionic bonding
   3. hydrogen bonding
   4. dispersion forces

3. 1. The amino acid has a polar side chain capable of engaging in hydrogen bonding; serine (answers will vary).
   2. The amino acid has a negatively charged side chain; aspartic acid (answers will vary).
   3. The amino acid has a polar side chain capable of engaging in hydrogen bonding; asparagine (answers volition vary).
   4. The amino acid has a nonpolar side chain; isoleucine (answers volition vary).

Source: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28McMurry%29/26:_Biomolecules-_Amino_Acids_Peptides_and_Proteins/26.11:_Enzymes_and_Coenzymes

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