Explain how the sequence of amino acids in a polypeptide determines a protein's shape and function, and why heat and pH can destroy that shape.
Each is in the Biology glossary, with every other term in the subject.
A secondary structure in which a polypeptide coils into a spiral held by hydrogen bonds between the C=O of one peptide bond and the N–H four amino acids along.
The –NH₂ group of an amino acid, which forms part of a peptide bond; it can gain a hydrogen ion to become –NH₃⁺.
A molecule with a central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R-group; the monomer of polypeptides.
A secondary structure in which sections of polypeptide lie side by side in a pleated sheet, held by hydrogen bonds between backbone C=O and N–H groups of neighbouring sections.
The –COOH group of an amino acid or fatty acid; in amino acids it forms part of a peptide bond, and it can lose a hydrogen ion to become –COO⁻.
A fibrous structural protein made of three polypeptides wound into a triple helix; it gives tendons, skin and bone high tensile strength and is the most abundant protein in mammals.
The precise three-dimensional shape of a protein, produced by the folding of its polypeptide chain or chains.
A protein that contains a non-polypeptide component, a prosthetic group, in addition to its polypeptides; haemoglobin, with its haem groups, is an example.
A change in the three-dimensional conformation of a protein, caused by heat or pH breaking the bonds that hold its shape, so that it loses its function; usually permanent.
A molecule made of two amino acids joined by a peptide bond in a condensation reaction.
A covalent bond between the sulfur atoms of two cysteine R-groups, which stabilises the tertiary or quaternary structure of a protein.
An amino acid that an organism cannot synthesise, so it must be obtained from the diet; adult humans have nine.
A protein with a long, thin, strand-like shape, usually insoluble in water and with a structural function, such as collagen or keratin.
A protein folded into a compact, rounded shape, usually soluble in water, such as an enzyme, insulin or haemoglobin.
A ring-shaped prosthetic group with an iron ion (Fe²⁺) at its centre; each haem group in haemoglobin binds one oxygen molecule.
A conjugated globular protein in red blood cells made of four polypeptides, each with a haem group containing iron, which binds oxygen for transport.
The clustering of non-polar R-groups away from water in a protein, caused by water molecules hydrogen bonding with each other and excluding the non-polar groups.
A globular protein hormone secreted by β cells of the pancreas that lowers blood glucose; it has two polypeptides (21 and 30 amino acids) linked by disulfide bonds.
An attraction between oppositely charged ions or groups; in proteins, between positively charged (–NH₃⁺) and negatively charged (–COO⁻) R-groups.
An amino acid that an organism can synthesise from other amino acids or compounds, so it need not be obtained from the diet.
The covalent C–N bond between the carboxyl carbon of one amino acid and the amine nitrogen of the next, formed by a condensation reaction.
An unbranched chain of many amino acids joined by peptide bonds, made on ribosomes; one or more polypeptides fold to form a protein.
The sequence of amino acids in a polypeptide, linked by peptide bonds; it determines the higher levels of structure and therefore the conformation of the protein.
A non-polypeptide component permanently bound to a protein and needed for its function, such as the haem group of haemoglobin.
A large molecule made of one or more polypeptides folded into a specific three-dimensional shape that determines its function.
The arrangement of two or more polypeptide subunits, and of any non-polypeptide prosthetic groups, in a protein.
The variable side chain attached to the central carbon of an amino acid; the 20 amino acids differ only in their R-groups, which give each its properties.
Regular coiling or folding of the polypeptide backbone into alpha helices or beta-pleated sheets, stabilised by hydrogen bonds between C=O and N–H groups of the backbone.
The overall three-dimensional folding of a single polypeptide, held by hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions between R-groups.
Proteins are large molecules built from chains of amino acids. They carry out more different jobs than any other class of molecule: enzymes catalyse reactions, haemoglobin carries oxygen, antibodies bind to pathogens, and collagen gives skin and bone their strength.
Each protein can do its job because it has a particular shape, and that shape is set by the order of amino acids in its chain. Twenty amino acids, joined in different orders and numbers, give an almost unlimited range of shapes and therefore of functions. A change in conditions, such as a rise in temperature, can destroy the shape and stop the protein working.
40 exam-style questions on this subtopic, written in the style of the IB papers. The first has its markscheme open. Counted on
Many biological processes depend on the specific shapes of molecules.
Outline how amino acids are linked together to form polypeptides and why an immense variety of polypeptides is possible.
4 marks
a. amino acids are joined by condensation (reactions);
b. water is released / removed (in each reaction);
c. between the amine group of one amino acid and the carboxyl group of another;
d. forming a peptide bond;
e. on ribosomes / during translation, in a sequence set by the gene / mRNA;
f. there are 20 different amino acids (coded for by the genetic code);
g. any number of amino acids can be joined in any sequence;
h. 20n possible sequences for a chain of n amino acids;
[4 max]
Explain how the structure of the active site of an enzyme determines its specificity and how denaturation affects this.
7 marks
a. enzymes are globular proteins;
b. the active site is a small region / is formed by only a few amino acids;
c. its shape and chemical properties depend on the folding of the whole polypeptide / tertiary structure;
d. interactions between amino acids throughout the enzyme hold the active site in shape;
One of 1 short multiple-choice questions on B1.2. Pick an answer to see why it's right or wrong.
Which bond joins amino acids in a polypeptide?
The official deck for this part of the syllabus.
What is a protein?
Give four examples of proteins and state the function of each.
With a free account, Augment remembers where you stopped and what you have practiced.
A protein is made of one or more chains of amino acids. Every amino acid has the same basic structure. A central carbon atom, called the alpha (α) carbon, is bonded to four different groups:
an amine group, –NH₂;
a carboxyl group, –COOH;
a hydrogen atom, –H;
an R-group, also called the side chain, which is different in each amino acid.
Living organisms use 20 different amino acids to build proteins. They differ only in their R-groups. The smallest R-group is a single hydrogen atom, in glycine. Others are long chains or rings, some carry an electric charge, and some are non-polar. These differences give each amino acid different chemical properties, and they are the reason proteins are so varied.
Amino acids are joined by condensation reactions, in the same way that monosaccharides are joined to make polysaccharides (B1.1 Carbohydrates and lipids). The –OH of the carboxyl group of one amino acid and an –H from the amine group of the next are removed as a molecule of water. A covalent C–N bond, called a peptide bond, forms between the carbon of the carboxyl group and the nitrogen of the amine group.
Two amino acids linked in this way form a dipeptide. The dipeptide still has a free amine group at one end and a free carboxyl group at the other, so further amino acids can be added at either end. A chain of a few amino acids (up to about 20) is called an oligopeptide, and a longer chain is a polypeptide. A protein consists of one or more polypeptides folded into a particular shape.
In cells, polypeptides are made on ribosomes during translation, with one amino acid added at a time in the order coded by messenger RNA (D1.2 Protein synthesis). Energy from ATP is used to attach each amino acid to its transfer RNA before it is added to the chain. Peptide bonds are broken by hydrolysis, for example by protease enzymes during digestion in the stomach and small intestine.
Animals obtain amino acids by digesting proteins in their food. Some amino acids can also be made in the body by converting other amino acids, but others cannot. The 20 amino acids are therefore divided into two groups.
A non-essential amino acid can be synthesised in the body from other amino acids or from other compounds, so it does not have to be eaten. Adult humans cannot make nine of the 20 amino acids, so these nine are essential amino acids. If the diet lacks even one of them, proteins that contain it cannot be made in the quantities needed, which limits growth and repair. Amino acids cannot be stored in the body in large amounts; any surplus is broken down, so essential amino acids are needed regularly.
Animal proteins, such as meat, fish, eggs and milk, usually contain all the essential amino acids in the proportions humans need. Many plant proteins are low in one or more of them: cereals such as wheat and rice are low in lysine, while legumes such as beans and lentils are low in methionine. A vegan diet contains no animal products, so it must combine different plant protein sources, such as cereals with legumes, to provide every essential amino acid. A few plant foods, including soya beans and quinoa, contain all of them in good proportions.
The genetic code specifies 20 amino acids, and ribosomes can link them in any order. A polypeptide can be any length from a few amino acids to many thousands. These two facts give an almost limitless number of possible polypeptides.
At each position in the chain, any of the 20 amino acids can be placed. For a chain of amino acids, the number of possible sequences is therefore:
Real polypeptides vary greatly in length. Some examples:
Polypeptide | Number of amino acids | Function |
|---|---|---|
Oxytocin | 9 | Hormone that stimulates contraction of the uterus during birth |
Insulin | 51 (in two chains of 21 and 30) | Hormone that lowers blood glucose concentration |
β-globin chain of haemoglobin | 146 | Part of the oxygen-carrying protein of red blood cells |
Lysozyme (from hen egg white) | 129 | Enzyme that digests bacterial cell walls |
Titin | about 34,000 | Elastic protein in muscle, the largest known polypeptide |
Frederick Sanger was the first to work out the complete amino acid sequence of a protein. He showed in 1955 that insulin consists of 51 amino acids in two chains, and that insulin from any cow always has exactly the same sequence. This was the first evidence that each protein has a precise, repeatable sequence, coded by a gene, and it earned Sanger the Nobel Prize in Chemistry in 1958.
After it is made, a polypeptide folds into a particular three-dimensional shape. The shape of a protein is called its conformation. The conformation is held in place by many weak bonds and interactions between the R-groups of amino acids in different parts of the chain. Most proteins can do their job only when they have the right conformation: an enzyme must fit its substrate, and an antibody must fit its antigen.
The bonds that hold a protein's conformation are much weaker than the covalent peptide bonds of the chain. They can be broken by heat or by a change in pH, and the protein then loses its shape. This is denaturation. A denatured protein usually cannot carry out its function, and in most cases the change is permanent: when conditions return to normal, the chain does not fold back into its original shape.
As temperature rises, the atoms in a protein vibrate more and more. Above a certain temperature the vibrations break the weak bonds holding the conformation, and the chain unfolds. Most human proteins are stable at body temperature (37 °C), but many begin to denature at temperatures not far above it. Cooking an egg is a familiar example: heat denatures the proteins in egg white, which unfold and tangle together, so the clear liquid becomes white and solid, and it cannot become clear again when it cools.
The curve of the percentage of protein molecules still folded against temperature shows that denaturation is not gradual over the whole range. The protein stays almost fully folded until a critical range of temperatures is reached, and then most of the molecules unfold over a few degrees. Proteins from different organisms denature at different temperatures, matching the conditions in which the organism lives: proteins from bacteria in hot springs stay folded at temperatures that would denature human proteins.
pH is a measure of the concentration of hydrogen ions (H⁺). Some R-groups carry a positive or negative charge, and their charges depend on pH. In acidic conditions (high H⁺ concentration) negatively charged groups gain H⁺ and lose their charge; in alkaline conditions positively charged groups lose H⁺. Changing the charges breaks the attractions between oppositely charged R-groups and alters hydrogen bonding, so the conformation changes. Each protein has a range of pH in which it keeps its shape. Pepsin, a protease in the stomach, works at about pH 2, while most proteins in the cytoplasm are denatured at such a low pH.
YouTubeIf the amino acid sequence of a polypeptide determines how it folds, then it should be possible to work out the conformation of a protein from its sequence alone. This "protein-folding problem" was one of the great unsolved problems in biology for about 50 years. Working out structures by experiment, using X-ray crystallography or cryogenic electron microscopy, can take months or years for a single protein, while genome sequencing has produced the sequences of hundreds of millions of proteins.
Since 1994, the Critical Assessment of Structure Prediction (CASP) has tested computer programs every two years. The programs are given the sequences of proteins whose structures have just been worked out by experiment but not yet published. Their predictions are then compared with the real structures using a score called GDT, from 0 to 100, where 100 means a perfect match. For many years, predictions for proteins unlike any of known structure were poor.
At CASP14 in November 2020, the program AlphaFold, developed by the company DeepMind, scored a median GDT of 92.4 across all the targets. On average its predicted positions of atoms were within about 1.6 Å (0.16 nm) of the real positions, close to the accuracy of experimental methods. AlphaFold uses a neural network trained on the structures of about 170,000 proteins already known, together with the patterns of amino acid changes seen in related proteins from different species.
Milestone | Result |
|---|---|
CASP14 assessment, November 2020 | Median GDT score of 92.4 across all targets; about 1.6 Å average error in atom positions |
AlphaFold Protein Structure Database, July 2022 | Over 200 million predicted structures (214,684,311), covering almost every known protein sequence |
Nobel Prize in Chemistry, 2024 | Half to Demis Hassabis and John Jumper for protein structure prediction; half to David Baker for computational protein design |
Sources: DeepMind CASP14 report, 2020; EMBL-EBI, 2022; Royal Swedish Academy of Sciences, 2024.
With the AlphaFold Protein Structure Database, built with the European Bioinformatics Institute (EMBL-EBI), researchers can look up the predicted shape of almost any protein whose sequence is known. Knowing the shape of a protein helps biologists understand what it does, how a mutation that changes one amino acid might disrupt it, and how a drug could be designed to fit into it. Computational design goes in the other direction: David Baker's group designs new amino acid sequences that fold into shapes never seen in nature, such as new enzymes and proteins that bind to viruses.
The predictions are models. They are tested against experimental structures and can be wrong, especially for proteins that change shape when they bind other molecules or that have no fixed shape at all. AlphaFold shows how technology, both computing and microscopy, allows scientists to study structures far too small to be seen directly, and it is strong evidence for the idea that sequence determines conformation and conformation determines function.
YouTubeThe R-groups of the 20 amino acids differ in size, shape and chemical properties. When amino acids are linked into a polypeptide, their R-groups project from the backbone and determine how the chain interacts with water, with other parts of itself and with other molecules. The diversity of R-groups is therefore the basis of the immense diversity in protein form and function.
R-groups are classified by how they interact with water:
Type of R-group | Properties | Behaviour in a protein |
|---|---|---|
Hydrophobic (non-polar) | Mainly C and H; no charge, little or no polarity | Avoid water; cluster together inside a protein or in contact with the hydrophobic core of a membrane |
Hydrophilic, polar | Contain polar groups such as –OH, but carry no overall charge | Form hydrogen bonds with water and with other polar groups |
Hydrophilic, charged: acidic | Contain a carboxyl group that loses H⁺ to become –COO⁻ (negative) | Attract positive groups and water; charge depends on pH |
Hydrophilic, charged: basic | Contain an amine group that gains H⁺ to become –NH₃⁺ (positive) | Attract negative groups and water; charge depends on pH |
Because the R-groups vary so much, a protein can have a surface with exactly the right pattern of charges, polar groups and non-polar patches to bind one particular molecule. This is the basis of enzyme specificity, of antibodies recognising antigens and of receptors binding hormones.
The primary structure of a protein is the sequence of amino acids in its polypeptide chain or chains, held together by peptide bonds. It is coded by a gene. The primary structure determines every other level of structure: the type and position of each R-group determine where the chain coils or folds, which parts are attracted to each other and which parts face water. As a result, every copy of a particular polypeptide folds into the same conformation, so protein structures are precise, predictable and repeatable.
A change of even one amino acid can change the conformation. In sickle cell disease, valine replaces glutamic acid at position 6 of the β-globin chain of haemoglobin. Valine is hydrophobic while glutamic acid is charged, so at low oxygen concentrations the altered haemoglobin molecules stick to each other and form long fibres that distort the red blood cells. The mutation that causes this change is described in D1.2 Protein synthesis.
The secondary structure is the regular coiling or folding of parts of the polypeptide backbone. It is stabilised by hydrogen bonds between the C=O group of one peptide bond and the N–H group of another peptide bond further along the chain. The R-groups are not involved. Because peptide bonds occur at regular intervals along the backbone, the hydrogen bonds also form at regular positions, which gives regular structures.
In an alpha helix (α-helix), the chain coils into a spiral, and each C=O forms a hydrogen bond with the N–H four amino acids further along. The R-groups point outwards from the helix.
In a beta-pleated sheet (β-pleated sheet), two or more sections of the chain lie side by side and are joined by hydrogen bonds between the C=O of one section and the N–H of the next. The sheet has a pleated, zigzag shape, with R-groups pointing above and below it.
Each hydrogen bond is weak, but a helix or sheet contains many of them, so the structure is stable. Most proteins contain some sections of α-helix, some of β-sheet and some less regular loops. In ribbon diagrams of proteins, α-helices are drawn as coiled ribbons and β-sheets as broad flat arrows, so they can be recognised in images of protein structures.
The tertiary structure is the overall three-dimensional shape of a single polypeptide, formed when the chain, with its helices and sheets, folds further. It is held in place by interactions between the R-groups of amino acids that may be far apart in the sequence but close together in the folded chain:
Hydrogen bonds between polar R-groups, for example between an –OH and a C=O.
Ionic bonds between positively and negatively charged R-groups. Amine groups in R-groups can gain H⁺ to become –NH₃⁺, and carboxyl groups can lose H⁺ to become –COO⁻; the opposite charges attract. An ionic bond is the attraction between oppositely charged ions or groups.
Disulfide bonds between the R-groups of two cysteine amino acids, each of which contains a sulfur atom. The two sulfur atoms form a covalent S–S bond. A disulfide bond is the strongest of the four.
Hydrophobic interactions between non-polar R-groups. These R-groups are pushed together, away from water, and cluster in the centre of the protein. A hydrophobic interaction is not a bond between the groups themselves but the result of water molecules hydrogen bonding with each other and excluding them.
Hydrogen bonds, ionic bonds and hydrophobic interactions are weak individually, so heat breaks them easily, and ionic bonds are also broken when a change in pH alters the charges on R-groups. This is why proteins are denatured by heat and extreme pH. Disulfide bonds are covalent and survive moderate heating, which is why proteins that need to be very stable, such as the keratin of hair and nails, contain many of them.
Interaction | Between | Strength | Broken by |
|---|---|---|---|
Hydrogen bond | Polar R-groups (and backbone C=O and N–H in secondary structure) | Weak | Heat |
Ionic bond | Positively and negatively charged R-groups | Weak in water | Change in pH; heat |
Disulfide bond | Sulfur atoms in two cysteine R-groups | Strong (covalent) | Reducing agents; not usually by heat |
Hydrophobic interaction | Non-polar R-groups | Weak | Heat; organic solvents |
Proteins fold so that their R-groups are in the most stable positions. In a watery environment, non-polar R-groups are kept away from water and polar and charged R-groups are kept in contact with it. Where the polar and non-polar amino acids lie in the primary structure therefore controls the folded shape.
In water-soluble globular proteins, such as enzymes in the cytoplasm and haemoglobin in red blood cells, the hydrophobic amino acids cluster in the centre of the molecule, forming a hydrophobic core. The hydrophilic amino acids are on the surface, where they form hydrogen bonds with water. This arrangement makes the protein soluble, and the folding that buries the hydrophobic groups is one of the main forces that give the protein its compact shape.
Integral proteins of membranes, such as channel proteins and pumps, have one or more regions of hydrophobic amino acids, often forming α-helices about 20 amino acids long. These regions are in contact with the hydrophobic fatty acid tails in the core of the phospholipid bilayer, which holds the protein firmly in the membrane. The parts of the protein outside the bilayer, in contact with the water on either side, have hydrophilic amino acids. Channel proteins also have hydrophilic amino acids lining a pore through the centre, so that ions and polar molecules can pass through the membrane. Membrane proteins are covered in B2.1 Membranes and membrane transport.
Many proteins consist of a single polypeptide, but others are made of two or more polypeptides, called subunits, linked together. The quaternary structure is the way these subunits are arranged and held together. The same interactions that hold tertiary structure (hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions) also link the subunits.
A protein made only of polypeptides is a non-conjugated protein. A conjugated protein also contains a non-polypeptide part, called a prosthetic group, which is permanently bound and needed for the protein to work.
Insulin is non-conjugated. Its two polypeptides, the A chain (21 amino acids) and the B chain (30 amino acids), are linked by two disulfide bonds, and the A chain has a third disulfide bond within it.
Collagen is non-conjugated. Three long polypeptides, each about 1,000 amino acids long, are wound around each other into a triple helix.
Haemoglobin is conjugated. It has four polypeptides (two α-globin and two β-globin chains), and each is bound to a haem group, a ring-shaped prosthetic group with an iron ion (Fe²⁺) at its centre. Each haem group binds one O₂ molecule, so one haemoglobin molecule carries up to four.
Without its haem groups, the globin polypeptides alone cannot bind oxygen. The prosthetic groups are therefore an essential part of the working protein, which is why haemoglobin is classed as conjugated. The haem groups also give blood its red colour.
The ribbon model of haemoglobin shows its four subunits and four haem groups. Each subunit is folded mainly into α-helices, which wrap around a pocket lined with hydrophobic R-groups that holds the haem group, so all four levels of structure can be seen in one molecule.
Proteins can be grouped by shape into globular and fibrous proteins. The difference in shape comes from their primary structure, and each shape suits a different kind of function.
Insulin is a small hormone secreted by β cells in the pancreas when the blood glucose concentration is high. Its two polypeptides fold into a compact, rounded shape with hydrophilic R-groups on the surface, so it is soluble in water and is carried dissolved in blood plasma. Part of its surface has a precise shape and pattern of charges that fits a binding site on the insulin receptor in the plasma membranes of target cells, such as liver and muscle cells. Binding of insulin to its receptor triggers the uptake of glucose from the blood. The control of blood glucose is studied in D3.3 Homeostasis.
Collagen is the most abundant protein in mammals, making up roughly a quarter to a third of all the protein in the body. Each of its three polypeptides has a repeating sequence in which every third amino acid is glycine, the amino acid with the smallest R-group. Glycine is small enough to fit in the centre of the triple helix, so the three chains can pack very tightly. Many collagen molecules then line up side by side, held by covalent cross-links, to form fibres.
Collagen fibres have very high tensile strength and do not stretch much, so collagen is found where tissues must resist pulling forces: in tendons, ligaments, skin, the walls of blood vessels and the matrix of bone. Its long shape and few exposed hydrophilic groups make it insoluble, which suits a structural protein that must stay in place.
The strength of collagen depends on its precise structure. To form stable triple helices, some of the proline amino acids in collagen must be modified by an enzyme that needs vitamin C. In scurvy, caused by a lack of vitamin C in the diet, this step fails, so the collagen made is weak; wounds heal poorly, gums bleed and blood vessels become fragile.
Feature | Globular protein (insulin) | Fibrous protein (collagen) |
|---|---|---|
Overall shape | Compact, rounded | Long, thin, rope-like |
Polypeptides | Two short chains (21 and 30 amino acids) | Three long chains (about 1,000 each) |
Primary structure | Irregular sequence | Repeating sequence with glycine every third amino acid |
Solubility in water | Soluble | Insoluble |
Function | Hormone that binds a specific receptor | Structural: tensile strength |
How form suits function | Soluble so carried in blood; precise shape fits its receptor | Triple helix and fibres resist pulling; insolubility keeps it in place |
The comparison of insulin and collagen shows form and function at the level of molecules: both are made from the same 20 amino acids by the same kind of reaction, yet their different sequences give one a shape for signalling and the other a shape for support.
Proteins are assessed in Paper 1A through identification questions (which part of an amino acid varies, which bond is broken by heat), in Paper 1B through data on enzyme activity or denaturation at different temperatures and pH values, and in Paper 2 through drawings of amino acids and dipeptides and through extended responses. At HL, extended responses often ask you to explain how the levels of protein structure produce a specific shape, or to compare globular and fibrous proteins with named examples.
Marks are given for precise terms used correctly: "peptide bond", "condensation", "R-group", "conformation" and "denaturation" at SL, and "primary", "secondary", "tertiary", "quaternary", "disulfide bond", "hydrophobic interaction", "prosthetic group" and "conjugated" at HL. Diagrams must have every atom bonded correctly; carbon has four bonds and nitrogen three.
I can draw and label the generalised structure of an amino acid.
I can draw a dipeptide, label the peptide bond and write the word equation for its formation.
I can distinguish essential and non-essential amino acids and explain why vegan diets combine protein sources.
I can calculate the number of possible sequences of a polypeptide using 20ⁿ.
I can explain how heat and pH denature proteins and why denatured proteins lose their function.
I can classify R-groups as hydrophobic, polar or charged. (HL)
I can describe primary, secondary, tertiary and quaternary structure and the bonds that hold each. (HL)
I can explain how polar and non-polar amino acids position a protein in water or in a membrane. (HL)
I can compare globular and fibrous proteins and conjugated and non-conjugated proteins using insulin, collagen and haemoglobin. (HL)
[7 max]
Describe how the rate of an enzyme-catalysed reaction changes as substrate concentration increases.
4 marks
a. at low substrate concentrations the rate rises steeply / (almost) in proportion to concentration;
b. (because) collisions between substrate molecules and active sites become more frequent;
c. so more enzyme–substrate complexes form per unit time;
d. the rise in rate becomes smaller as concentration increases / curve flattens;
e. at high concentrations the rate reaches a maximum / plateau;
f. (because) all / nearly all active sites are occupied / enzyme is saturated;
g. enzyme concentration then limits the rate;
[4 max]
Accept a correctly labelled sketch graph for marks a, d and e.
One additional mark is available for the construction of the answer.
Which groups are bonded to the alpha (central) carbon atom of every amino acid?
When two amino acids are linked by a condensation reaction, which atoms form the molecule of water that is released?