Enzymes Flashcards
True or false: enzymes are catalysts, that are not consumed in processes.
True
Nearly all enzymes are _________ __________, although a few catalytically-active RNA molecules (ribozymes) have been identified.
Globular proteins
What is an enzyme’s active site?
The region of the enzyme that binds into the substrate, and converts it into product
The active site is often a cleft or crevice on the surface of the enzyme, that forms a predominately non-polar, or ______________, environment, to enhance substrate binding.
Hydrophobic
List four forces which bind the substrate to the active site.
Electrostatic interactions, hydrogen bonds, and hydrophobic forces, and in some instances, reversible covalent bonds
What adjective describes the molecule that the enzyme substrate complex creates?
Bound
Catalytically-active _______ ______ residues within the active site act on the substrate molecule, to transform it into product, which is then released.
Amino acid
At a molecular level, the __________ fits into the active site.
Substrate
Specific interactions with the amino acid side groups at the _______ ______ hold components in place.
Active site
Describe the enzymatic ‘lock-and-key’ model.
The lock-and-key model elucidates the theory that the shape of the substrate and the active site are rigid, perfectly complementary, and fit together exactly, when in the right alignment
Describe the enzymatic ‘induced fit’ model.
In the induced-fit model, the binding of the substrate induces a conformational change in the active site of the enzyme
True or false: some enzymes show features of both models, with some complementarity.
True
To what does ‘enzyme specificity’ refer?
How the properties and spatial arrangement of the amino acid residues forming the active site determine which molecules can bind and become substrates
List three enzymes in which enzyme specificity is observed.
Trypsin, chymotrypsin, and elastase
To what family do trypsin, chymotrypsin, and elastase belong?
Serine proteases
Explain the etymology of ‘serine proteases’.
‘Serine’ comes from the serine residue in the active site that plays a critical role in catalysis, and ‘protease’, from the catalysis of the hydrolysis of peptide bonds in proteins
Describe the enzymatic action of chymotrypsin.
Chymotrypsin cleaves on the carboxyl side of bulky, aromatic, and hydrophobic amino acid residues. Amino acid residues with small side chains in its active site allow for access to bulky side chains of substrates
Describe the action of trypsin.
Cleaves on the carboxyl side of positively charges lysine or arginine residues, and has a negatively charged Asp residue in its active site, to react with Lys and Arg
Describe the action of elastase.
Cleaves on the carboxyl side of amino acid residues with small, uncharged side chains. It has relatively large uncharged side chains of Val and Thr protruding into its active site, preventing access of all but the small side chains of Gly and Ala
What is ‘energy of activation’?
The changes that take place during the course of a particular process
In the first stage, ___ must be converted to the S† transition state.
S
In the second stage, S† is converted to ___.
P
True or false: the S† is at a lower free energy level than S.
False
The free energy change for S to S† is __________, and termed the ‘energy of activation’.
Positive
The _________ ______ constitutes a barrier to chemical reactions occurring.
Energy hump
______ __________ ___________ between the starting substrate and final product impacts the reaction.
Free energy difference
What is the enzymatic ‘turnover number’?
The maximum number of molecules of substrate that an enzyme can convert to product, per catalytic site, per unit time (kcat)
In enzyme nomenclature, the first number has _____ classification numbers.
Six
The second and third numbers in enzyme naming are __________.
Subclasses
True or false: in enzyme naming, the fourth digit is a serial number.
True
Many enzymes require the presence of small, nonprotein units, called __________ to carry out their particular reaction.
Cofactors
Cofactors may be either one or more _________ ions, such as Zn2+ or Fe2+, or a complex organic molecule called a coenzyme.
Inorganic
A metal or coenzyme that is covalently attached to the enzyme is called a ____________ ________.
Prosthetic group
A complete catalytically-active enzyme together with its prosthetic group is called a ______________.
Holoenzyme
The _________ part of the enzyme on its own, without its prosthetic group, is called an apoenzyme.
Protein
Some coenzymes, such as NAD+, are bound and released by the enzyme during its catalytic cycle and so function as ____-___________.
Co-substrates
Many coenzymes are derived from _________ precursors.
Vitamin
Each enzyme has an __________ pH.
Optimum
Rate of reaction is optimum at at an enzyme’s special pH, and ________ is at its prime.
Efficacy
Small deviations from pH optimum cause decreased activity, due to changes in the ____________ of groups at the active site of the enzyme.
Ionisation
Larger deviations from pH optimum causes _______________ of the enzyme itself, due to interference with the many weak noncovalent bonds maintaining its 3D structure.
Denaturation
Many enzymes have an optimum pH around ____________ pH.
Physiological
What is pepsin’s optimum pH?
~2
Small rises in temperature increase the thermal energy of substrate molecules, ____________ the energy of activation, increasing rate reaction.
Lowering
True or false: large rises in temperature denature the enzyme.
True
What are enzyme inhibitors?
Molecules that are capable of interfering with the activity of an individual enzyme
A molecule which acts directly on an enzyme to lower its ___________ ______ is an inhibitor.
Catalytic rate
List two examples of inhibitors.
Normal body metabolites, which inhibit a particular enzyme as part of the normal metabolic control of a reaction, and foreign substances, such as drugs or toxins - the effect of enzyme inhibition could be either therapeutic or, at the other extreme, lethal
True or false: inhibition is always reversible.
False