What Is Enzyme Structure and Function?
Learn more about this metabolic process
Improve your understanding of enzyme structure and function with this definition of the process, complete with enzyme classifications and examples.
To understand the structure and function of enzymes, it's first important to know what exactly enzymes are. Simply put, enzymes are proteins that participate in cellular metabolic processes with the ability to enhance the rate of reaction between bio-molecules.
Life would not exist without the presence of enzymes.
What Makes Enzymes Special
Some enzymes can even reverse a reaction (or reactants) from the direction it would normally take by reducing the activation energy (Ea) to the extent that the reaction favors the reverse direction. Similarly, enzymes can catalyze reactions that might not otherwise occur by lowering the activation energy to a more "affordable" level for the cell.
Enzymes can be isolated using various protein purification methods. The purity of an enzyme preparation is measured by determining its specific activity.
The Composition of Enzymes
Enzymes can have molecular weights ranging from about 10,000 to more than 1 million. Molecular weight is the sum of the atomic weights of a molecule's atoms. In chemistry, the term tends to be used interchangeably with molecular mass. Some enzymes have molecular weights that fall below the range listed above.
A small number of enzymes are not proteins but consist of small catalytic RNA molecules. Often, enzymes are multi-protein complexes made up of a number of individual protein sub-units.
Many enzymes catalyze reactions without help, but some require an additional non-protein component called a co-factor.
Co-factors may be inorganic ions such as Fe2+, Mg2+, Mn2+, or Zn2+, or consist of organic or metallo-organic molecules known as co-enzymes.
How Enzymes Are Classified
Enzymes are classified according to the reactions they catalyze. The six classes are as follows: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.
Hydrolases, oxidoreductases, and transferases are the most numerous forms of enzymes, while the remaining forms of enzymes are less common.
Oxidoreductases catalyze oxidation reactions in which electrons travel from one molecule to another. Transferases catalyze the transportation of a functional group from one molecule to another. Hydrolase enzymes catalyze hydrolysis or the process by which single bonds are broken down upon exposure to water.
Lyases catalyze lysis reactions that result in a double bond. Isomerases catalyze structural shifts in a molecule, causing a change in shape. Ligases are in charge of the catalysis of ligation, also known as the combination of a pair of substrates, or reactants.
Examples of Enzymes
Alcohol dehydrogenase is an oxidoreductase enzyme that converts alcohols to aldehydes or ketones. This enzyme makes alcohol less toxic as it breaks it down.
It also plays a key role in the fermentation process.
Aminotransferases are transferases that catalyze the amino acid degradation by removing amino groups.
Glucose-6-phosphatase is a hydrolase that removes the phosphate group from glucose-6-phosphate, leaving glucose and H3PO4. Pyruvate decarboxylase is a lyase that removes CO2 from pyruvate.
Ribulose phosphate epimerase is an isomerase that catalyzes the interconversion of ribulose-5-phosphate and xylulose-5-phosphate. And, finally, hexokinaseis a ligase that catalyzes the interconversion of glucose and ATP with glucose-6-phosphate and ADP.
Proteins are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes. They function as catalysts, they transport and store other molecules such as oxygen, they provide mechanical support and immune protection, they generate movement, they transmit nerve impulses, and they control growth and differentiation. Indeed, much of this text will focus on understanding what proteins do and how they perform these functions.
Several key properties enable proteins to participate in such a wide range of functions.
Proteins are linear polymers built of monomer units called amino acids. The construction of a vast array of macromolecules from a limited number of monomer building blocks is a recurring theme in biochemistry. Does protein function depend on the linear sequence of amino acids? The function of a protein is directly dependent on its threedimensional structure (Figure 3.1). Remarkably, proteins spontaneously fold up into three-dimensional structures that are determined by the sequence of amino acids in the protein polymer. Thus, proteins are the embodiment of the transition from the one-dimensional world of sequences to the three-dimensional world of molecules capable of diverse activities.
Proteins contain a wide range of functional groups. These functional groups include alcohols, thiols, thioethers, carboxylic acids, carboxamides, and a variety of basic groups. When combined in various sequences, this array of functional groups accounts for the broad spectrum of protein function. For instance, the chemical reactivity associated with these groups is essential to the function of enzymes, the proteins that catalyze specific chemical reactions in biological systems (see Chapters 8–10).
Proteins can interact with one another and with other biological macromolecules to form complex assemblies. The proteins within these assemblies can act synergistically to generate capabilities not afforded by the individual component proteins (Figure 3.2). These assemblies include macro-molecular machines that carry out the accurate replication of DNA, the transmission of signals within cells, and many other essential processes.
Some proteins are quite rigid, whereas others display limited flexibility. Rigid units can function as structural elements in the cytoskeleton (the internal scaffolding within cells) or in connective tissue. Parts of proteins with limited flexibility may act as hinges, springs, and levers that are crucial to protein function, to the assembly of proteins with one another and with other molecules into complex units, and to the transmission of information within and between cells (Figure 3.3).
Crystals of human insulin. Insulin is a protein hormone, crucial for maintaining blood sugar at appropriate levels. (Below) Chains of amino acids in a specific sequence (the primary structure) define a protein like insulin. These chains fold into well-defined (more...)
Structure Dictates Function. A protein component of the DNA replication machinery surrounds a section of DNA double helix. The structure of the protein allows large segments of DNA to be copied without the replication machinery dissociating from the (more...)
A Complex Protein Assembly. An electron micrograph of insect flight tissue in cross section shows a hexagonal array of two kinds of protein filaments. [Courtesy of Dr. Michael Reedy.]
Flexibility and Function. Upon binding iron, the protein lactoferrin undergoes conformational changes that allow other molecules to distinguish between the iron-free and the iron-bound forms.
3.1 Proteins Are Built from a Repertoire of 20 Amino Acids
3.2 Primary Structure: Amino Acids Are Linked by Peptide Bonds to Form Polypeptide Chains
3.3 Secondary Structure: Polypeptide Chains Can Fold Into Regular Structures Such as the Alpha Helix, the Beta Sheet, and Turns and Loops
3.4 Tertiary Structure: Water-Soluble Proteins Fold Into Compact Structures with Nonpolar Cores
3.5 Quaternary Structure: Polypeptide Chains Can Assemble Into Multisubunit Structures
3.6 The Amino Acid Sequence of a Protein Determines Its Three-Dimensional Structure
Appendix: Acid-Base Concepts